Post by: jamonwindeyer on July 27, 2015, 10:19:48 am
Motion and Kinematics
Spoiler
Hello everyone! Welcome to the first in a series of guides which will give you some quick revision of the core topics of HSC Physics. These guides will aim to go over the content very quickly, so reading them should jog your memory of a whole lot of content (a very good thing with exams just over 3 months away). This, combined with some exam question examples, will hopefully prove an awesome resource for your study for Trials and the HSC.
I wanted to write this first guide to revise some content which is covered more deeply in Year 11. Kinematics, and analysis of motion, is something which is sort of hidden away in HSC Physics. But it is an essential part of the course, since pretty much all First Year Physics courses at uni are heavily based on kinematics, and will always be the basis of at least a decent amount of marks in the HSC exam. This guide will revise the key concepts from Preliminary Physics, and address what you need to know in HSC, to understand why things move the way they do.
Remember to register for an account and ask any questions you have below!
Okay, so let's quickly revise the basics. We understand that there are units of distance, and units of time. Speed is the distance travelled per unit time (i.e., speed is a measure of the rate of change of distance). Similarly, acceleration is the measure of the rate of change of speed). While you don't have to do any calculus to analyse these relationships, it is important to at least know of them. It is how all the formulas are derived (again, something you don't have to do).
But why do things move the way they do? This is where forces come in. Forces are pushing, pulling or twists motions which cause the acceleration of an object. Isaac Newton formulated the infamous three laws:
1- An object will not experience an acceleration unless a non zero net external force acts on the object.
2-
3- For every action force, there is an equal and opposite reaction force.
Forces are an example of a vector quantity . Vectors are simply quantities where you must consider direction as well as magnitude. Going 3km north is clearly different to going 3km south, for example. Quantities where no direction is involved are called scalars.
In Year 11 you also are introduced to two other ideas. First, the idea of energy. Energy is the the capacity to do work , and the energy associated with motion is called kinetic energy. You are also taught about momentum. This is a property of a moving mass similar to inertia, and is defined as
. Both of these are conservative quantities . That is, neither energy nor momentum can be created or destroyed. They can only be transferred, or in the case of energy, transformed.
These are the basic ideas you should have some familiarity with from Year 11 (there are more, but they have less relevance). They don't come into questions specifically, but they are a vital stepping stone to accessing the questions in the HSC.
In the HSC, you focus on the idea of projectiles. A projectile can be defined in a number of ways, but in the HSC, think of a projectile only in terms of its motion.
We consider the motion of projectiles in terms of separate x and y axis components. The motion of all objects can be considered this way, and using trigonometry, it is actually fairly easy to do. Consider the example below, which shows how a velocity can be resolved into vertical and horizontal components using simple right angled trigonometry.
(http://i.imgur.com/tZvRvZOm.png)
A projectile is acted on by a single force; gravity, in the vertical direction. Since we know that forces are required to produce acceleration, this means that only the vertical motion of the object experiences an acceleration, in this case equal to
, or 
(on earth). The horizontal component experiences no acceleration; thus, the horizontal motion remains constant. This is what creates the parabolic path of a projectile through the air. Galileo proved all of this mathematically based on observations made using an incline plain.
Putting all of this together, we get the following formulae for analysing projectile motion. For the x axis, or any scenario with constant velocity and no acceleration;
And for the y axis, or any scenario with a constant acceleration;
Where v is instantaneous velocity, u is initial velocity, s is displacement, t is time elapsed, and a is acceleration.
Questions concerning projectiles usually involve choosing these formulas carefully to predict a value such as range, time of flight, time of impact, etc. When tackling these questions, it is important to remember a few key facts:
Most HSC exams will have a projectile question worth at least 4 marks. The last few papers have done slightly stranger questions, referring to practicals or asking for graphs. This could mean a new trend has started. Or, it could guarantee that we will see a return to tradition. Either way, questions like this are much more common:
(http://i.imgur.com/Oga6LoUl.png)
There are numerous ways to approach this question. First, however, we should resolve the velocity into horizontal and vertical components. We wish to find the initial velocity,
.
The horizontal velocity can be expressed as
, and the vertical, 
. This can be easily seen by drawing a triangle similar to that above. Don't round anything yet, I personally leave the trig ratios there, but you can also put it in surd form if you like. Just don't round until the end!
Next, consider the range. We know that it will travel 45 metres horizontally. Therefore, since the horizontal velocity is constant, we can use the simple formula:
We can substitute this value in when considering vertical motion, at a height of 35m, to solve for V.
 \\\\ 34=90\sin{60}+4.9\left(\frac{8100}{{V}^{2}}\right) \\\\ {V}^{2}=\frac{8100\times4.9}{\left(34-90\sin{60}\right)} \\\\ V=30{ms}^{-1})
We take the positive value of the square root in that last line, we are asked for a magnitude (no direction, so no sign necessary, nor does it make sense to make it negative).
Something else which can pop up is quadratics, particularly for solving for time of flight. Be careful when this occurs. Time can obviously not take a negative value. In general, when solving quadratics, only one answer will be of use to you; think carefully about which. Also be careful of differing values for g (they will always give you these in the question).
The other thing which is essential to understand in the HSC is uniform circular motion. Uniform circular motion occurs when a force is applied perpendicular to the instantaneous velocity of an object. This results in circular motion, with a centripetal force applied towards the centre. In the HSC, this centripetal force is applied by gravitational force, but we'll look at that in another guide. For now, just understand why uniform circular motion occurs, and remember these formulae for uniform circular motion in general:
Kinematics doesn't play as large a role as other areas of Physics in the HSC, but given how important it is for tertiary study, and the fact that a projectile question of some kind always appears, it is definitely worth looking over. Practice some questions, be sure you understand how it works!
That's all for this guide. Stay tuned for more, and remember to register for an account and questions below! This is my study area at Uni, and helping you guys helps me revise as well, so fire away!
I wanted to write this first guide to revise some content which is covered more deeply in Year 11. Kinematics, and analysis of motion, is something which is sort of hidden away in HSC Physics. But it is an essential part of the course, since pretty much all First Year Physics courses at uni are heavily based on kinematics, and will always be the basis of at least a decent amount of marks in the HSC exam. This guide will revise the key concepts from Preliminary Physics, and address what you need to know in HSC, to understand why things move the way they do.
Remember to register for an account and ask any questions you have below!
Okay, so let's quickly revise the basics. We understand that there are units of distance, and units of time. Speed is the distance travelled per unit time (i.e., speed is a measure of the rate of change of distance). Similarly, acceleration is the measure of the rate of change of speed). While you don't have to do any calculus to analyse these relationships, it is important to at least know of them. It is how all the formulas are derived (again, something you don't have to do).
But why do things move the way they do? This is where forces come in. Forces are pushing, pulling or twists motions which cause the acceleration of an object. Isaac Newton formulated the infamous three laws:
1- An object will not experience an acceleration unless a non zero net external force acts on the object.
2-
3- For every action force, there is an equal and opposite reaction force.
Forces are an example of a vector quantity . Vectors are simply quantities where you must consider direction as well as magnitude. Going 3km north is clearly different to going 3km south, for example. Quantities where no direction is involved are called scalars.
In Year 11 you also are introduced to two other ideas. First, the idea of energy. Energy is the the capacity to do work , and the energy associated with motion is called kinetic energy. You are also taught about momentum. This is a property of a moving mass similar to inertia, and is defined as
These are the basic ideas you should have some familiarity with from Year 11 (there are more, but they have less relevance). They don't come into questions specifically, but they are a vital stepping stone to accessing the questions in the HSC.
In the HSC, you focus on the idea of projectiles. A projectile can be defined in a number of ways, but in the HSC, think of a projectile only in terms of its motion.
We consider the motion of projectiles in terms of separate x and y axis components. The motion of all objects can be considered this way, and using trigonometry, it is actually fairly easy to do. Consider the example below, which shows how a velocity can be resolved into vertical and horizontal components using simple right angled trigonometry.
(http://i.imgur.com/tZvRvZOm.png)
A projectile is acted on by a single force; gravity, in the vertical direction. Since we know that forces are required to produce acceleration, this means that only the vertical motion of the object experiences an acceleration, in this case equal to
Putting all of this together, we get the following formulae for analysing projectile motion. For the x axis, or any scenario with constant velocity and no acceleration;
And for the y axis, or any scenario with a constant acceleration;
Where v is instantaneous velocity, u is initial velocity, s is displacement, t is time elapsed, and a is acceleration.
Questions concerning projectiles usually involve choosing these formulas carefully to predict a value such as range, time of flight, time of impact, etc. When tackling these questions, it is important to remember a few key facts:
- The vertical velocity of a projectile at its highest point (i.e., the peak of motion), is zero. This can be a useful start for many projectile questions.
- The time taken to reach the maximum height is half that taken to return to the initial height
- The maximum range of a projectile is achieved when it is launched at an angle of 45/degree
- Any x position for impact with the ground (or initial height) less than the maximum range can be achieved with two different launch angles (you don't need to prove this, and it rarely comes up, but good to keep in mind)
Most HSC exams will have a projectile question worth at least 4 marks. The last few papers have done slightly stranger questions, referring to practicals or asking for graphs. This could mean a new trend has started. Or, it could guarantee that we will see a return to tradition. Either way, questions like this are much more common:
(http://i.imgur.com/Oga6LoUl.png)
There are numerous ways to approach this question. First, however, we should resolve the velocity into horizontal and vertical components. We wish to find the initial velocity,
The horizontal velocity can be expressed as
Next, consider the range. We know that it will travel 45 metres horizontally. Therefore, since the horizontal velocity is constant, we can use the simple formula:
We can substitute this value in when considering vertical motion, at a height of 35m, to solve for V.
We take the positive value of the square root in that last line, we are asked for a magnitude (no direction, so no sign necessary, nor does it make sense to make it negative).
Something else which can pop up is quadratics, particularly for solving for time of flight. Be careful when this occurs. Time can obviously not take a negative value. In general, when solving quadratics, only one answer will be of use to you; think carefully about which. Also be careful of differing values for g (they will always give you these in the question).
The other thing which is essential to understand in the HSC is uniform circular motion. Uniform circular motion occurs when a force is applied perpendicular to the instantaneous velocity of an object. This results in circular motion, with a centripetal force applied towards the centre. In the HSC, this centripetal force is applied by gravitational force, but we'll look at that in another guide. For now, just understand why uniform circular motion occurs, and remember these formulae for uniform circular motion in general:
Kinematics doesn't play as large a role as other areas of Physics in the HSC, but given how important it is for tertiary study, and the fact that a projectile question of some kind always appears, it is definitely worth looking over. Practice some questions, be sure you understand how it works!
That's all for this guide. Stay tuned for more, and remember to register for an account and questions below! This is my study area at Uni, and helping you guys helps me revise as well, so fire away!
A GUIDE BY JAMON WINDEYER
Rocket Launches
Spoiler
Hello again everyone! Time for another HSC Physics Guide! This one is going to cover a few dot points, all concerning the physics of rocket launches. This means all those associated dot points, and I'll mix in gravity as well. Like the other guides, this guide will aim to summarise all the related content and address a few common exam style questions. Essentially, the guides will form something you can read to revise the whole course in a couple of hours. Covering this amount of content means that I can't go into as much detail as you may need. While I'll try to slow down to cover the hard stuff, you may have questions. In that case:
Remember to register for an account and ask any questions you have below!
Let's begin. I'll cover gravity first!
Gravity is one of the four fundamental forces, caused by the mass of objects. Gravitational fields surround all masses, and are theoretically infinite in size. They decrease rapidly in strength with distance from the object (inverse square law), and are stronger for greater masses. Any mass in a gravitational field will experience an attractive force, according to Newton's Law of Universal Gravitation
.
Gravitational potential energy (GPE) is the energy possessed by an object due to its position in a gravitational field. In HSC Physics, we take the zero point for GPE (when the GPE of an object is zero) as when the distance from the origin from the field is infinite. As the object moves closer, it loses GPE, and so the value becomes negative. Thus, GPE is a negative quantity. See the diagram below if that is a little unclear. The formula for GPE is
.
(http://i.imgur.com/BYMuv7zl.jpg)
Questions on gravity alone are unusual, but questions on GPE are quite common.
Example One: What is the gravitational potential energy of the moon with respect to the earth? The mass of the moon is
kilograms and the mass of the earth is 
kilograms. The earth moon distance is 
kilometres.
This is simply a formula question. Now, this question doesn't require it, but a lot of questions have an extra trick; remember the distance is taken from the centre of the earth. Add the radius of the earth to your distance if necessary (EG- if the question says altitude). Other tricks, ensure you convert kilometres to metres, and check your data sheets for seemingly missing quantities. Regardless, we substitute:
Next, the physics of rocket launches. You should understand (in a little more detail than I give here) the following:
Any of these concepts can be asked in a describe/explain question, or where applicable, a mathematical question. Be sure to know enough to explain what is going on for each concept. The most common question asked, which I'll answer below, concerns a final concept: An Analysis of a Rocket Launch.
Example Two: Analyse the changing acceleration of a rocket during launch in terms of the Law of Conservation of Momentum and the forces experienced by astronauts
This is a direct syllabus dot point, and if this was asked in an exam, it would be worth a bare minimum of 6 marks. Likely 8. You would answer similar to this:
Obviously this is a quite demanding question, and would likely not be so broad in the exam. But it does pop up in some form a lot , so be ready. You should also be ready for the stranger questions, usually concerning your practical investigations, and/or your research into a chosen rocket scientist . That last point is in over half the papers from the last few years, they like to punish the people who forget it ;)
The final area which requires understanding is orbits. An orbit occurs when the object is travelling at a certain speed; not fast enough to escape a gravitational field, but fast enough to not be sucked in to the centre. In this course, we consider orbits as uniform circular motion, with the centripetal force provided by the gravitational force. By equating these two formulae, we can obtain many quantities. The orbital velocity formula is one of these:
We can also equate centripetal and gravitational force, and with some clever substitutions, derive Kepler's Law of Periods:
Mathematical questions concerning Kepler's Law of Periods are common, such as the one below:
Example Three: A space probe is placed in an orbit at an altitude of 188 km above Earth. Given Earth has a radius of 6380 km, calculate the period of this orbit.
Kepler's law of periods makes this an easy question, but be careful with your value for
!
Also note the use of SI UNITS . Rearrange to get an answer of 88.11 minutes for T (we take the positive value).
