HSC Physics Question Thread

[Maz]

Hey!

I think that you would definitely need to be a little bit clearer with your answer, just to make sure you fully get across to the marker that you know what you're talking about.

How would you explain this situation to the traveler?

Firstly, I would start by explaining that his situation is impossible. If something is 4.367 light years from earth, then it takes light 4.367 years to get from Alpha Centauri to Earth. Therefore, it is impossible to get there in less than 4 years as this would be faster than the speed of light.

Maybe his clock reads more than 4.367 years (let's just say 5 years). In that case, the fact that he was traveling near the speed of light would explain why he got there so quickly. Length contraction occurs at high speeds, meaning that, to the traveler, the distance between himself and Alpha Centauri has decreased. This allows him to get there quicker. Remember that to the traveler, time operates exactly as normal. To the traveler, it is the outside world that appears to be moving more slowly. So time dilation should not actually make a difference to a traveler viewing a clock on the space ship: to him, the clock moves as normal.

Observer on Earth

To an observer on earth, time dilation would mean that the time experienced by a traveler appears to be slower. To the person on Earth, the clock on the spaceship will move more slowly. However, time will move forward as normal on Earth, and so it will appear that the traveler takes longer than 5 years to get there. The observer on earth will also notice the mass of the spaceship increase (due to mass dilation) and the length of the spaceship decrease (based on length contraction).

I hope this helps! Take a look at the formulas in order to know exactly what's going on when something moves very quickly. Remember that a person moving near to the speed of light will still experience everything (time, length etc.) to be the same; it is only the outside world that appears to be different.

Jake

Thankyou so so much- especially for explaining it so well

[Loki98]

Sorry for such a broad question, could u  please explain the differences and similarities between ac and dc motors and ac and dc generators. I'm mainly having trouble with the purpose of the commutator in each case.

[jamonwindeyer]

hello
Can someone please help me with this?
Alpha Centauri is a star 4.367light year from earth. A time traveller taking this journey at near light speed in a spaceship was confused by the fact that he made the journey in less than 4 years based on his clock. how would you explain this situation to the traveller? How would this dilemma be viewed by an observer on Earth.
As an answer i put: To the observer on Earth, time dilation has affected their view of time taken by the the ship to be more than time taken relative to the traveler. Time dilation is more noticeable as speed increases.
i was wandering if that would be enough as an answer- or if i was even right and how you would calculate that?
Thankyou so much in advance 

Hey mq123! I want to add my interpretation to Jake's answer above. I think this situation would be possible, and here is my reasoning.

We remember that 4.367 light years is a measure of distance. As the time traveller moves at relativistic speeds, length contraction will occur. We normally consider the effects of length contraction on the moving object as viewed by an observer (EG - a moving spaceship compressing to external view). However, length contraction also occurs as viewed from the moving object. From the frame of reference of the time traveller, the 4.367 light years of distance will shrink by length contraction. It actually will travel a smaller distance, and thus, arrive sooner than you would expect without considering Special Relativity! This, in my opinion, causes the anomaly described in Part 1.

As for Part 2, I agree with Jake's reasoning. If the clock on the ship reads 4 years, this will actually represent much longer in earth years, since time passes more slowly for moving observers. This is by the normal effects of time dilation.

I hope this interpretation adds to Jake's and that between us we've given you a solid understanding of the question! The amount of detail you would respond with would depend on how many marks it is worth in the HSC Exam, but any response which covers Length Contraction of the distance travelled, and time dilation causing the clocks to fall out of sync, to a decent level of detail, would be right on track 

[RuiAce]

Sorry for such a broad question, could u  please explain the differences and similarities between ac and dc motors and ac and dc generators. I'm mainly having trouble with the purpose of the commutator in each case.

There are tons of differences. But in terms of the commutator:

In an AC generator, the commutator ONLY exists to ensure current flow exists. But in a DC generator, there exists an actual need for the polarity of the current to be reversed so that the current produced is always the same in polarity (conversely, for a DC motor it's to ensure that the DC motor maintains torque in the same direction).