There are other concepts to do with orbit which can be asked in describe/explain style questions. Be sure you are familiar with:
There is much less here than above, but I guarantee that Kepler's Law will show up, and one of these other concepts is likely to pop up as well, perhaps as a quick 2 mark question. Further, few questions in these sections would require any diagrams, so this makes things a little easier to prepare for (getting full marks for diagrams is tricky). However, be sure to draw them anyway if it helps your explanations, and the marker will love it.
That's about it for gravity and rocket launches! Hopefully this guide proves useful. Read over it, see if the summaries jog memories. If things are a bit iffy, do some practice questions, and of course, feel free to register for an account and ask any questions you have in the comments. It is difficult to cover everything for Physics in a guide which doesn't become excessively long, so ask away! I'm happy to help.
Remember to register for an account and ask any questions you have below!
Let's begin. I'll cover gravity first!
Gravity is one of the four fundamental forces, caused by the mass of objects. Gravitational fields surround all masses, and are theoretically infinite in size. They decrease rapidly in strength with distance from the object (inverse square law), and are stronger for greater masses. Any mass in a gravitational field will experience an attractive force, according to Newton's Law of Universal Gravitation
Gravitational potential energy (GPE) is the energy possessed by an object due to its position in a gravitational field. In HSC Physics, we take the zero point for GPE (when the GPE of an object is zero) as when the distance from the origin from the field is infinite. As the object moves closer, it loses GPE, and so the value becomes negative. Thus, GPE is a negative quantity. See the diagram below if that is a little unclear. The formula for GPE is
(http://i.imgur.com/BYMuv7zl.jpg)
Questions on gravity alone are unusual, but questions on GPE are quite common.
Example One: What is the gravitational potential energy of the moon with respect to the earth? The mass of the moon is
This is simply a formula question. Now, this question doesn't require it, but a lot of questions have an extra trick; remember the distance is taken from the centre of the earth. Add the radius of the earth to your distance if necessary (EG- if the question says altitude). Other tricks, ensure you convert kilometres to metres, and check your data sheets for seemingly missing quantities. Regardless, we substitute:
Next, the physics of rocket launches. You should understand (in a little more detail than I give here) the following:
- The idea of escape velocity. Newton envisaged this as firing a cannonball increasingly quickly, thus causing it to travel further. Eventually, it would travel all the way around the earth, and then eventually beyond. This velocity is called escape velocity , and is defined as
- The idea of G forces, used to define the reaction forces on an astronaut in terms of the force felt due to gravity on the earth's surface
- How the earth's rotational and orbital motion can be used to increase the velocity of a rocket, and the associated benefits for fuel economy. We can launch rockets to the east to gain the earth's rotational velocity (picture a catapult). Similar reasoning applies to the earths orbit around the sun.
- The slingshot effect, whereby a rockets velocity is increased due to interaction with the gravitational field of a planet or other astronomical body. Picture that scene in Ferris Bueller's Day Off, where Ferris grabs on to the cars while he is skateboarding. He speeds up, because he is being 'dragged' along.
- The ideal re-entry angle (5-7 degrees), and the consequences of failing to meet this angle. Too shallow, and the rocket will bounce back into space, since excessive kinetic energy remains. Too steep, and the rocket will overheat and disintegrate. There are also other re-entry issues, including ionisation blackout and heat, and shedding the adequate amount of energy to land
Any of these concepts can be asked in a describe/explain question, or where applicable, a mathematical question. Be sure to know enough to explain what is going on for each concept. The most common question asked, which I'll answer below, concerns a final concept: An Analysis of a Rocket Launch.
Example Two: Analyse the changing acceleration of a rocket during launch in terms of the Law of Conservation of Momentum and the forces experienced by astronauts
This is a direct syllabus dot point, and if this was asked in an exam, it would be worth a bare minimum of 6 marks. Likely 8. You would answer similar to this:
- The law of conservation of momentum states that the momentum within a system (eg- a rocket) must remain constant.
- Since the rocket is initially stationary, this means the momentum of the rocket system (rocket + fuel) must remain equal to zero.
- As the rocket is launched, the fuel is accelerated downwards as thrust, and thus has momentum downwards. Therefore, the rocket has momentum upwards (and equal in magnitude)
-
- Assuming constant thrust, the momentum of the fuel remains constant. However, the mass of the rocket decreases as fuel is burnt, so the velocity of the rocket must increase (since momentum must remain constant). Thus, the rocket accelerates upwards
- The force to accelerate the rocket upwards is provided by the reaction to the thrust force. We assume this force is constant. However, according to
- This means that, as acceleration increases, the astronauts experience increasingly large G-forces throughout the initial stages of launch
- Assuming multi stage rockets are used, these G forces briefly drop to zero as the stages are jettisoned (since acceleration is zero at this time). This is essential, as prolonged exposure to large G forces can be fatal
Obviously this is a quite demanding question, and would likely not be so broad in the exam. But it does pop up in some form a lot , so be ready. You should also be ready for the stranger questions, usually concerning your practical investigations, and/or your research into a chosen rocket scientist . That last point is in over half the papers from the last few years, they like to punish the people who forget it ;)
The final area which requires understanding is orbits. An orbit occurs when the object is travelling at a certain speed; not fast enough to escape a gravitational field, but fast enough to not be sucked in to the centre. In this course, we consider orbits as uniform circular motion, with the centripetal force provided by the gravitational force. By equating these two formulae, we can obtain many quantities. The orbital velocity formula is one of these:
We can also equate centripetal and gravitational force, and with some clever substitutions, derive Kepler's Law of Periods:
Mathematical questions concerning Kepler's Law of Periods are common, such as the one below:
Example Three: A space probe is placed in an orbit at an altitude of 188 km above Earth. Given Earth has a radius of 6380 km, calculate the period of this orbit.
Kepler's law of periods makes this an easy question, but be careful with your value for
Also note the use of SI UNITS . Rearrange to get an answer of 88.11 minutes for T (we take the positive value).
There are other concepts to do with orbit which can be asked in describe/explain style questions. Be sure you are familiar with:
- Comparing low earth and geostationary orbits. Low earth orbits have a higher velocity and shorter period that geostationary ones (the closer an object to the centre of a field, the faster it must travel to remain in orbit)
- Accounting for orbital decay for satellites. Simply, it is caused by ozone particles, which impart a frictional force which decelerates the satellite, causing it to fall from its orbital path.
There is much less here than above, but I guarantee that Kepler's Law will show up, and one of these other concepts is likely to pop up as well, perhaps as a quick 2 mark question. Further, few questions in these sections would require any diagrams, so this makes things a little easier to prepare for (getting full marks for diagrams is tricky). However, be sure to draw them anyway if it helps your explanations, and the marker will love it.
That's about it for gravity and rocket launches! Hopefully this guide proves useful. Read over it, see if the summaries jog memories. If things are a bit iffy, do some practice questions, and of course, feel free to register for an account and ask any questions you have in the comments. It is difficult to cover everything for Physics in a guide which doesn't become excessively long, so ask away! I'm happy to help.
A GUIDE BY JAMON WINDEYER
Relativity
Spoiler
Hey everyone! Time for another Physics guide. This one is going to cover relativity; and all the associated content, equations, explanations, as well as visiting some potential exam questions. Relativity is undoubtedly the most confusing and troubling part of HSC Physics, with some of the hardest concepts, most difficult formulae, and will generally prove a bit of a pain in the exam. The trick is to understand the ideas of relativity; if you understand it, potentially confusing questions become easy marks.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
I'll first cover the idea of relativity in general. It is to do with reference frames . These can be defined a number of ways, but in HSC Physics, simply consider them as a reference point for measurements, or a 'zero' point. For example, from my reference frame sitting at my desk, I consider myself stationary. However, from the reference frame of the sun, I am actually moving quite quickly in an orbit. The idea to understand is that all measurements are made with reference to some zero point. This was the initial idea of relativity, that all motion is relative to something else.
There are two kinds of reference frames; inertial and non inertial . Again, these can be defined in depth, but for this course you need only remember that non-inertial frames of reference are accelerating . Inertial frames are not accelerating. In non inertial frames, we have to invent fictitious forces to account for the physical behaviour of objects.
Questions on these initial concepts of relativity are quite rare. However, if they do pop up, it is most likely to ask you something similar to one of the following:
In the 1800's, the idea of an absolute rest frame developed, something against which all other motion could be measured. This frame was called The Aether , an undetectable material which permeated all matter in the universe, and also acted as the medium by which light was transmitted. At this stage, it was thought all waves permeated through a medium; the light was thought to permeate through the Aether.
Michelson and Morley attempted to detect the Aether by detecting the earth's movement through it. To do this, they tried to detect changes in the interference patterns of two beams of light. A beam was split, and the effect of the Aether searched for in interference patterns between them. The apparatus is shown below. But this experiment yielded a null result, discrediting the Aether model and inspiring perhaps the greatest piece of mathematics/science literature since the Principia Mathematica by Isaac Newton.
(http://i.imgur.com/3XzVS9Yl.gif)
Questions on this experiment are VERY common. Be sure you can replicate this diagram, and talk about the purpose correctly. They were NOT trying to measure the velocity of light. They were NOT trying to detect the Aether. They were attempting to measure the velocity of the earth through the Aether . Be very careful there. Besides this, quick explanations of the logic behind the experiment, should form the basis of most questions here.
Einstein published his Special Theory of Relativity in 1905. In it, he suggested that ALL inertial frames of reference are equally valid. Further, he suggested that the speed of light (
) was an absolute constant, the same in all frames of reference. His reasoning was based on thought experimentation. Consider yourself on a train travelling very close to the speed of light. You bounce a light off the ground and to the roof. You see it go straight up and down, but another observer sees it differently. Something has to change...
(http://i.imgur.com/CC9MeZAl.png)
The astounding consequences of this theory are that time, mass and length all become relative quantities. Simulteneity becomes a relative statement also. Mass dilation, time dilation, and length contraction can be described by the following quantities.
The important thing to note here is that mass and time quantities increase as the velocity of a reference frame increases. Length decreases. This is an easy way to check answers to common questions like this (one pops up in almost every HSC exam):
Example One: The distance between the cathode and screen in a cathode ray tube is 40cm. If an electron travels through the tube at
, what is the apparent distance from the cathode to the screen in the electron’s frame of reference?
Don't be confused about the wording, this is simply a sub and go. Be careful with SI units and substituting in the correct places!
These questions are usually muddled in interpretation. Re read the question. Be sure you understand what it is asking, and substitute in the right place.
The other consequence of this theory is that mass and energy are equivalent, a very interesting idea. This of course links to the infamous equation,
. Interestingly, this equation is not asked as a mathematical question very often at all. Probably because it is one of the easiest formula in the course. Much more common is an explanation of the equivalence of mass and energy.
Accounting for this is simple. Consider an object travelling at close to the speed of light. If we apply a force, kinetic energy increases. But the velocity cannot exceed the speed of light. Thus, mass must increase. Therefore, we can deduce that mass and energy must be equivalent, since we increase the mass of an object by doing work on it.
The trickiest questions in a HSC exam often come from this topic, and usually concern Einstein. There are even questions which link to later topics on his political opinions! Tricky questions here, however, often concern Einstein's thought experiments. Be sure you can explain them, and draw simple diagrams like the one above, to make your meaning clear. Further, be prepared to discuss (quite loosely):
Questions on relativity in the HSC are punishing; they ask a HSC student to describe and explain one of the most complex ideas in modern physics. My biggest tips; be VERY careful with your wording, leave no room for interpretation. For example, do NOT write: "Special relativity means that the speed of light is constant, which means that mass, time and length are relative."... No mention of reference frames, no explanation, it screams "I MEMORISED THIS SENTENCE." Don't do this. Understand why these quantities are relative. Understand the role of reference frames. It is worth the time investment.
That is actually about all for relativity. Not much in terms of content; but very demanding subject area. Invest time to truly understand what is going on, and questions in these areas will become easy marks for you! Be sure to ask any questions you have below. It doesn't take long to register for an account , and it is an awesome way to pick the brains of the students around you. They are your best resource.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
I'll first cover the idea of relativity in general. It is to do with reference frames . These can be defined a number of ways, but in HSC Physics, simply consider them as a reference point for measurements, or a 'zero' point. For example, from my reference frame sitting at my desk, I consider myself stationary. However, from the reference frame of the sun, I am actually moving quite quickly in an orbit. The idea to understand is that all measurements are made with reference to some zero point. This was the initial idea of relativity, that all motion is relative to something else.
There are two kinds of reference frames; inertial and non inertial . Again, these can be defined in depth, but for this course you need only remember that non-inertial frames of reference are accelerating . Inertial frames are not accelerating. In non inertial frames, we have to invent fictitious forces to account for the physical behaviour of objects.
Questions on these initial concepts of relativity are quite rare. However, if they do pop up, it is most likely to ask you something similar to one of the following:
- Differentiate between inertial and non inertial frames of reference
- Describe the idea of relativity.
- What is a frame of reference?
In the 1800's, the idea of an absolute rest frame developed, something against which all other motion could be measured. This frame was called The Aether , an undetectable material which permeated all matter in the universe, and also acted as the medium by which light was transmitted. At this stage, it was thought all waves permeated through a medium; the light was thought to permeate through the Aether.
Michelson and Morley attempted to detect the Aether by detecting the earth's movement through it. To do this, they tried to detect changes in the interference patterns of two beams of light. A beam was split, and the effect of the Aether searched for in interference patterns between them. The apparatus is shown below. But this experiment yielded a null result, discrediting the Aether model and inspiring perhaps the greatest piece of mathematics/science literature since the Principia Mathematica by Isaac Newton.
(http://i.imgur.com/3XzVS9Yl.gif)
Questions on this experiment are VERY common. Be sure you can replicate this diagram, and talk about the purpose correctly. They were NOT trying to measure the velocity of light. They were NOT trying to detect the Aether. They were attempting to measure the velocity of the earth through the Aether . Be very careful there. Besides this, quick explanations of the logic behind the experiment, should form the basis of most questions here.
Einstein published his Special Theory of Relativity in 1905. In it, he suggested that ALL inertial frames of reference are equally valid. Further, he suggested that the speed of light (
(http://i.imgur.com/CC9MeZAl.png)
The astounding consequences of this theory are that time, mass and length all become relative quantities. Simulteneity becomes a relative statement also. Mass dilation, time dilation, and length contraction can be described by the following quantities.
The important thing to note here is that mass and time quantities increase as the velocity of a reference frame increases. Length decreases. This is an easy way to check answers to common questions like this (one pops up in almost every HSC exam):
Example One: The distance between the cathode and screen in a cathode ray tube is 40cm. If an electron travels through the tube at
Don't be confused about the wording, this is simply a sub and go. Be careful with SI units and substituting in the correct places!
These questions are usually muddled in interpretation. Re read the question. Be sure you understand what it is asking, and substitute in the right place.
The other consequence of this theory is that mass and energy are equivalent, a very interesting idea. This of course links to the infamous equation,
Accounting for this is simple. Consider an object travelling at close to the speed of light. If we apply a force, kinetic energy increases. But the velocity cannot exceed the speed of light. Thus, mass must increase. Therefore, we can deduce that mass and energy must be equivalent, since we increase the mass of an object by doing work on it.
The trickiest questions in a HSC exam often come from this topic, and usually concern Einstein. There are even questions which link to later topics on his political opinions! Tricky questions here, however, often concern Einstein's thought experiments. Be sure you can explain them, and draw simple diagrams like the one above, to make your meaning clear. Further, be prepared to discuss (quite loosely):
- The advantages of thought experiments, in terms of limitations on current technologies.
- The link between theory and proof
- The proof of special relativity. This has been observed in atomic clocks, and in the lifespan of small, sub-atomic particles.
Questions on relativity in the HSC are punishing; they ask a HSC student to describe and explain one of the most complex ideas in modern physics. My biggest tips; be VERY careful with your wording, leave no room for interpretation. For example, do NOT write: "Special relativity means that the speed of light is constant, which means that mass, time and length are relative."... No mention of reference frames, no explanation, it screams "I MEMORISED THIS SENTENCE." Don't do this. Understand why these quantities are relative. Understand the role of reference frames. It is worth the time investment.
That is actually about all for relativity. Not much in terms of content; but very demanding subject area. Invest time to truly understand what is going on, and questions in these areas will become easy marks for you! Be sure to ask any questions you have below. It doesn't take long to register for an account , and it is an awesome way to pick the brains of the students around you. They are your best resource.
A GUIDE BY JAMON WINDEYER
Motors and Motor Effect
Spoiler
Hello again everyone! Time once again for another Physics guide. We are done with the Space part of the course, so if you have any questions there, feel free to post it in the forums and either myself, another moderator, or one of your fellow HSC’ers will be sure to help you out. Now we move on to motors and generators. The first will cover motors; a little shorter than the next two guides, but it forms the basis of knowledge in some key areas. I’m going to assume a basic understanding of current, resistance and voltage; there are heaps of resources available if you need a refresh!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
So, firstly, we need to define a very vital effect; the motor effect . The motor effect is the force experienced by a current carrying conductor in a magnetic field. We look at this in more detail in Ideas to Implementation, but essentially, a moving electric charge (ie, a current) will experience a force in a magnetic field. This causes a force on the conductor.
The force on the conductor due to a motor effect is dependent on the strength of the magnetic field
, the current 
, the length of conductor in the field 
, and the angle 
between the conductor and the magnetic field lines.
This formula makes it clear that the force is maximised when the conductor is perpendicular to the field, and is zero if it is parallel.
Before we look at motors, one more thing. Parallel conductors carrying a current will experience a force between them. If the current is in the same direction, this force is attractive. Otherwise, it is repulsive. This is a common mathematical question in the HSC, which normally makes for easy marks. Just use the formula, where
:
The motor effect is the basis for the operation of motors. The simple DC motor studied in the course looks like this. In terms of the motor effect, current is shown in blue, magnetic field in purple, force in green. All the elements are perpendicular. This is the right hand slap rule! Hold up your right hand like you are waving, with your thumb pointed out. If the magnetic field lines are your fingers, and the current is your thumb, you will slap in the direction of the force
(http://i.imgur.com/i1mj7CS.png)
It consists of two main parts.
The stator creates the required magnetic field. This may be provided by permanent magnets, or by electromagnets (current carrying coils). It needs only to produce a field of the required strength.
The rotor is the part of the motor that spins. This normally consists of an axle which is attached to some external load (EG- a wheel), and the armature. The armature consists of the coils, usually wrapped around an iron core. A current flows through this coil, connected to a power source through brushes which maintain contact with the circuitry, as the motor spins.
The coils are what makes the motor turn. According to the motor effect, the two sides of the coil perpendicular to the field will experience a force. The current flows in opposite directions on either side, so this force is in opposite directions. This causes the motor to spin.
We don’t normally consider the individual forces on the sides of the coil. We consider the resultant torque. Torque is, put simply, turning force. Forces act linearly, torque acts rotationally. The simple formula for torque we need for this course is
, where 
is a force applied at a distance 
from the rotation point. We also have a formula for the torque experienced by a coil in a motor, which is dependent on magnetic field , the area of the coil 
, the current , the number of turns 
in the coil, and the angle with the field.
What we notice about this formula is that the torque on the coil will change direction every half turn (180 degrees). Obviously, for a DC motor, we want the torque to be in a single direction. This is where the role of the split ring commutator proves vital. In DC motors, this commutator is connected to both sides of the coil, with a split between each section. This commutator spins as the axle spins, and the brushes come into contact with the opposite side of the commutator every half turn. This reverses the direction of current through the coil every half turn, thus maintaining a constant direction of torque.
Now, all of this is quite hard to picture, and very easily confusing. I found the easiest way to understand how it all worked was video content. There are lots of resources available, with animations or active diagrams, which will prove very useful if you are having trouble. Thinking back to your practicals will undoubtedly help too. Just try a whole bunch of things until you wrap your head around it, and of course, if you need more detailed explanations, just ask! Happy to provide if they are needed.
Let’s look at a very common mathematical question:
(http://i.imgur.com/OkMFDael.png)
Part One : Determine the force needed to lift the mass.
This ties back into our Space topic. To lift the mass, we need a force equal to the weight force of the object. Therefore,
Part Two : Calculate the minimum current required in the coil to lift the mass.
This questions asks us to come up with the torque required to generate this required force. We equate our two formulae for torque to find the required current. I’ll leave out the rearranging at the end:
Explaining the operation of a DC motor is a VERY common question. Always draw a diagram, label the key parts, and explain what they do. Set your information out clearly; there is a lot to cover, and the markers don’t want to see a big mess of scribbles and extra dot points in a confusing layout. Even if you add stuff, keep everything neat and as logical as possible.
Just as common, if not more so, is explaining how the motor effect is applied in either a loudspeaker or a galvanometer. These can be doozy questions. The trick with these is to make sure you explain how it works! Marks are lost by describing the part and their function, but not explaining how it contributes to the function. For example, consider the difference between these two sample responses:
A galvanometer uses a radial magnetic field created by a permanent magnet. A coil is attached to the needle, and anchored to a spring as shown. When a current goes through the coil, the needle moves to match the scale.
... and ...
A galvanometer utilises the motor effect for its operation. It uses a radial magnetic field created by a permanent magnet; this means that the torque experienced by the coil is constant at all points of its rotation. The torque experienced can therefore be defined as
, where B, A and n are constant. Thus, torque is directly proportional to current. A spring provides an opposing torque, so that when a current flows through the coil, it rotates to the appropriate scale on the meter for the user.
Neither are perfect, but this a 2 mark versus a 4 mark response, roughly speaking. In the exam you would be better off using dot points for these. Keep this in mind! So, information on a galvanometer and a loudspeaker. Be sure you have a simple diagram in mind to draw (some good ones are shown):
Electric meters utilise the motor effect to measure current. Such meters utilise a radial magnetic field, meaning an identical torque is experienced at any angle within the housing. Thus, if the same coil is used, torque is directly proportional to the current flowing through the meter. The armature is designed to spin and stretch a spring; a greater current is required to generate enough torque to spin the armature against this opposing spring torque. This setup is calibrated so that the current changes produce appropriate changes in the position of a needle.
(http://i.imgur.com/QjGzML7.png)
The loudspeaker also works due to the motor effect. An electromagnet is fed an Alternating Current, causing it to experience a force in interaction with the permanent magnetic field. The electromagnet vibrates with the same frequency as the input AC voltage, vibrating a sound cone which produces sound waves.
(http://www.s-cool.co.uk/assets/test_its/alevel/physics/forces-in-magnetic-fields/dia01.jpg)
Besides this, some other miscellaneous questions which could be asked:
For this last part, you'll have to read the next guide on Generators, stay tuned!
Thanks for reading! If you have any questions, take 30 seconds to register and ask any questions you have below! I am happy to help, and it's a great chance to get some help and advice from the community. Happy study!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
So, firstly, we need to define a very vital effect; the motor effect . The motor effect is the force experienced by a current carrying conductor in a magnetic field. We look at this in more detail in Ideas to Implementation, but essentially, a moving electric charge (ie, a current) will experience a force in a magnetic field. This causes a force on the conductor.
The force on the conductor due to a motor effect is dependent on the strength of the magnetic field
This formula makes it clear that the force is maximised when the conductor is perpendicular to the field, and is zero if it is parallel.
Before we look at motors, one more thing. Parallel conductors carrying a current will experience a force between them. If the current is in the same direction, this force is attractive. Otherwise, it is repulsive. This is a common mathematical question in the HSC, which normally makes for easy marks. Just use the formula, where
The motor effect is the basis for the operation of motors. The simple DC motor studied in the course looks like this. In terms of the motor effect, current is shown in blue, magnetic field in purple, force in green. All the elements are perpendicular. This is the right hand slap rule! Hold up your right hand like you are waving, with your thumb pointed out. If the magnetic field lines are your fingers, and the current is your thumb, you will slap in the direction of the force
(http://i.imgur.com/i1mj7CS.png)
It consists of two main parts.
The stator creates the required magnetic field. This may be provided by permanent magnets, or by electromagnets (current carrying coils). It needs only to produce a field of the required strength.
The rotor is the part of the motor that spins. This normally consists of an axle which is attached to some external load (EG- a wheel), and the armature. The armature consists of the coils, usually wrapped around an iron core. A current flows through this coil, connected to a power source through brushes which maintain contact with the circuitry, as the motor spins.
The coils are what makes the motor turn. According to the motor effect, the two sides of the coil perpendicular to the field will experience a force. The current flows in opposite directions on either side, so this force is in opposite directions. This causes the motor to spin.
We don’t normally consider the individual forces on the sides of the coil. We consider the resultant torque. Torque is, put simply, turning force. Forces act linearly, torque acts rotationally. The simple formula for torque we need for this course is
What we notice about this formula is that the torque on the coil will change direction every half turn (180 degrees). Obviously, for a DC motor, we want the torque to be in a single direction. This is where the role of the split ring commutator proves vital. In DC motors, this commutator is connected to both sides of the coil, with a split between each section. This commutator spins as the axle spins, and the brushes come into contact with the opposite side of the commutator every half turn. This reverses the direction of current through the coil every half turn, thus maintaining a constant direction of torque.
Now, all of this is quite hard to picture, and very easily confusing. I found the easiest way to understand how it all worked was video content. There are lots of resources available, with animations or active diagrams, which will prove very useful if you are having trouble. Thinking back to your practicals will undoubtedly help too. Just try a whole bunch of things until you wrap your head around it, and of course, if you need more detailed explanations, just ask! Happy to provide if they are needed.
Let’s look at a very common mathematical question:
(http://i.imgur.com/OkMFDael.png)
Part One : Determine the force needed to lift the mass.
This ties back into our Space topic. To lift the mass, we need a force equal to the weight force of the object. Therefore,
Part Two : Calculate the minimum current required in the coil to lift the mass.
This questions asks us to come up with the torque required to generate this required force. We equate our two formulae for torque to find the required current. I’ll leave out the rearranging at the end:
Explaining the operation of a DC motor is a VERY common question. Always draw a diagram, label the key parts, and explain what they do. Set your information out clearly; there is a lot to cover, and the markers don’t want to see a big mess of scribbles and extra dot points in a confusing layout. Even if you add stuff, keep everything neat and as logical as possible.
Just as common, if not more so, is explaining how the motor effect is applied in either a loudspeaker or a galvanometer. These can be doozy questions. The trick with these is to make sure you explain how it works! Marks are lost by describing the part and their function, but not explaining how it contributes to the function. For example, consider the difference between these two sample responses:
A galvanometer uses a radial magnetic field created by a permanent magnet. A coil is attached to the needle, and anchored to a spring as shown. When a current goes through the coil, the needle moves to match the scale.
... and ...
A galvanometer utilises the motor effect for its operation. It uses a radial magnetic field created by a permanent magnet; this means that the torque experienced by the coil is constant at all points of its rotation. The torque experienced can therefore be defined as
Neither are perfect, but this a 2 mark versus a 4 mark response, roughly speaking. In the exam you would be better off using dot points for these. Keep this in mind! So, information on a galvanometer and a loudspeaker. Be sure you have a simple diagram in mind to draw (some good ones are shown):
Electric meters utilise the motor effect to measure current. Such meters utilise a radial magnetic field, meaning an identical torque is experienced at any angle within the housing. Thus, if the same coil is used, torque is directly proportional to the current flowing through the meter. The armature is designed to spin and stretch a spring; a greater current is required to generate enough torque to spin the armature against this opposing spring torque. This setup is calibrated so that the current changes produce appropriate changes in the position of a needle.
(http://i.imgur.com/QjGzML7.png)
The loudspeaker also works due to the motor effect. An electromagnet is fed an Alternating Current, causing it to experience a force in interaction with the permanent magnetic field. The electromagnet vibrates with the same frequency as the input AC voltage, vibrating a sound cone which produces sound waves.
(http://www.s-cool.co.uk/assets/test_its/alevel/physics/forces-in-magnetic-fields/dia01.jpg)
Besides this, some other miscellaneous questions which could be asked:
- Getting the direction of rotation from given information. Use the right hand slap rule for each side of the coil to check. Remember, magnetic field lines go from N to S, and conventional current flows from + to -
- Describing the function of particular parts of the motor. Easy marks if you know your stuff
- Comparisons between motors and generators
For this last part, you'll have to read the next guide on Generators, stay tuned!
Thanks for reading! If you have any questions, take 30 seconds to register and ask any questions you have below! I am happy to help, and it's a great chance to get some help and advice from the community. Happy study!
A GUIDE BY JAMON WINDEYER
Generators and Induction
Spoiler
Hello again everyone! Time again for another guide on HSC Physics. This one is going to cover generators and electromagnetic induction. Quite a few concepts here, but nothing overly complicated, which is good news! Be sure to go check out my guide on Motors to give this stuff some context.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
First, electromagnetic induction and associated theory. There is a lot to cover here before we even look at Generators.