Some other differences include:
 
The slip ring commutator, due to not having the obvious split, will not be as susceptible to sparking as opposed to that of the split ring commutator making it more safer to use. The split also causes wear and tear to be more frequent. So these mean that AC generators will typically be safer and require less maintenance.

AC generators, of course, can have their voltages stepped up or down by transformers and thus can be:
a) Transmitted more effectively, as heat losses are minimised according to P_loss=RI^2
b) High voltage requiring appliances (TVs) and low voltage requiring ones (computers) can both be used simultaneously.
c) Power stations can be built together in one rural site, instead of several 100s of rural AND urban regions.

Yet, for only a very specific voltage range (that range I have forgotten), DC is more powerful than AC. Furthermore, AC requires synchronisation between power stations to ensure power transmission is effective.

As far as I recall, a lot more information on this I obtained from Physics in Focus during my study.

[Happy Physics Land]

Hey Loki:

Ok so essentially the most significant difference between an AC motor and a DC motor is indeed their commutators, and the mechanism of AC motor can vary depending on the type of motor you are talking about. In this case I will talk to you about the difference between an AC universal motor and a DC motor (If Jake would like to spot out the difference between AC induction motor or synchronous motor and DC motor, please feel free to do so too!).

A DC motor requires a current flow being supplied to the coil placed inside a magnetic field. As a result of the motor effect, which states that a current-carrying current will experience a force in magnetic field, the coil will experience a torque because of the two forces acting on both sides of the coil. As the coil is rotating, it encounters the problem of not being able to rotate consistently in one direction because torque direction will reverse every half a turn. This problem is effectively overcome through the implementation of split-ring commutators, which reverses the direction of current flow in each coil every half-revolution. This ensures that the torque on the coil is ALWAYS in the same direction. Do keep in mind that change of current direction is only due to the change in position of each half of the split ring (i.e. if one half of the split ring originally is in contact with the positive brush, then after half a turn it will come in contact with negative brush). This ensures torque in constant direction.

Similarly, universal motor operating on AC also employs a split ring commutator to keep the direction of current in the rotor coils the same. In contrast to a DC motor, an AC universal motor involves the reversing of the direction of the magnetic field produced by the stator coils (electromagnets) every half-turn, whereas in an DC motor, the magnetic field is kept permanent. The reason for doing so is that, because of the commutator, we have effectively kept the direction of AC current in the rotor coils the same, and to keep torque in a constant direction, it is necessary to change the polarity of the electromagnets every half turn.

All generators involves the input of external mechanical force and torque applied which causes the armature and coil to rotate in a magnetic field. As the rotor coil experiences a change in flux due to change in relative motion, alternating EMF and current are induced in the coil (Faraday's Law).

In an AC generator, the stator coils produces a magnetic field that is similar to that provided by a pair of permanent magnets. Its crucial difference from DC generators is the utilisation of SLIP rings. Each end of the coil is connected to one of the two slip rings on the axle. Brushes press against the slip rings providing a sliding electrical contact. The slip rings remain in contact with the same brush and the same end of the coil throughout the rotation of an AC generator.

Similarly, in a DC generator, the stator coils are also producing a magnetic field that is similar to that provided by a pair of permanent magnets. However, the difference between DC and AC generator is its use of SPLIT ring commutators. The commutator provides points of contact between thr rotor coils and the external electric circuit, it serves to reverse the direction of current flow in the external circuit once every half turn of the generator. This ensures that the current produced is always in the same direction (which is essentially DC which flows only in one direction).

I really hoped that my explanation would help you Loki and if you have any doubts or questions, please dont hesitate to ask!

Best Regards
Happy Physics Land

[jamonwindeyer]

Sorry for such a broad question, could u  please explain the differences and similarities between ac and dc motors and ac and dc generators. I'm mainly having trouble with the purpose of the commutator in each case.

Sorry for such a broad question, could u  please explain the differences and similarities between ac and dc motors and ac and dc generators. I'm mainly having trouble with the purpose of the commutator in each case.

Not at all Loki98! Broad questions are awesome, I get more freedom to write what I want to say 

So, a big question, I could write pages, I'll try and keep it fairly concise by focusing only on Universal AC Motors/Generators.