Faraday was the first to observe electromagnetic induction; he detected an induced current in a solenoid when it was exposed to a changing magnetic field. This is an experiment you are likely to have replicated; it is quite simple. Faraday discovered that the induced current was stronger when the rate of change of magnetic flux density (magnetic field strength) was greater.
Here, it is important to distinguish between magnetic flux and magnetic flux density. Magnetic Flux is simply the number of magnetic field lines passing through a given area. Magnetic flux density, however, is how we measure magnetic fields, and is found by dividing magnetic flux by the area.
(http://i.imgur.com/ZWwSkzH.png)
Next to cover is Lenz's Law . This law states that an induced emf (electromotive force, potential difference, all essentially the same) always generates a magnetic field which opposes the change that created it. This makes sense in terms of the Conservation of Energy; if the new magnetic field assisted the change, then energy would be created. In motors, we remember that the coil spins inside a magnetic field. Thus, the coil experiences a changing magnetic flux density, and thus, a current is induced. Lenz's law says that this new current/emf acts in the opposite direction, thus generating an opposing torque. This is called back emf .
Back emf opposes the supply emf in a motor. Back emf increases as the motor spins faster; thus, a stationary motor contains much more current than a spinning one!
Eddy currents are a common source of confusion. Eddy currents are circular flows of current which occur in conductive materials exposed to a changing magnetic field. According to Lenz’s Law, this changing magnetic flux will induce an opposing emf and magnetic field in the conductor. We can use the right hand grip rule to determine the orientation of the resultant magnetic field, and indeed, this a very common question in HSC exams. Remember the rule; clench your right hand in a fist with your thumb pointed. Your fingers point in the direction of current, and your thumb points to the North Pole of the resultant field.
(Excuse the poor quality image editing on my behalf)
(http://i.imgur.com/ZX5jsta.jpg)
Let's look at a general induction question to practice:
(http://i.imgur.com/qeDUMoFl.png)
Right, so we know that the induced current will oppose the change. Thus, the south pole will be at the bottom. So, the north pole is at the top; point your thumb upwards and clench your fist. Your fingers point anticlockwise. Now, inside the coil, the magnetic field lines go from South to North (the easiest way to remember this is just the opposite of what happens on the outside, it can be a little confusing). So, the answer is B!
Another common question will ask you to explain how induction is useful in induction stovetops or induction braking. I'll give a sample response to both.
Example One : Explain how induction cooktops heat food.
Induction cooktops use electromagnetic induction to heat food. Beneath the stovetop there are solenoids. When the stove is switched on, they are fed an AC current. This AC current causes a constantly changing magnetic flux in the saucepan above, inducing eddy currents (circular flows of current in a conductor). The resistance of the pan causes it to heat up when these currents flow, thus heating the food.
Example Two : Explain the role of eddy currents in electromagnetic braking.
Eddy currents are circular flows of current which occur in a conductor exposed to a changing magnetic field. These currents are utilised in electromagnetic braking. The moving carriage is fitted with super magnets or electromagnets. Copper sheets are placed where the carriage should stop. As the carriage approaches, eddy currents are induced in the copper, opposing the motion that created them according. This is due to Lenz's Law, which states that an induced emf will oppose the change which created it. Thus, the carriage is decelerated.
Questions on Lenz's Law, eddy currents, and induction used in these ways, are common. Be sure you are ready for them, and remember, diagrams are an awesome way to demonstrate your understanding.
Right, now to generators. Generators are essentially identical to a motor in structure (again, go see my other guide if you need a refresher). In a generator, instead of being attached to a load, the axle is attached to some kind of spinning energy source (EG- a windmill). This spinning energy source causes the coil to move in the magnetic field, thus inducing an electromotive force which can be fed to an external circuit. DC generators, like motors, use a split ring commutator to reverse the direction of current every half turn. AC Generators simply use slip rings , maintaining the changing current for the external circuitry.
Look at the slip rings in this MC question from 2014.
(http://i.imgur.com/15B2WIwl.png)
The question says it rotates one rotation with the switch open. The slip rings maintain contact with the coil, but with the switch open, no current flows on this first rotation. The answer is immediately restricted to be B!
Comparisons between motors and generators are a common question in the HSC. Essentially, the structure is nearly identical, but the devices fulfil opposite functions.
There are several arguments regarding AC vs DC generators. AC Generators are easier to construct, experience less wear, and can use transformers (this will be covered later). However, they require extensive shielding and insulation to account for eddy currents and induction in surrounding metals. DC Generators also produce more energy per volt. AC Generators are traditionally used for more large scale power production and distribution, DC generators are useful for small scale power production (EG- in car batteries).
You are also asked to understand the AC vs DC argument which occurred between the respective systems of Westinghouse and Edison. This is one of those ones that a lot of students ignore, and therefore, one that BOSTES loves to ask. Be prepared!
Westinghouse was invested in AC current, Edison in DC.
Edison’s DC system:
• Recorded power losses of over one third with limited transmission distance
• Would require a great number of generators, causing higher costs and more pollution
• Could not promise efficiency
While Westinghouse’s AC system:
• Could transmit power over large distances through use of transformers, with less than one percent power loss
• Generators could be constructed near fuel source and power transmitted
• Was arguably more dangerous, but could be cordoned off and protected for small cost
Needless to say, the Westinghouse AC system prevailed and is what we use today.
There are a few more dot points to cover in terms of generators and power distribution, but I am leaving them for the next guide, which will be a big one covering all the "impact on society" questions, transformers, and AC induction motors.
So, that's all for this guide! Stay tuned for more on the M&G topic, and then Ideas to Implementation! Be sure to register and ask questions, as many as you like, I am happy to help! Happy study!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
First, electromagnetic induction and associated theory. There is a lot to cover here before we even look at Generators.
Faraday was the first to observe electromagnetic induction; he detected an induced current in a solenoid when it was exposed to a changing magnetic field. This is an experiment you are likely to have replicated; it is quite simple. Faraday discovered that the induced current was stronger when the rate of change of magnetic flux density (magnetic field strength) was greater.
Here, it is important to distinguish between magnetic flux and magnetic flux density. Magnetic Flux is simply the number of magnetic field lines passing through a given area. Magnetic flux density, however, is how we measure magnetic fields, and is found by dividing magnetic flux by the area.
(http://i.imgur.com/ZWwSkzH.png)
Next to cover is Lenz's Law . This law states that an induced emf (electromotive force, potential difference, all essentially the same) always generates a magnetic field which opposes the change that created it. This makes sense in terms of the Conservation of Energy; if the new magnetic field assisted the change, then energy would be created. In motors, we remember that the coil spins inside a magnetic field. Thus, the coil experiences a changing magnetic flux density, and thus, a current is induced. Lenz's law says that this new current/emf acts in the opposite direction, thus generating an opposing torque. This is called back emf .
Back emf opposes the supply emf in a motor. Back emf increases as the motor spins faster; thus, a stationary motor contains much more current than a spinning one!
Eddy currents are a common source of confusion. Eddy currents are circular flows of current which occur in conductive materials exposed to a changing magnetic field. According to Lenz’s Law, this changing magnetic flux will induce an opposing emf and magnetic field in the conductor. We can use the right hand grip rule to determine the orientation of the resultant magnetic field, and indeed, this a very common question in HSC exams. Remember the rule; clench your right hand in a fist with your thumb pointed. Your fingers point in the direction of current, and your thumb points to the North Pole of the resultant field.
(Excuse the poor quality image editing on my behalf)
(http://i.imgur.com/ZX5jsta.jpg)
Let's look at a general induction question to practice:
(http://i.imgur.com/qeDUMoFl.png)
Right, so we know that the induced current will oppose the change. Thus, the south pole will be at the bottom. So, the north pole is at the top; point your thumb upwards and clench your fist. Your fingers point anticlockwise. Now, inside the coil, the magnetic field lines go from South to North (the easiest way to remember this is just the opposite of what happens on the outside, it can be a little confusing). So, the answer is B!
Another common question will ask you to explain how induction is useful in induction stovetops or induction braking. I'll give a sample response to both.
Example One : Explain how induction cooktops heat food.
Induction cooktops use electromagnetic induction to heat food. Beneath the stovetop there are solenoids. When the stove is switched on, they are fed an AC current. This AC current causes a constantly changing magnetic flux in the saucepan above, inducing eddy currents (circular flows of current in a conductor). The resistance of the pan causes it to heat up when these currents flow, thus heating the food.
Example Two : Explain the role of eddy currents in electromagnetic braking.
Eddy currents are circular flows of current which occur in a conductor exposed to a changing magnetic field. These currents are utilised in electromagnetic braking. The moving carriage is fitted with super magnets or electromagnets. Copper sheets are placed where the carriage should stop. As the carriage approaches, eddy currents are induced in the copper, opposing the motion that created them according. This is due to Lenz's Law, which states that an induced emf will oppose the change which created it. Thus, the carriage is decelerated.
Questions on Lenz's Law, eddy currents, and induction used in these ways, are common. Be sure you are ready for them, and remember, diagrams are an awesome way to demonstrate your understanding.
Right, now to generators. Generators are essentially identical to a motor in structure (again, go see my other guide if you need a refresher). In a generator, instead of being attached to a load, the axle is attached to some kind of spinning energy source (EG- a windmill). This spinning energy source causes the coil to move in the magnetic field, thus inducing an electromotive force which can be fed to an external circuit. DC generators, like motors, use a split ring commutator to reverse the direction of current every half turn. AC Generators simply use slip rings , maintaining the changing current for the external circuitry.
Look at the slip rings in this MC question from 2014.
(http://i.imgur.com/15B2WIwl.png)
The question says it rotates one rotation with the switch open. The slip rings maintain contact with the coil, but with the switch open, no current flows on this first rotation. The answer is immediately restricted to be B!
Comparisons between motors and generators are a common question in the HSC. Essentially, the structure is nearly identical, but the devices fulfil opposite functions.
There are several arguments regarding AC vs DC generators. AC Generators are easier to construct, experience less wear, and can use transformers (this will be covered later). However, they require extensive shielding and insulation to account for eddy currents and induction in surrounding metals. DC Generators also produce more energy per volt. AC Generators are traditionally used for more large scale power production and distribution, DC generators are useful for small scale power production (EG- in car batteries).
You are also asked to understand the AC vs DC argument which occurred between the respective systems of Westinghouse and Edison. This is one of those ones that a lot of students ignore, and therefore, one that BOSTES loves to ask. Be prepared!
Westinghouse was invested in AC current, Edison in DC.
Edison’s DC system:
• Recorded power losses of over one third with limited transmission distance
• Would require a great number of generators, causing higher costs and more pollution
• Could not promise efficiency
While Westinghouse’s AC system:
• Could transmit power over large distances through use of transformers, with less than one percent power loss
• Generators could be constructed near fuel source and power transmitted
• Was arguably more dangerous, but could be cordoned off and protected for small cost
Needless to say, the Westinghouse AC system prevailed and is what we use today.
There are a few more dot points to cover in terms of generators and power distribution, but I am leaving them for the next guide, which will be a big one covering all the "impact on society" questions, transformers, and AC induction motors.
So, that's all for this guide! Stay tuned for more on the M&G topic, and then Ideas to Implementation! Be sure to register and ask questions, as many as you like, I am happy to help! Happy study!
A GUIDE BY JAMON WINDEYER
Post by: jamonwindeyer on July 27, 2015, 10:22:53 am
Spoiler
Hello everyone! Welcome to another HSC Physics guide; this one is going to cover the remaining parts of the Motors and Generators topic. This means transformers, AC induction motors, power distribution, and the role of electricity and transformers in society. It’s a fair bit of stuff, but thankfully, nothing too difficult to grasp! So sit back, relax, and let’s do some revision.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Okay, so let’s begin with transformers. Transformers are used to change the voltage of electricity in AC circuits. The voltage can be increased or decreased as required. You actually don’t have to know how these work! But it is handy to know the basic premise.
All transformers consist of two coils wrapped around a ferromagnetic core. AC current fed to the primary coil generates a changing magnetic flux, which induces an EMF output in the secondary coil. The core amplifies the effects of the changing flux.
Now comes the bit you only need to be able to describe, not explain why it happens. The size of the emf (and thus, the voltage) in a coil is dependent on the number of turns in that coil. Thus, step up transformers have more coils in the secondary coil, and allow the voltage to be increased. Step down transformers have less coils in the secondary coil, allowing the voltage to be decreased.
This is a step up transformer. There is more coils in the secondary coil, so the emf (voltage) increases.
(http://i.imgur.com/X57GYvy.png)
The formula which describes this is below. The ratio of voltages in primary/secondary is equal to the ratio of turns.
This a common exam question!
Example One: The primary winding of a transformer contains 2000 turns. The primary AC voltage is 23 000 volts and the output voltage is 660 000 volts. Calculate the number of turns in the secondary winding.
Easy. Sub and go:
The important thing to remember about transformers, is that no energy is created or destroyed. The electrical energy in each core must be equal (assuming 100% efficiency), so
.
Of course, no transformer is 100% efficient. Where does the energy go? The answer is in the core. The ferromagnetic core is super important to allow the changing flux to actually have a noticeable effect on the secondary coil. Without it, transformers would barely work. However, the core (usually made of iron) also experiences this changing flux, thus inducing eddy currents. . Be sure to check out my guide on induction if you need a refresher here.
These eddy currents waste energy in the form of heat which builds up in the core. This is obviously a bad thing, and to minimise it, laminated cores are used. Insulated laminations are used to divide the core into smaller sections, significantly reducing the size of eddy currents and thus reducing the energy loss due to heat.
(http://i.imgur.com/Eg9gAOd.png)
Transformers are absolutely vital to modern power distribution networks. Let’s look at why.
First, they minimise power loss. Power losses in transmission wires occur due to heating, according to the formula
. This energy loss is costly and can cause damage to infrastructure. Transformers allow this power loss to be minimised, by increasing voltage and thus minimising current. This becomes increasingly important over large distances, and so is ultimately the reason AC is used for power distribution. Electricity for Australian homes is normally generated with a voltage of 23 kV, then stepped up to 330 kV for long range transmission, then gradually stepped down to the 240 V we use in household appliances.