Motors and generators should be viewed as similar in design, but opposite in function. Generators take kinetic energy and transform it into electrical energy, via the principle of electromagnetic induction. Motors take electrical energy and convert it into kinetic energy via the Motor Effect. Opposite in function, however, they have quite a few similarities. All generators and motors (as studied in this course) have:

• A stator, normally a permanent magnet. This creates a magnetic field, inside which is placed:
• A rotor, the spinning part of the apparatus, which consists of coils wrapped around an iron core.

In motors, the rotor spins due to the Motor Effect, which causes a rotational force due to the interaction between the magnetic field and the electric current in the armature. In generators, we spin the rotor and a current is induced in the coils, due to the changing magnetic field.

Now, the commutators. This is a little confusing. Commutators are little metal rings which allow the external circuit to maintain contact with the coils as they spin. There are two kinds, which I'll go into below, but to understand the explanations we first need to go through something else.

For both the Motor Effect and Electromagnetic Induction, the resultant force or current is dependent on the direction of the magnetic field. You should have learned in class that, as the motor/generator spins, this actually results in the direction of the force or current changing every half turn (180 degrees). This can be deduced by applying the "Right Hand Rule."

So, the two commutators:

Slip ring commutators simply maintain contact, nothing else. For AC Generators, this means that the direction of the generated current is unaffected, and so, continues to change every half turn. This creates an AC current with no further complications.

Split ring commutators are, as the name implies, split in half. This means the connection with the external circuit (either load or supply) is interrupted every half turn. In a DC motor, this is vital, as it actually allows the direction of current in the coil (say, clockwise/anticlockwise) to be reversed every half turn. It is quite hard to explain how this works with words or even pictures, I found some awesome videos on Youtube, like this one. This was literally just a bookmark I had leftover in my HSC folder, it helped me heaps back when I did the course.

And the principle in a DC Generator is the same. The induced current reverses every half turn normally, and the split ring commutator reverses it again, so the polarity remains constant!

This is just a quick run down of the concepts. Not everything is explained in full detail (feel free to ask if you need more), but hopefully it helps (especially in combination with that posted by those above) 

[Loki98]

Thanks for all the help guys. Helped me out heaps.

[brenden]

Thanks for all the help guys. Helped me out heaps.

are you checking ATAR Notes at school? if so you're my favourite type of person

[Maz]

Hey
Can someone please help me out with this?
Two trains are moving at different but constant velocities. Are the any conditions under which they are
in the same inertial reference frame?
Do 2 non-inertial reference frames imply that one frame is accelerating?
i'm not really understanding relativity...
Thankyou so much in advance

[Happy Physics Land]

Hey
Can someone please help me out with this?
Two trains are moving at different but constant velocities. Are the any conditions under which they are
in the same inertial reference frame?
Do 2 non-inertial reference frames imply that one frame is accelerating?
i'm not really understanding relativity...
Thankyou so much in advance

Hey mq123:

Ok so hmm let's start with defining inertial frames of reference and non-inertial frames of reference shall we?

An inertial frame of reference is one that is either at rest or travelling at constant velocity. Within the frame of reference, the occupant cannot perform any experiments to detect whether he is travelling at a constant velocity or at rest because he experiences no force. Imagine yourself being in a black box. You cannot determine whether the black box is travelling at a constant speed or at rest because you dont feel anything (IF the box is accelerating, you would feel a force according to Newton's second law F = ma). So in regards to your first question, if you are an occupant inside the train, then both trains would be the same inertial frame of reference because you cant perform any experiment to detect that the two trains are actually travelling at different velocities. If you are a person inside the train, you wouldnt feel a single difference between travelling at 20km/h and travelling at 2000km/h.

Non-inertial frames of references are basically the opposite. The one thing you'll have to know is that if the train is accelerating, then it cant be an inertial frame of reference (i.e. it will have to be a non-inertial frame of reference). In a non-inertial frame of reference, the occupant can actually detect if he is decelerating or accelerating because he feels a force. So umm in response to your second question I'm not too sure what you meant but I would tell you that both non-inertial frames of reference are accelerating, because the occupants in both frames would be experiencing a force that is created by acceleration.