In addition, many household appliances contain miniature transformers. Appliances are designed to operate on a specific voltage, AC or DC. This is particularly true for battery operated devices, which run on low voltage DC. Not operating at the correct voltage can cause overheating and endanger the user. Transformers and rectifiers (converts AC to DC) are used to moderate the power supply to the appliance, or even the specific part in use. For example, the computer I am typing this on is likely using a step-up transformer to power my big screen, while stepping down the voltage to turn on my little webcam light. This allows appliances to be designed for the most efficient voltage, and the mains supply can just be transformed to suit this.
Analysing the role of transformers in electrical power in some way is a common question. The above paragraphs would fit nicely into an extended response on this subject. Even more common is evaluating the impact of transformers. The important point is that transformers ARE the reason we use electricity so much today. So, is this good or bad?
It could definitely be considered a good thing, because:
• Shift from DC to AC for mass power production, resulting in lower costs, and less urban pollution.
• Minimising energy wastage has reduced pollution in the atmosphere, reduced the cost of electricity, and maximized the efficiency of electrical systems
• Allows power stations to be located far from urban centres
• Allows development of appliances which use different voltages from 240V outlets
• Minimised costs of production, thus reducing costs of products for consumers, and improving quality of life
• Less reliance on steam power created a greener/cleaner urban scape, thus improving general health
Negative impacts surround health issues due to pollution, massive increases in obesity rates and other lifestyle related illnesses, and increased levels of unemployment due to replacing manual labour with machinery. Use your imagination! There is lots to argue.
However, impacts on the environment are almost completely negative. Increased reliance on electrical power increases reliance on fossil fuels, in turn contributing to global warming and increasing temperatures, raising sea levels, thus reducing available agricultural land, increasing food costs and cost of living.
Absolutely, in these questions I recommend doing an argument similar to: "While the development of the AC Generator/transformer (whichever is asked) has improved our quality of life in many ways, the negative impacts on the environment cannot be ignored" . Put your own spin on it! You might argue that the environmental damage is worth it, you might argue that it isn't. Make it your own and express yourself clearly, and it is hard to lose marks!
Two last little bits of theory to cover. The first, AC induction motors! These can be a little confusing to understand, but questions will not go into too much depth. Essentially, these motors work based on Lenz's Law. Instead of a constant magnetic field, AC induction motors have a rotating magnetic field. This is done with an arrangement of electromagnets; think a hula hoop covered in Christmas lights or something, and the lights turn on one by one, in order! It's essentially the same idea.
This rotating field exposes the rotor to a changing flux (note, induction motors do not have a coil!). The rotor is shaped like one of those wheels that hamsters run on. Eddy currents flow in response to the changing field, and according to Lenz's Law, these currents will oppose the change which created them. The net result? The rotor chases the magnetic field, like a mouse chasing the Christmas light! That's as much detail as you need, but try and think back to your practical to get a better picture.
Another terribly edited image, this one of a squirrel cage rotor! ;)
(http://i.imgur.com/QT91Pr3.png)
Finally, we look at energy transformations in the home and industry. You just need a few examples here, to answer common questions like: Identify three energy transformations which make use of electrical energy in the home. You should specify what new energy is created and what purpose it serves.
Easy marks, just say things like this, and then add what each appliance you choose is used for!
• Electrical energy is transformed into radiant (electromagnetic) energy in toasters, microwaves, radios, and light globes
• Electrical energy is converted into kinetic energy (movement) in blenders, speakers, and electric tooth- brushes
• Electrical energy is converted into chemical potential energy in rechargeable batteries
For industry, it might be harder to think of examples. Here are a couple:
Electrical energy is transformed into electromagnetic waves as X Rays in medical imaging, light in laser printing, heat in large industrial ovens, and radio waves for communication
• Electrical energy is transformed into kinetic energy in cars, trucks and plans, as well as in essentially any kind of manufacturing process
And, that's it! You can cross Motors and Generators off your list! Many would say it is the easiest topic in the course, and I'd agree, though there are some slightly difficult concepts. With that in mind, if anything in this or any of the other guides has proven complicated, please feel free to register and ask me any questions you had! Or leave some feedback, say hey, share photos of your pet kitten, anything! Stay tuned for more guides, but until next time, happy study!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Okay, so let’s begin with transformers. Transformers are used to change the voltage of electricity in AC circuits. The voltage can be increased or decreased as required. You actually don’t have to know how these work! But it is handy to know the basic premise.
All transformers consist of two coils wrapped around a ferromagnetic core. AC current fed to the primary coil generates a changing magnetic flux, which induces an EMF output in the secondary coil. The core amplifies the effects of the changing flux.
Now comes the bit you only need to be able to describe, not explain why it happens. The size of the emf (and thus, the voltage) in a coil is dependent on the number of turns in that coil. Thus, step up transformers have more coils in the secondary coil, and allow the voltage to be increased. Step down transformers have less coils in the secondary coil, allowing the voltage to be decreased.
This is a step up transformer. There is more coils in the secondary coil, so the emf (voltage) increases.
(http://i.imgur.com/X57GYvy.png)
The formula which describes this is below. The ratio of voltages in primary/secondary is equal to the ratio of turns.
This a common exam question!
Example One: The primary winding of a transformer contains 2000 turns. The primary AC voltage is 23 000 volts and the output voltage is 660 000 volts. Calculate the number of turns in the secondary winding.
Easy. Sub and go:
The important thing to remember about transformers, is that no energy is created or destroyed. The electrical energy in each core must be equal (assuming 100% efficiency), so
Of course, no transformer is 100% efficient. Where does the energy go? The answer is in the core. The ferromagnetic core is super important to allow the changing flux to actually have a noticeable effect on the secondary coil. Without it, transformers would barely work. However, the core (usually made of iron) also experiences this changing flux, thus inducing eddy currents. . Be sure to check out my guide on induction if you need a refresher here.
These eddy currents waste energy in the form of heat which builds up in the core. This is obviously a bad thing, and to minimise it, laminated cores are used. Insulated laminations are used to divide the core into smaller sections, significantly reducing the size of eddy currents and thus reducing the energy loss due to heat.
(http://i.imgur.com/Eg9gAOd.png)
Transformers are absolutely vital to modern power distribution networks. Let’s look at why.
First, they minimise power loss. Power losses in transmission wires occur due to heating, according to the formula
In addition, many household appliances contain miniature transformers. Appliances are designed to operate on a specific voltage, AC or DC. This is particularly true for battery operated devices, which run on low voltage DC. Not operating at the correct voltage can cause overheating and endanger the user. Transformers and rectifiers (converts AC to DC) are used to moderate the power supply to the appliance, or even the specific part in use. For example, the computer I am typing this on is likely using a step-up transformer to power my big screen, while stepping down the voltage to turn on my little webcam light. This allows appliances to be designed for the most efficient voltage, and the mains supply can just be transformed to suit this.
Analysing the role of transformers in electrical power in some way is a common question. The above paragraphs would fit nicely into an extended response on this subject. Even more common is evaluating the impact of transformers. The important point is that transformers ARE the reason we use electricity so much today. So, is this good or bad?
It could definitely be considered a good thing, because:
• Shift from DC to AC for mass power production, resulting in lower costs, and less urban pollution.
• Minimising energy wastage has reduced pollution in the atmosphere, reduced the cost of electricity, and maximized the efficiency of electrical systems
• Allows power stations to be located far from urban centres
• Allows development of appliances which use different voltages from 240V outlets
• Minimised costs of production, thus reducing costs of products for consumers, and improving quality of life
• Less reliance on steam power created a greener/cleaner urban scape, thus improving general health
Negative impacts surround health issues due to pollution, massive increases in obesity rates and other lifestyle related illnesses, and increased levels of unemployment due to replacing manual labour with machinery. Use your imagination! There is lots to argue.
However, impacts on the environment are almost completely negative. Increased reliance on electrical power increases reliance on fossil fuels, in turn contributing to global warming and increasing temperatures, raising sea levels, thus reducing available agricultural land, increasing food costs and cost of living.
Absolutely, in these questions I recommend doing an argument similar to: "While the development of the AC Generator/transformer (whichever is asked) has improved our quality of life in many ways, the negative impacts on the environment cannot be ignored" . Put your own spin on it! You might argue that the environmental damage is worth it, you might argue that it isn't. Make it your own and express yourself clearly, and it is hard to lose marks!
Two last little bits of theory to cover. The first, AC induction motors! These can be a little confusing to understand, but questions will not go into too much depth. Essentially, these motors work based on Lenz's Law. Instead of a constant magnetic field, AC induction motors have a rotating magnetic field. This is done with an arrangement of electromagnets; think a hula hoop covered in Christmas lights or something, and the lights turn on one by one, in order! It's essentially the same idea.
This rotating field exposes the rotor to a changing flux (note, induction motors do not have a coil!). The rotor is shaped like one of those wheels that hamsters run on. Eddy currents flow in response to the changing field, and according to Lenz's Law, these currents will oppose the change which created them. The net result? The rotor chases the magnetic field, like a mouse chasing the Christmas light! That's as much detail as you need, but try and think back to your practical to get a better picture.
Another terribly edited image, this one of a squirrel cage rotor! ;)
(http://i.imgur.com/QT91Pr3.png)
Finally, we look at energy transformations in the home and industry. You just need a few examples here, to answer common questions like: Identify three energy transformations which make use of electrical energy in the home. You should specify what new energy is created and what purpose it serves.
Easy marks, just say things like this, and then add what each appliance you choose is used for!
• Electrical energy is transformed into radiant (electromagnetic) energy in toasters, microwaves, radios, and light globes
• Electrical energy is converted into kinetic energy (movement) in blenders, speakers, and electric tooth- brushes
• Electrical energy is converted into chemical potential energy in rechargeable batteries
For industry, it might be harder to think of examples. Here are a couple:
Electrical energy is transformed into electromagnetic waves as X Rays in medical imaging, light in laser printing, heat in large industrial ovens, and radio waves for communication
• Electrical energy is transformed into kinetic energy in cars, trucks and plans, as well as in essentially any kind of manufacturing process
And, that's it! You can cross Motors and Generators off your list! Many would say it is the easiest topic in the course, and I'd agree, though there are some slightly difficult concepts. With that in mind, if anything in this or any of the other guides has proven complicated, please feel free to register and ask me any questions you had! Or leave some feedback, say hey, share photos of your pet kitten, anything! Stay tuned for more guides, but until next time, happy study!
A GUIDE BY JAMON WINDEYER
Cathode Rays
Spoiler
Hello once again everyone! Time again for another HSC Physics Guide. We are in to Ideas and Implementation now, an interesting topic, but a little difficult, lots of content to cover! Like the others, these guides will have a focus on summarising the content, and addressing the common exam questions likely to pop up in your exam! Squeezing everything in to these guides is tricky, so read slowly, and let me know if there are any areas you need a little extra help with. This guide will look at Cathode Rays.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Ok, so cathode ray tubes look like this:
(http://i.imgur.com/kXlfWwG.png)
Basically, they are an evacuated glass tube with an electrode at either end (the negatively charged plate is called the cathode , the positively charged plate is called the anode ). Due to the low air pressure, air does not act as an insulator inside these tubes. So, if a large enough potential difference is applied, electrons begin to flow from the cathode to the anode. These are cathode rays . These rays make different patterns, called striations , as they collide with the air particles still in the tube. These striations change based on the air pressure in the tube (you aren’t required to know details here, just identify that changes occurred with air pressure).
Now, you would have done several experiments with cathode rays. Some key ones to recall are listed below. These experiments were done in the 19th century to determine the nature of cathode rays.
There was argument as to whether they were a particle, or a wave. Indeed, this is a very common HSC question. You may be asked to compare or present arguments concerning the nature of cathode rays. Whatever the wording, this is what thy mean and here are some arguments you can list.
Some arguments for the ‘wave’ theory:
Arguments for the ‘particle’ theory:
Before we move on, we need to do a bit of electric and magnetic field theory.
Magnetic fields surround all, well, magnetic substances (shocker!). Now, any ferromagnetic material, or other magnet, in this field will experience a force in response to the field. However, there is a relationship between electricity and magnetism (something you don’t need to know about). What this means, is that a moving electric charge will experience a force in a magnetic field. Note that, it MUST be moving. This is something which links to the Motor Effect studied earlier.
The formula we use is as follows, where
is the charge on the object in coulombs, 
is speed, is magnetic field strength, and is the angle made with the field lines.
A common way this formula is asked is the following.
(http://i.imgur.com/IlBR9Ks.png)
We have to equate centripetal and magnetic force to get our radius:
Not too bad, and again, very common. Be on the lookout.
Electric fields are a little simpler. The force experienced by a charged particle is equal to the electric field strength, multiplied by the charge on the particle.
You also have to be able to analyse the electric field between two charged plates, simiilar to the situation in a cathode ray tube. The important thing here: the electric field is constant in strength at all points between the plates . This is not like the field surrounding point charges, which diminish in strength over distance.
The electric field strength between two large plates is
, where V is the potential difference across the plates, and d is the distance between them. Obviously, the plates must have opposite charges for this to be valid, otherwise a potential difference won’t exist. As the plates get closer, the field is stronger, but no matter how strong, it is constant at all points!
Let’s combine these ideas to look at this multiple choice question:
(http://i.imgur.com/6HReQxFl.png)
Now, this is a multiple choice question, no need for complicated math, let’s make the very accurate approximation that neutrons and protons have the same mass. Now, the electric field strength between the plates is, and the force experienced is . So, 
. Read that over if you need to.
Now, let’s consider the proton.
, but , so 
.
Therefore, by rearranging,
.
Now, the same formula applies to the alpha particle. Except, the charge is twice as much as the proton, and the mass is four times as much. So, replace with 
, and 
with 
, and we get:
So, the answer is B!
Right, now electric field theory is out of the way, we can more easily understand Thompson’s experiment. JJ Thompson conducted an experiment to determine the charge/mass ratio of cathode rays. He had observed that the rays were deflected by electric fields, and thus must have been negatively charged particles. He wanted their charge/mass ratio.
First, Thompson set up a cathode ray tube, with the rays subjected to both electric and magnetic fields. These fields would create a force, but Thompson set them up so the force acted in opposite directions. So, if he modified the fields so that the ray maintained it’s original path, then the two forces must have been equal. So, he can equate magnetic and electric forces:
Next, Thompson removes the electric field. The magnetic field creates a force perpendicular to the motion of the particle. Sound familiar? Uniform circular motion occurs. So Thompson can equate magnetic and centripetal force.
Thompson knows the velocity from the previous set, he knows the magnetic field strength, and he can measure the radius. So, he has the charge/mass ratio! This was, in essence, the discovery of the electron.
This is a complicated experiment, but it is asked frequently. Read this over, ask me any questions, make sure you remember the process and can do these mathematical manipulations yourself.
The final part of understanding cathode rays is to understand how they are actually used. There are three key parts to understand.
The first is the electron gun. This is the part that actually generates the cathode rays. It consists of a cathode and anode as normal. Normally, in addition, the cathode is heated to increase the number of electrons ejected. This is called thermionic emission . The electrons accelerate towards the anode, and pass through a hole, out the end of the electron gun.
Next comes the deflection plates. These are sets of plates which create a set of either magnetic or electric fields (not both). These manipulate the moving electrons. One set of plates deflects the beam vertically. The other deflects it horizontally. Thus, the beam can be moved up and down, as well as left to right, to hit any point on…
The fluorescent screen. Screens in appliances with cathode ray tubes utilise phosphors, which fluoresce for a short time after being struck by an electron. By manipulating the electron gun to sweep across the screen very quickly, a picture can be shown!
For example, if you were to slow down a cathode ray television and look at it 1000 times slower, you would see the electron gun hit the pixel in the bottom left corner. Then the next one. Then the next one. Then the next one. All the way to the top, where it loops back and repeats. This happens so fast, that you only see a moving picture.
Putting all of this in one question is a big ask. The electron gun or deflection plates are normally the ones specified, so be prepared to draw a diagram of a cathode ray tube, explain how the plates are used to deflect the beam (and why), and just describe how they work in general. It can be easy to lose marks here, be careful that you explain things clearly and logically!
So, that’s Cathode Rays! Stay tuned for more on Ideas and Implementation, next up is the Photoelectric Effect. Be sure to register and ask questions, as many as you like, I am happy to help! Happy study!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Ok, so cathode ray tubes look like this:
(http://i.imgur.com/kXlfWwG.png)
Basically, they are an evacuated glass tube with an electrode at either end (the negatively charged plate is called the cathode , the positively charged plate is called the anode ). Due to the low air pressure, air does not act as an insulator inside these tubes. So, if a large enough potential difference is applied, electrons begin to flow from the cathode to the anode. These are cathode rays . These rays make different patterns, called striations , as they collide with the air particles still in the tube. These striations change based on the air pressure in the tube (you aren’t required to know details here, just identify that changes occurred with air pressure).
Now, you would have done several experiments with cathode rays. Some key ones to recall are listed below. These experiments were done in the 19th century to determine the nature of cathode rays.
There was argument as to whether they were a particle, or a wave. Indeed, this is a very common HSC question. You may be asked to compare or present arguments concerning the nature of cathode rays. Whatever the wording, this is what thy mean and here are some arguments you can list.
Some arguments for the ‘wave’ theory:
- If you place a barrier in the path of the cathode rays, that barrier creates a ‘shadow,’ much like you would expect for visible light. It shows that the rays, like waves, travel in straight lines.
- The rays are observed to induce fluorescence in phosphors, such as those on certain screens. This is a property of electromagnetic waves.
Arguments for the ‘particle’ theory:
- The rays can be observed to cause the movement of a paddle wheel (or similar) where friction is minimised. Thus, it can be deduced that the rays have momentum.
- The rays are deflected by both magnetic and electric fields, although, the latter was not proved immediately. Both of these suggested that the rays were negatively charged particles
Before we move on, we need to do a bit of electric and magnetic field theory.
Magnetic fields surround all, well, magnetic substances (shocker!). Now, any ferromagnetic material, or other magnet, in this field will experience a force in response to the field. However, there is a relationship between electricity and magnetism (something you don’t need to know about). What this means, is that a moving electric charge will experience a force in a magnetic field. Note that, it MUST be moving. This is something which links to the Motor Effect studied earlier.
The formula we use is as follows, where
A common way this formula is asked is the following.
(http://i.imgur.com/IlBR9Ks.png)
We have to equate centripetal and magnetic force to get our radius:
Not too bad, and again, very common. Be on the lookout.
Electric fields are a little simpler. The force experienced by a charged particle is equal to the electric field strength, multiplied by the charge on the particle.
You also have to be able to analyse the electric field between two charged plates, simiilar to the situation in a cathode ray tube. The important thing here: the electric field is constant in strength at all points between the plates . This is not like the field surrounding point charges, which diminish in strength over distance.
The electric field strength between two large plates is
Let’s combine these ideas to look at this multiple choice question:
(http://i.imgur.com/6HReQxFl.png)
Now, this is a multiple choice question, no need for complicated math, let’s make the very accurate approximation that neutrons and protons have the same mass. Now, the electric field strength between the plates is
Now, let’s consider the proton.
Therefore, by rearranging,
Now, the same formula applies to the alpha particle. Except, the charge is twice as much as the proton, and the mass is four times as much. So, replace
So, the answer is B!
Right, now electric field theory is out of the way, we can more easily understand Thompson’s experiment. JJ Thompson conducted an experiment to determine the charge/mass ratio of cathode rays. He had observed that the rays were deflected by electric fields, and thus must have been negatively charged particles. He wanted their charge/mass ratio.
First, Thompson set up a cathode ray tube, with the rays subjected to both electric and magnetic fields. These fields would create a force, but Thompson set them up so the force acted in opposite directions. So, if he modified the fields so that the ray maintained it’s original path, then the two forces must have been equal. So, he can equate magnetic and electric forces:
Next, Thompson removes the electric field. The magnetic field creates a force perpendicular to the motion of the particle. Sound familiar? Uniform circular motion occurs. So Thompson can equate magnetic and centripetal force.
Thompson knows the velocity from the previous set, he knows the magnetic field strength, and he can measure the radius. So, he has the charge/mass ratio! This was, in essence, the discovery of the electron.
This is a complicated experiment, but it is asked frequently. Read this over, ask me any questions, make sure you remember the process and can do these mathematical manipulations yourself.
The final part of understanding cathode rays is to understand how they are actually used. There are three key parts to understand.
The first is the electron gun. This is the part that actually generates the cathode rays. It consists of a cathode and anode as normal. Normally, in addition, the cathode is heated to increase the number of electrons ejected. This is called thermionic emission . The electrons accelerate towards the anode, and pass through a hole, out the end of the electron gun.
Next comes the deflection plates. These are sets of plates which create a set of either magnetic or electric fields (not both). These manipulate the moving electrons. One set of plates deflects the beam vertically. The other deflects it horizontally. Thus, the beam can be moved up and down, as well as left to right, to hit any point on…
The fluorescent screen. Screens in appliances with cathode ray tubes utilise phosphors, which fluoresce for a short time after being struck by an electron. By manipulating the electron gun to sweep across the screen very quickly, a picture can be shown!
For example, if you were to slow down a cathode ray television and look at it 1000 times slower, you would see the electron gun hit the pixel in the bottom left corner. Then the next one. Then the next one. Then the next one. All the way to the top, where it loops back and repeats. This happens so fast, that you only see a moving picture.
Putting all of this in one question is a big ask. The electron gun or deflection plates are normally the ones specified, so be prepared to draw a diagram of a cathode ray tube, explain how the plates are used to deflect the beam (and why), and just describe how they work in general. It can be easy to lose marks here, be careful that you explain things clearly and logically!
So, that’s Cathode Rays! Stay tuned for more on Ideas and Implementation, next up is the Photoelectric Effect. Be sure to register and ask questions, as many as you like, I am happy to help! Happy study!
A GUIDE BY JAMON WINDEYER
Photoelectric Effect
Spoiler
Hey everyone! Only a few HSC Physics guides to go, and coincidentally, only a few weeks until Trials! Scary scary stuff, although hopefully these guides are proving at least a little helpful. This one is going to cover the photoelectric effect, something that is a little difficult to understand in parts, so I’ll try to go a little slower than normal. And as always, I’ll be sure to mention potential exam questions and go through them where I think it will help.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
So, the photoelectric effect. It was first observed by Hertz in 1887. He was investigating radio waves; he induced sparking in one coil, and found that coils near it sparked as well. This was the first observation of radio waves. Hertz calculated the speed of the waves using the wave equation,
. This, of course, was the speed of light we know today, 
.
Hertz also noticed that the sparking occurred more readily when the coils were exposed to UV light. He never investigated it, but he had observed the photoelectric effect!
Before we explain it, we need to know a bit about black bodies.
A black body is, theoretically, a perfect absorber and emitter of radiation. There is more complex theory here, but it isn’t related to this course. Classical theory falters when attempting to explain black body radiation. Classical theory suggests that, as intensity of radiation increases, frequency should increase also. This means that as the frequencies approach the ultraviolet and gamma end of the EM spectrum, the intensity escalates rapidly, approaching infinity. This makes no sense, and violates the conservation of energy.
Instead, experimental data suggested that radiation was spread over multiple frequencies. This contradiction is known as the ultraviolet catastrophe .
Planck suggested a solution which, at first, he believed to be just a mathematical trick. He proposed that energy was not released in waves, but in packets, which he called quanta. The energy in each quantum is determined by the frequency of the radiation. Thus, the radiation emitted by a black body is quantised, with the total energy determined based on the number of quanta n, the frequency f, and Planck’s Constant h (6.63 × 10−34). Energy per quanta simply removes the n.
A very common HSC question will show you a graph like this, and ask a question on the ultraviolet catastrophe:
(https://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Blackbody-lg.png/303px-Blackbody-lg.png)
(Image taken from Wikipedia)
Again, simply discuss how classical theory (for extra points, it is actually called Rayleigh Jeans Law) fails to explain intensity at short wavelengths, and how quantum theory proposes a solution.
But how does this tie in to the photoelectric effect? This is where Einstein stepped in. Einstein applied Planck’s quantum theories to his model of light, and proposed that light consists of quanta packets, which he called photons . He also suggested the All or Nothing Principle , the idea that if a proton collides with something, it either gives up all its energy, or none. Essentially, he maintained that quanta could not be divided.
The particle and wave models of light are linked in a fairly simple way. The particle model of light considers light as small wave packets called photons. An increase in frequency corresponds to an increased amount of energy per photon. The number of photons, rather, is associated with the amplitude of the light waves in wave models. So essentially, higher frequency equals more energy. Larger amplitude means more photons.
Einstein also was able to finally explain the photoelectric effect. The photoelectric effect states that when light of an appropriate frequency is shone onto a metal surface, electrons are emitted from that surface. This occurs due to photons passing their energy to the electrons, allowing them to be ejected from the metal. Ejecting the electron takes energy; this is the work function φ of the metal. Thus, the kinetic energy K of an emitted electron is:
where E is the energy of the photon.
Every metal has a threshold frequency , the minimum frequency required for the photons to have enough energy to allow electrons to escape the metal (ie, this is the frequency when the energy per photon is equal to the work function of the metal). Below this frequency, the photoelectric effect does not occur.
Considering the equation K = hf − φ, all of this comes together. It is a linear relationship, where the x intercept is the threshold frequency, the y intercept is the work function, and the gradient is Planck’s Constant.
(http://dev.physicslab.org/img/17a0d8fb-7882-4256-9531-9883e39d1557.gif)
Explaining the photoelectric effect is a common question, as is analysing Einstein’s contributions to the photoelectric effect. Essentially, without Einstein, quantum theory would have remained a mathematical trick with no significance. Einstein brought it into mainstream scientific acceptance.
Let's look at a common, simple math question that I got asked last year:
Example: Calculate the energy of a photon of wavelength 415 nm.
This is just a substitution question utilising Planck's formula and the wave formula:
Remember that photons travel at the speed of light, and make sure you know your more unusual units of measurement. They are a common trick.
Now this topic is all about how ideas are used in everyday stuff. For the photoelectric effect, we need to look at how it is applied in photocells. Let's answer a sample 4 mark question:
Example Two: Explain how the photoelectric effect is applied in a photo cell, and what applications it has.
A photo cell is a device which can detect or measure the amount of incoming light. It works through the photoelectric effect in conjunction with two charged electrodes (a cathode and an anode). The photoelectric effect causes increased electron emission from the cathode, as exposure to light increases. A sensitive circuit measures the photoelectric current generated, and thus, a quantitative measure of light is achieved. Photo cells have applications in light meters (photography), electric eyes, and scientific measuring equipment.
(http://i.imgur.com/qNimOFp.png)
This is about how much detail you would need to guarantee 4 marks; it could absolutely be done in less words. I prefer taking extra words for clearer responses.
And the final little part of this is a little strange. It is also a common HSC question, purely because so few students study it properly. The syllabus asks us to analyse the relationship between science and politics, with reference to Planck and Einstein. Weird huh? Well, here we go...
Much of scientific development is funded by government and politically driven agendas. In the 20th century, this meant the World Wars. Einstein and Planck had different opinions regarding the role of science in politics.
Einstein was a noted pacifist who regularly spoke out against national militarism. He preached separation between science and politics. It is extremely ironic, therefore, that Einstein played a pivotal role in Project Manhattan, the development of the atomic bomb which ended World War 2.
Planck, rather, was a patriot. He accepted that sociopolitical forces guided his science, and indeed, played a large role in the German efforts to develop nuclear arms. We can see therefore, that despite best intentions, science always seems to be rooted in politics and policy.
It is interesting to consider the ”atomic race.” Germany and America conducted identical research towards atomic weapons. However, the political situation at the time meant that Planck and his counterparts were criticised for aiding in war efforts. Einstein was praised for assisting in an ingenious advancement in physics. Again, we see that even with identical work, the response is entirely different.
There are a few way questions can be framed, but this should be more than enough evidence to make a solid discussion point. Questions here are commonly worth 4-6 marks.
And that's it! A few tricky concepts covered here, so be sure to register and ask questions, as many as you like, I am happy to help! Stay tuned for the last few Physics guides over the coming days. Happy study!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
So, the photoelectric effect. It was first observed by Hertz in 1887. He was investigating radio waves; he induced sparking in one coil, and found that coils near it sparked as well. This was the first observation of radio waves. Hertz calculated the speed of the waves using the wave equation,
Hertz also noticed that the sparking occurred more readily when the coils were exposed to UV light. He never investigated it, but he had observed the photoelectric effect!
Before we explain it, we need to know a bit about black bodies.