If you have further doubts or questions, please dont hesitate to ask. If you would like to read more about special relativity, I found a site for you that covers everything you'd need to know http://physics.mq.edu.au/~jcresser/Phys378/LectureNotes/VectorsTensorsSR.pdf. You probably would only need to read chapters 1-4 because thats all what you are going to be assessed on.

Best Regards
Happy Physics Land

[Maz]

Hey mq123:

Ok so hmm let's start with defining inertial frames of reference and non-inertial frames of reference shall we?

An inertial frame of reference is one that is either at rest or travelling at constant velocity. Within the frame of reference, the occupant cannot perform any experiments to detect whether he is travelling at a constant velocity or at rest because he experiences no force. Imagine yourself being in a black box. You cannot determine whether the black box is travelling at a constant speed or at rest because you dont feel anything (IF the box is accelerating, you would feel a force according to Newton's second law F = ma). So in regards to your first question, if you are an occupant inside the train, then both trains would be the same inertial frame of reference because you cant perform any experiment to detect that the two trains are actually travelling at different velocities. If you are a person inside the train, you wouldnt feel a single difference between travelling at 20km/h and travelling at 2000km/h.

Non-inertial frames of references are basically the opposite. The one thing you'll have to know is that if the train is accelerating, then it cant be an inertial frame of reference (i.e. it will have to be a non-inertial frame of reference). In a non-inertial frame of reference, the occupant can actually detect if he is decelerating or accelerating because he feels a force. So umm in response to your second question I'm not too sure what you meant but I would tell you that both non-inertial frames of reference are accelerating, because the occupants in both frames would be experiencing a force that is created by acceleration.

If you have further doubts or questions, please dont hesitate to ask. If you would like to read more about special relativity, I found a site for you that covers everything you'd need to know http://physics.mq.edu.au/~jcresser/Phys378/LectureNotes/VectorsTensorsSR.pdf. You probably would only need to read chapters 1-4 because thats all what you are going to be assessed on.

Best Regards
Happy Physics Land

thankyou so much

[Maz]

Hey humans...again
can i please get some help on another question? I've done part a...and the rest...i just don't know if it's right cos i
don't have any answers...
it's attached...my teachers are having a debate on what lo and l are- i think l0 is 600...

any help would be appreciated

[jakesilove]

Hey humans...again
can i please get some help on another question? I've done part a...and the rest...i just don't know if it's right cos i
don't have any answers...
it's attached...my teachers are having a debate on what lo and l are- i think l0 is 600...

any help would be appreciated

Hey MQ!

THIS QUESTION IS NOT APPLICABLE TO HSC STUDENTS

Questions like this aren't found in the HSC, as they are slightly harder than standard length contraction/time dilation questions. The issue is that the distance the light needs to travel actually changes based on whether it is traveling to the nose of the aircraft, or the tail of the aircraft. Anyway, below are my answers! I only did them quickly, so feel free to correct me





Jake

[JellyBeanz]

Hey MQ!

THIS QUESTION IS NOT APPLICABLE TO HSC STUDENTS

Questions like this aren't found in the HSC, as they are slightly harder than standard length contraction/time dilation questions. The issue is that the distance the light needs to travel actually changes based on whether it is traveling to the nose of the aircraft, or the tail of the aircraft. Anyway, below are my answers! I only did them quickly, so feel free to correct me

(Image removed from quote.)
(Image removed from quote.)
(Image removed from quote.)

Jake

Haha sorry Jake for us VCE kids posting you questions at times XD, thanks heaps for answering them.

You guys are awesome.

[Maz]

Hey MQ!

THIS QUESTION IS NOT APPLICABLE TO HSC STUDENTS

Questions like this aren't found in the HSC, as they are slightly harder than standard length contraction/time dilation questions. The issue is that the distance the light needs to travel actually changes based on whether it is traveling to the nose of the aircraft, or the tail of the aircraft. Anyway, below are my answers! I only did them quickly, so feel free to correct me

(Image removed from quote.)
(Image removed from quote.)
(Image removed from quote.)

Jake

thankyou so much- you guys are so lucky not having to do these types of questions we have sooo many like them
really appreciate the help