A black body is, theoretically, a perfect absorber and emitter of radiation. There is more complex theory here, but it isn’t related to this course. Classical theory falters when attempting to explain black body radiation. Classical theory suggests that, as intensity of radiation increases, frequency should increase also. This means that as the frequencies approach the ultraviolet and gamma end of the EM spectrum, the intensity escalates rapidly, approaching infinity. This makes no sense, and violates the conservation of energy.
Instead, experimental data suggested that radiation was spread over multiple frequencies. This contradiction is known as the ultraviolet catastrophe .
Planck suggested a solution which, at first, he believed to be just a mathematical trick. He proposed that energy was not released in waves, but in packets, which he called quanta. The energy in each quantum is determined by the frequency of the radiation. Thus, the radiation emitted by a black body is quantised, with the total energy determined based on the number of quanta n, the frequency f, and Planck’s Constant h (6.63 × 10−34). Energy per quanta simply removes the n.
A very common HSC question will show you a graph like this, and ask a question on the ultraviolet catastrophe:
(https://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Blackbody-lg.png/303px-Blackbody-lg.png)
(Image taken from Wikipedia)
Again, simply discuss how classical theory (for extra points, it is actually called Rayleigh Jeans Law) fails to explain intensity at short wavelengths, and how quantum theory proposes a solution.
But how does this tie in to the photoelectric effect? This is where Einstein stepped in. Einstein applied Planck’s quantum theories to his model of light, and proposed that light consists of quanta packets, which he called photons . He also suggested the All or Nothing Principle , the idea that if a proton collides with something, it either gives up all its energy, or none. Essentially, he maintained that quanta could not be divided.
The particle and wave models of light are linked in a fairly simple way. The particle model of light considers light as small wave packets called photons. An increase in frequency corresponds to an increased amount of energy per photon. The number of photons, rather, is associated with the amplitude of the light waves in wave models. So essentially, higher frequency equals more energy. Larger amplitude means more photons.
Einstein also was able to finally explain the photoelectric effect. The photoelectric effect states that when light of an appropriate frequency is shone onto a metal surface, electrons are emitted from that surface. This occurs due to photons passing their energy to the electrons, allowing them to be ejected from the metal. Ejecting the electron takes energy; this is the work function φ of the metal. Thus, the kinetic energy K of an emitted electron is:
where E is the energy of the photon.
Every metal has a threshold frequency , the minimum frequency required for the photons to have enough energy to allow electrons to escape the metal (ie, this is the frequency when the energy per photon is equal to the work function of the metal). Below this frequency, the photoelectric effect does not occur.
Considering the equation K = hf − φ, all of this comes together. It is a linear relationship, where the x intercept is the threshold frequency, the y intercept is the work function, and the gradient is Planck’s Constant.
(http://dev.physicslab.org/img/17a0d8fb-7882-4256-9531-9883e39d1557.gif)
Explaining the photoelectric effect is a common question, as is analysing Einstein’s contributions to the photoelectric effect. Essentially, without Einstein, quantum theory would have remained a mathematical trick with no significance. Einstein brought it into mainstream scientific acceptance.
Let's look at a common, simple math question that I got asked last year:
Example: Calculate the energy of a photon of wavelength 415 nm.
This is just a substitution question utilising Planck's formula and the wave formula:
Remember that photons travel at the speed of light, and make sure you know your more unusual units of measurement. They are a common trick.
Now this topic is all about how ideas are used in everyday stuff. For the photoelectric effect, we need to look at how it is applied in photocells. Let's answer a sample 4 mark question:
Example Two: Explain how the photoelectric effect is applied in a photo cell, and what applications it has.
A photo cell is a device which can detect or measure the amount of incoming light. It works through the photoelectric effect in conjunction with two charged electrodes (a cathode and an anode). The photoelectric effect causes increased electron emission from the cathode, as exposure to light increases. A sensitive circuit measures the photoelectric current generated, and thus, a quantitative measure of light is achieved. Photo cells have applications in light meters (photography), electric eyes, and scientific measuring equipment.
(http://i.imgur.com/qNimOFp.png)
This is about how much detail you would need to guarantee 4 marks; it could absolutely be done in less words. I prefer taking extra words for clearer responses.
And the final little part of this is a little strange. It is also a common HSC question, purely because so few students study it properly. The syllabus asks us to analyse the relationship between science and politics, with reference to Planck and Einstein. Weird huh? Well, here we go...
Much of scientific development is funded by government and politically driven agendas. In the 20th century, this meant the World Wars. Einstein and Planck had different opinions regarding the role of science in politics.
Einstein was a noted pacifist who regularly spoke out against national militarism. He preached separation between science and politics. It is extremely ironic, therefore, that Einstein played a pivotal role in Project Manhattan, the development of the atomic bomb which ended World War 2.
Planck, rather, was a patriot. He accepted that sociopolitical forces guided his science, and indeed, played a large role in the German efforts to develop nuclear arms. We can see therefore, that despite best intentions, science always seems to be rooted in politics and policy.
It is interesting to consider the ”atomic race.” Germany and America conducted identical research towards atomic weapons. However, the political situation at the time meant that Planck and his counterparts were criticised for aiding in war efforts. Einstein was praised for assisting in an ingenious advancement in physics. Again, we see that even with identical work, the response is entirely different.
There are a few way questions can be framed, but this should be more than enough evidence to make a solid discussion point. Questions here are commonly worth 4-6 marks.
And that's it! A few tricky concepts covered here, so be sure to register and ask questions, as many as you like, I am happy to help! Stay tuned for the last few Physics guides over the coming days. Happy study!
A GUIDE BY JAMON WINDEYER
Semiconductors
Spoiler
Hello everyone! Time for the second last Physics guide :( We're almost there! I hope they have proved a helpful resource in summarising course content and that they will continue to be a little useful in studying for your Trials and HSC. This guide is going to cover semiconductors, with a final quick guide to superconductors to round everything off 8) Semiconductors introduce some weird concepts, but be consistent, and the understanding will come.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Okay, so first we have to venture a little bit into chemistry. Of course, we know that electrons orbit the nucleus of an atom. These take place in arrangements called electron shells . The outermost sell is called the valence band , with the electrons occupying it appropriately called valence electrons.
In certain solids, atoms join together in a structure called a lattice, where valence electrons are shared between the different atoms. This means that they are, to a certain extent, free to move throughout the lattice. These electrons form a new band of electrons called the conduction band .
(http://i.imgur.com/aDIxykS.png)
This highlights a model for atomic structure called the band structure model. Electrons exist at certain energy levels around the nucleus, which in a lattice, broaden to become energy bands . We can consider this model through diagrams such as this:
(http://i.imgur.com/zuZpQDum.png)
In this diagram, we see a gap between the valence and conduction band. Extra energy is required to bridge this gap; such as that supplied by a potential difference. It is electrons jumping this gap to the conduction band, and thus being free to move, which causes conduction in metals.
In conductors, the valence and conduction bands overlap, meaning very little energy is required for conduction to occur. In insulators, the gap is large, therefore resistance is high, since lots of energy is required for conduction to occur. Semiconductors , rather, are somewhere in between, and a lot more interesting.
(http://i.imgur.com/lBYasWBl.png)
To understand semiconductors, we need to understand holes . An electron that jumps to the conduction band in a semiconductor leaves behind an empty space, called a hole. The hole behaves like a positive charge. Electrons in the lattice move to occupy it, leaving behind other holes to repeat the cycle, kind of like a line of taxis moving up to occupy an empty space outside the airport (you will have done some kind of similar demonstration in class). This extra movement effectively acts as an additional method of carrying current. Thus, both holes and electrons act as current carriers.
Right, so semiconductors. Semiconductors form a crystal lattice structure much like metals. However, semi-conductive materials are Group 4 elements, meaning they are missing an electron from their valence shell. Thus, these lattices form in structures that share the electrons between the atoms via covalent bonds (they are called tetrahedral structures).
(http://i.imgur.com/2R39S6X.png)
Germanium was initially the material used in semiconductors. The semiconductor crystal had to be pure to be effective, and germanium was able to be purified. Silicon was the superior material; cheaper and easier to dope (see below). But it could not be purified. Only recently has this technology been developed.
Semiconductors are not overly useful by themselves; but doping changes that.
Doping involves introducing a foreign element to alter how a semiconductor behaves.
N type Semiconductors are formed when Group 5 impurity atoms are added, such as Arsenic or Phosphorus. This leaves additional electrons, free to move in the conduction band.
P type semiconductors are formed when Group 3 impurity atoms are added, such as Aluminium or Boron. This leaves the lattice with an additional hole, again, increasing conductivity.
What doping does is, effectively, adds an additional energy level between the valence and conduction bands. This means that conduction can occur more readily, as the electrons can make two smaller jumps, rather than one big one, thus requiring less energy. This extra level would simply be identified as a dotted line between the two lines on a band diagram.
Doped semiconductors are arguably the foundation of the modern age of computing. Computers rely on sophisticated switches called transistors, which are made up of doped semiconductors. We'll go into more detail in a second, but first, lets look at the precursor to the transistor, the thermionic device .
Thermionic devices and solid state devices (transistors) both allow the manipulation of a larger current with a much smaller one. During World War 2, thermionic devices were used for communication purposes. Several issues arose. Signals were weak, technology was too bulky for the field, and the systems were unreliable and unable to meet increasing demand for the military. Scientists sought to replace the thermionic devices (which were based on cathode ray tubes) with a new technology. This lead to the development of the solid state device. They were cheaper, lighter, more durable, more reliable, required no warm up time, and were energy efficient; superior to thermionic devices in every sense.
The common transistor is a thin piece of P type silicon sandwiched between two layers of N type silicon. This NPN junction creates what is called a depletion zone at the edges of the junction, creating an electric field which allows current to flow only in a single direction. Essentially, the junction has an inbuilt potential difference, which can be controlled with a small current. This fulfils the function of the valves in a much more efficient manner.
Note, you don't have to know how the depletion zone works. But if you are interested, in the basic sense, the extra holes in the P type semiconductor are filled by the extra electrons in the N type semiconductor. This leaves the P type with a negative charge and the N type with a positive charge, thus creating an electric field.
This depletion zone is utilised in solar cells. Solar cells utilise a simple PN diode with a single depletion zone. Oncoming light hits special material in the solar panels which ensures as much of the light is possible is ’absorbed’ into the semiconductor. This releases electrons via the photoelectric effect. The diode forces these electrons to flow in a single direction, into an external circuit to power a load or to charge a battery.
And the final part of the topic; another topic another impact on society question.
Example: Assess the impacts of transistors on society in terms of their use in microchips and microprocessors.
Transistors lead to the development of the microchip and microprocessors, arguably the foundation of modern computer technology.
This has had obvious positive impacts including:
• Revolutions in the manufacturing sector
• More powerful computers with more widespread use
• Increased standard of living
• Faster developments in science and technology
• New communication methods (internet)
However, there are also negative impacts stemming from the automation of manual labour:
• Increased obesity rates
• Higher unemployment rates
• Poorly trained employees
Thus, the transistor has definitely revolutionised computer science, but has had both a positive and negative impact on the growth of the modern world.
And that's pretty much all the content. I've left potential exam questions to the end, since more than most other topics, semiconductor question really require knowledge of the whole topic. The good news; no formulas in this topic!! Which is of course super awesome. However, many questions in this topic require diagrams. Take a look at the ones supplied, they are the ones you may have to draw, make sure you know them and can replicate them.
Some common questions:
And one final example.
Example: Explain how doping affects conductivity in semiconductors.
To answer this for the full 5 marks, you need to mention a whole bunch of things.
Current in semiconductors is carried by both holes and electrons. Holes are electron vacancies, left behind by electrons, which then act as positive charges. As current flows, electrons move one away, holes move the other, and both act as current carriers. However, un-doped semiconductors have limited conductivity, with the valence band and conduction bands still separated by a small energy gap (INSERT DIAGRAM HERE).
The conductivity of the semiconductor can be increased by doping, if either Group III or Group V impurity is added to the semiconductor. Group V impurities add an extra electron to the lattice structure forming an n-type semiconductor. These electrons have a smaller energy gap and electrons become the dominant charge carriers. If a Group III impurity is added, a p-type semiconductor is formed and an additional hole is introduced into the lattice. Holes are then the dominant charge carriers.
Quite a mouthful for 5 marks, and it could absolutely be done in less. But this response is heavily based on the HSC Sample for this question; which is what I highly suggest you reference when in doubt as to how much detail you need.
So, that’s all for today! Stay tuned for the very last guide, and be sure to register and ask questions, head into Trials with all your questions answered! Happy study!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Okay, so first we have to venture a little bit into chemistry. Of course, we know that electrons orbit the nucleus of an atom. These take place in arrangements called electron shells . The outermost sell is called the valence band , with the electrons occupying it appropriately called valence electrons.
In certain solids, atoms join together in a structure called a lattice, where valence electrons are shared between the different atoms. This means that they are, to a certain extent, free to move throughout the lattice. These electrons form a new band of electrons called the conduction band .
(http://i.imgur.com/aDIxykS.png)
This highlights a model for atomic structure called the band structure model. Electrons exist at certain energy levels around the nucleus, which in a lattice, broaden to become energy bands . We can consider this model through diagrams such as this:
(http://i.imgur.com/zuZpQDum.png)
In this diagram, we see a gap between the valence and conduction band. Extra energy is required to bridge this gap; such as that supplied by a potential difference. It is electrons jumping this gap to the conduction band, and thus being free to move, which causes conduction in metals.
In conductors, the valence and conduction bands overlap, meaning very little energy is required for conduction to occur. In insulators, the gap is large, therefore resistance is high, since lots of energy is required for conduction to occur. Semiconductors , rather, are somewhere in between, and a lot more interesting.
(http://i.imgur.com/lBYasWBl.png)
To understand semiconductors, we need to understand holes . An electron that jumps to the conduction band in a semiconductor leaves behind an empty space, called a hole. The hole behaves like a positive charge. Electrons in the lattice move to occupy it, leaving behind other holes to repeat the cycle, kind of like a line of taxis moving up to occupy an empty space outside the airport (you will have done some kind of similar demonstration in class). This extra movement effectively acts as an additional method of carrying current. Thus, both holes and electrons act as current carriers.
Right, so semiconductors. Semiconductors form a crystal lattice structure much like metals. However, semi-conductive materials are Group 4 elements, meaning they are missing an electron from their valence shell. Thus, these lattices form in structures that share the electrons between the atoms via covalent bonds (they are called tetrahedral structures).
(http://i.imgur.com/2R39S6X.png)
Germanium was initially the material used in semiconductors. The semiconductor crystal had to be pure to be effective, and germanium was able to be purified. Silicon was the superior material; cheaper and easier to dope (see below). But it could not be purified. Only recently has this technology been developed.
Semiconductors are not overly useful by themselves; but doping changes that.
Doping involves introducing a foreign element to alter how a semiconductor behaves.
N type Semiconductors are formed when Group 5 impurity atoms are added, such as Arsenic or Phosphorus. This leaves additional electrons, free to move in the conduction band.
P type semiconductors are formed when Group 3 impurity atoms are added, such as Aluminium or Boron. This leaves the lattice with an additional hole, again, increasing conductivity.
What doping does is, effectively, adds an additional energy level between the valence and conduction bands. This means that conduction can occur more readily, as the electrons can make two smaller jumps, rather than one big one, thus requiring less energy. This extra level would simply be identified as a dotted line between the two lines on a band diagram.
Doped semiconductors are arguably the foundation of the modern age of computing. Computers rely on sophisticated switches called transistors, which are made up of doped semiconductors. We'll go into more detail in a second, but first, lets look at the precursor to the transistor, the thermionic device .
Thermionic devices and solid state devices (transistors) both allow the manipulation of a larger current with a much smaller one. During World War 2, thermionic devices were used for communication purposes. Several issues arose. Signals were weak, technology was too bulky for the field, and the systems were unreliable and unable to meet increasing demand for the military. Scientists sought to replace the thermionic devices (which were based on cathode ray tubes) with a new technology. This lead to the development of the solid state device. They were cheaper, lighter, more durable, more reliable, required no warm up time, and were energy efficient; superior to thermionic devices in every sense.
The common transistor is a thin piece of P type silicon sandwiched between two layers of N type silicon. This NPN junction creates what is called a depletion zone at the edges of the junction, creating an electric field which allows current to flow only in a single direction. Essentially, the junction has an inbuilt potential difference, which can be controlled with a small current. This fulfils the function of the valves in a much more efficient manner.
Note, you don't have to know how the depletion zone works. But if you are interested, in the basic sense, the extra holes in the P type semiconductor are filled by the extra electrons in the N type semiconductor. This leaves the P type with a negative charge and the N type with a positive charge, thus creating an electric field.
This depletion zone is utilised in solar cells. Solar cells utilise a simple PN diode with a single depletion zone. Oncoming light hits special material in the solar panels which ensures as much of the light is possible is ’absorbed’ into the semiconductor. This releases electrons via the photoelectric effect. The diode forces these electrons to flow in a single direction, into an external circuit to power a load or to charge a battery.
And the final part of the topic; another topic another impact on society question.
Example: Assess the impacts of transistors on society in terms of their use in microchips and microprocessors.
Transistors lead to the development of the microchip and microprocessors, arguably the foundation of modern computer technology.
This has had obvious positive impacts including:
• Revolutions in the manufacturing sector
• More powerful computers with more widespread use
• Increased standard of living
• Faster developments in science and technology
• New communication methods (internet)
However, there are also negative impacts stemming from the automation of manual labour:
• Increased obesity rates
• Higher unemployment rates
• Poorly trained employees
Thus, the transistor has definitely revolutionised computer science, but has had both a positive and negative impact on the growth of the modern world.
And that's pretty much all the content. I've left potential exam questions to the end, since more than most other topics, semiconductor question really require knowledge of the whole topic. The good news; no formulas in this topic!! Which is of course super awesome. However, many questions in this topic require diagrams. Take a look at the ones supplied, they are the ones you may have to draw, make sure you know them and can replicate them.
Some common questions:
- Describing the idea of a lattice structure and the conduction band. Draw the diagram above, and explain how valence electrons are shared between atoms, leaving others free to move. It is compared to a 'sea of electrons' surrounding a bunch of static nuclei.
- Drawing band diagrams and relating them to conduction. Draw the diagram above, make sure the gap between the two bands in the semiconductor is smaller than the one for the insulator! . Then simply state that the bigger gap means higher resistance, easy! (obviously with more sophisticated language)
- Explaining the different types of doping for semiconductors. Remember that P type is Group 3, and N type is Group 5. You can get your own examples by looking at the periodic table at the back of the exam! . Indeed, this often is useful for MC questions as well.
- Comparing thermionic and solid state devices (easy marks, SSD's are just all around way better)
- Explaining how a photocell works (see above)
And one final example.
Example: Explain how doping affects conductivity in semiconductors.
To answer this for the full 5 marks, you need to mention a whole bunch of things.
Current in semiconductors is carried by both holes and electrons. Holes are electron vacancies, left behind by electrons, which then act as positive charges. As current flows, electrons move one away, holes move the other, and both act as current carriers. However, un-doped semiconductors have limited conductivity, with the valence band and conduction bands still separated by a small energy gap (INSERT DIAGRAM HERE).
The conductivity of the semiconductor can be increased by doping, if either Group III or Group V impurity is added to the semiconductor. Group V impurities add an extra electron to the lattice structure forming an n-type semiconductor. These electrons have a smaller energy gap and electrons become the dominant charge carriers. If a Group III impurity is added, a p-type semiconductor is formed and an additional hole is introduced into the lattice. Holes are then the dominant charge carriers.
Quite a mouthful for 5 marks, and it could absolutely be done in less. But this response is heavily based on the HSC Sample for this question; which is what I highly suggest you reference when in doubt as to how much detail you need.
So, that’s all for today! Stay tuned for the very last guide, and be sure to register and ask questions, head into Trials with all your questions answered! Happy study!
A GUIDE BY JAMON WINDEYER
Superconductors
Spoiler
Hello once again for the final time everyone. Yes, that’s right, it is time for the very last Physics guide :( Yes I know, it’s very sad. But let’s make the most of the occasion! This guide will cover the last part of the core course, Superconductors. Some weird concepts, but on the whole, nothing overly difficult, as long as you are persistent. I’ll help you get prepared. This guide will be a little shorter than others, so a nice and easy finish.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Right, before we cover superconductors, we have to revise a few little things. Recall that metals have a crystal lattice structure , and can be described as a net of static nuclei, surrounded by a sea of electrons. It is the movement of these free electrons which allows conduction in metals. This lattice structure was discovered by the Braggs using X-ray crystallography. You don’t need to know much about this, but basically it involves bouncing X rays off of a metal and looking at the interference patterns. X rays, having a similar wavelength to the distance between atoms in a metal, give a good resolution picture of the atomic structure. The most you’ll have to do here is identify or describe the process, you won’t have to explain it.
So, we know what causes induction. But what causes resistance? Well, beyond the energy gaps covered in previous topics, resistance is actually caused by two things. Firstly, impurities in the lattice. If external elements are present in a metal, or other disruptions are present, they will interrupt the flow of electrons in various ways. Secondly, lattice vibrations contribute to resistance. These vibrations occur due to the temperature of the metal; heat energy causes vibration in the atomic structure of the metal. These can disrupt the flow of electrons and thus increase resistance.
How does all of this relate to superconductors? The best way to explain this is to explain the BCS Theory , the theory which explains superconductive behaviour. I’ll note, however, that this theory is outdated and now thought to be at least partially incorrect. However, it is an effective model in most circumstances.
The BCS Theory concerns lattice vibrations. As the temperature of an superconductor decreases, these vibrations decrease in intensity. Eventually, the superconductor reaches a low enough temperature that these vibrations become negligible. This is called the critical temperature . When this happens, the moving electrons actually are able to attract the positively charged nuclei around them. This creates a lattice distortion , where the lattice distorts inwards towards the electron. In quantum terms, this is called a phonon .
Now, these phonons are actually a region of focused positive charge. They attract other electrons, effectively creating electron pairs. These are called cooper pairs , and due to quantum energy effects you aren’t required to understand, they travel through the lattice completely unaffected. These pairs continually break and reform, but result in a net flow of electrons with zero resistance.
This explains how a superconductor works. Below the critical temperature, superconductors have zero resistance! Different materials have different critical temperatures, and physicists are continually developing complex compounds and alloys with higher critical temperatures. Typically, the critical temperatures are extremely low (usually below 150 Kelvin), and require liquid nitrogen/helium to reach this temperature.
(http://i.imgur.com/XoBOucvm.png)
You are also required to explain the Meissner Effect . You would have seen this in action in an experiment; basically, a magnet placed above a superconductor below critical temperature will levitate . But what is happening?
The Meissner Effect states that a superconductor below its critical temperature will exclude all magnetic flux. That is, magnetic fields cannot penetrate it. So, when we place a magnet above a superconductor, its magnetic field is excluded from the superconductor and so the magnet hovers in place. There are other quantum pinning effects at work too, but you don’t need to understand these.
Superconductors obviously prove extremely useful. Of course, zero resistance means that we can achieve 100% efficiency, no power loss occurs! This can be seen in the formula. At the moment, superconductors are primarily used to create extremely powerful electromagnets, used in Magnetic Resonance Imaging, and Maglev Trains . Explaining how superconductors are used in Maglev Trains is a common question…
Example: Explain how superconductors have allowed the development of new, more efficient means of transport, such as Maglev Trains.
Maglev Trains, currently used in parts of Asia, operate using extremely powerful electromagnets. Permanent magnetic fields are used to suspend the train above the track, while a set of changing magnetic fields are used to propel the train forward/backward. Superconductors, and their associated properties (namely, zero resistance) have allowed the development of electromagnets powerful enough to suit this purpose. This new technology allows friction to be completely removed, thus improving energy efficiency, reducing urban noise, and reducing commute times for travellers.
Note: The Meisner Effect has no role in the operation of a Maglev Train. BOSTES hates this error, don’t make it yourself!
However, there are serious limitations to how superconductors can be used, at least with the current technology available. For one, the extremely low critical temperature required for superconductive behaviour to occur. This requires use of dangerous liquid gases, and lots of energy to maintain the low temperature, which at the moment, completely offsets any benefits from using them beyond specific scenarios. Further, superconductors are not malleable, and thus are not suitable for things like transmission wires or similar. Finally, superconductors cannot carry AC current, only DC current. Thus, to use them, we would have to completely redesign our power infrastructure.
And that's actually about it for superconductors! Be sure you can explain the BCS Theory well. It is a little complex, so feel free to register and ask questions, head into Trials with all your questions answered! Best of luck for all your Trials, including Physics, happy study!
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.
Right, before we cover superconductors, we have to revise a few little things. Recall that metals have a crystal lattice structure , and can be described as a net of static nuclei, surrounded by a sea of electrons. It is the movement of these free electrons which allows conduction in metals. This lattice structure was discovered by the Braggs using X-ray crystallography. You don’t need to know much about this, but basically it involves bouncing X rays off of a metal and looking at the interference patterns. X rays, having a similar wavelength to the distance between atoms in a metal, give a good resolution picture of the atomic structure. The most you’ll have to do here is identify or describe the process, you won’t have to explain it.
So, we know what causes induction. But what causes resistance? Well, beyond the energy gaps covered in previous topics, resistance is actually caused by two things. Firstly, impurities in the lattice. If external elements are present in a metal, or other disruptions are present, they will interrupt the flow of electrons in various ways. Secondly, lattice vibrations contribute to resistance. These vibrations occur due to the temperature of the metal; heat energy causes vibration in the atomic structure of the metal. These can disrupt the flow of electrons and thus increase resistance.
How does all of this relate to superconductors? The best way to explain this is to explain the BCS Theory , the theory which explains superconductive behaviour. I’ll note, however, that this theory is outdated and now thought to be at least partially incorrect. However, it is an effective model in most circumstances.
The BCS Theory concerns lattice vibrations. As the temperature of an superconductor decreases, these vibrations decrease in intensity. Eventually, the superconductor reaches a low enough temperature that these vibrations become negligible. This is called the critical temperature . When this happens, the moving electrons actually are able to attract the positively charged nuclei around them. This creates a lattice distortion , where the lattice distorts inwards towards the electron. In quantum terms, this is called a phonon .
Now, these phonons are actually a region of focused positive charge. They attract other electrons, effectively creating electron pairs. These are called cooper pairs , and due to quantum energy effects you aren’t required to understand, they travel through the lattice completely unaffected. These pairs continually break and reform, but result in a net flow of electrons with zero resistance.
This explains how a superconductor works. Below the critical temperature, superconductors have zero resistance! Different materials have different critical temperatures, and physicists are continually developing complex compounds and alloys with higher critical temperatures. Typically, the critical temperatures are extremely low (usually below 150 Kelvin), and require liquid nitrogen/helium to reach this temperature.
(http://i.imgur.com/XoBOucvm.png)
You are also required to explain the Meissner Effect . You would have seen this in action in an experiment; basically, a magnet placed above a superconductor below critical temperature will levitate . But what is happening?
The Meissner Effect states that a superconductor below its critical temperature will exclude all magnetic flux. That is, magnetic fields cannot penetrate it. So, when we place a magnet above a superconductor, its magnetic field is excluded from the superconductor and so the magnet hovers in place. There are other quantum pinning effects at work too, but you don’t need to understand these.
Superconductors obviously prove extremely useful. Of course, zero resistance means that we can achieve 100% efficiency, no power loss occurs! This can be seen in the formula
Example: Explain how superconductors have allowed the development of new, more efficient means of transport, such as Maglev Trains.
Maglev Trains, currently used in parts of Asia, operate using extremely powerful electromagnets. Permanent magnetic fields are used to suspend the train above the track, while a set of changing magnetic fields are used to propel the train forward/backward. Superconductors, and their associated properties (namely, zero resistance) have allowed the development of electromagnets powerful enough to suit this purpose. This new technology allows friction to be completely removed, thus improving energy efficiency, reducing urban noise, and reducing commute times for travellers.
Note: The Meisner Effect has no role in the operation of a Maglev Train. BOSTES hates this error, don’t make it yourself!
However, there are serious limitations to how superconductors can be used, at least with the current technology available. For one, the extremely low critical temperature required for superconductive behaviour to occur. This requires use of dangerous liquid gases, and lots of energy to maintain the low temperature, which at the moment, completely offsets any benefits from using them beyond specific scenarios. Further, superconductors are not malleable, and thus are not suitable for things like transmission wires or similar. Finally, superconductors cannot carry AC current, only DC current. Thus, to use them, we would have to completely redesign our power infrastructure.
And that's actually about it for superconductors! Be sure you can explain the BCS Theory well. It is a little complex, so feel free to register and ask questions, head into Trials with all your questions answered! Best of luck for all your Trials, including Physics, happy study!