HSC Physics Question Thread

[nimasha.w]

hiiiii! i don't fully understand the depletion zone in a pn junction :-))

[jamonwindeyer]

Hello,

Regarding the Idea to Implementation topic - Cathode Rays, how would I assess the validity of the experiments on the nature of cathode rays (i.e paddle wheel, Maltese cross, etc)?

thanks.

Hey FallonXay! First and foremost, we must remind ourselves that validity refers to how well variables are controlled in an experiment. The independent variable should be altered systematically, the dependent variable should be measured correctly, and everything else should be kept constant  essentially, this amounts to the following question: Does the experiment test what it actually should?

For these experiments it's a little trickier, because the experiments are quantitative, not qualitative. There are no actual measurements made, so we step back a little bit, and just consider in general how well the variables were controlled. Would anything have affected the results of these experiments, and/or were results considered correctly?

My interpretation of this would be to discuss how correctly the results were interpreted. Obviously, some of the Cathode ray experiments suggested that the rays were waves, and other suggested they were particles. The ones suggesting particle behaviour were valid because the results were interpreted correctly, the others were (somewhat) invalid because the results, while controlled, were interpreted incorrectly.

This would be how I would approach it, as well as coming up with variables in the experiments that may not have been controlled correctly (air pressure in tube, voltage, etc.)  in saying that, this question is just a tad strange, I personally doubt it would be asked in the HSC hope this helps!

[jamonwindeyer]

hiiiii! i don't fully understand the depletion zone in a pn junction :-))

Hey there! So this is a little bit of a topic that you don't need to completely understand for the HSC, it is a reasonably complex piece of Physics, but we can get a rough idea quite effectively and that will do nicely.

I'm going to assume that you know the basics of extrinsic semiconductors: That p-type semiconductors have an excess of holes, and n-type semiconductors have an excess of electrons.

Therefore, when we join a P and N type semiconductor together, we have what is called electron and hole drift. The excess electrons drift from the N type to the P type, and the holes drift from the P type to the N type. On the N side, this therefore leaves behind a positive donor ion, and on the P side, this leaves behind negative acceptor ions. This is shown in the diagram below.

Note that we call this a depletion zone because the diffused electrons and holes are depleted. The electrons come into contact with the holes on the P side and disappear via recombination, and the same thing happens in reverse on the N side. This leaves behind no mobile charge carriers in the depletion zone. What does happen, however, is that the positive/negative ions set up an electric field, as shown below. This electric field is the basis for the modern transistor!

The very astute may notice that this electric field is what eventually prevents further diffusion of electrons and holes. Once this happens, we say the junction is in equilibrium, but this is beyond HSC Physics 

I hope this helps!!

[Aliceyyy98]

Heyyy~
Was wondering how the pn junction in solar cells create electricity? Confused about the direction of current and electron flow >< and also how does the induction motor - squirrel cage work? Like how does the rotor chase the magnetic field! Thank youuuu

[jamonwindeyer]

Heyyy~
Was wondering how the pn junction in solar cells create electricity? Confused about the direction of current and electron flow >< and also how does the induction motor - squirrel cage work? Like how does the rotor chase the magnetic field! Thank youuuu

Hey there!! For the PN junction in solar cells: In terms of how a PN junction works, check out the post I made right before your post, that should explain it for you! Now, for solar cells specifically:

The PN junction effectively works to ensure that any charge can only flow in the same direction as the electric field. Well, it can flow the other way if you really want it to, but that isn't relevant here. In solar cells, the photoelectric effect causes electrons to be released from the semiconductors in the junction. A special coating maximises the amount of photoelectric emission occurring. These released electrons are then forced to move according to the electric field in the junction: They move in the opposite direction. Remember, the electric field indicates the direction positive charges will flow: Electrons do the opposite.

So what we've established is that these electrons are forced to move in a single direction by the electric field in the depletion zone caused by the PN junction. The junction therefore becomes a current source! Electrons flow out of the junction, and through an external load or battery, and this process continues. Keep in mind that a complete circuit is required for this to continue; the electrons continually flow around the circuit and back into the PN junction ready to be re-emitted.

Note that current will just be in the opposite direction to electron flow, by definition 

Now to the induction squirrel cage rotor. The principle at play here is Lenz's Law, the idea that induced currents will oppose the change that created them. An induction motor is exposed to a rotating magnetic field. Essentially, this means that a North pole (several, but lets simplify) is spinning around the rotor. The changing field induces a current in the rotor, which will cause the rotor to generate its own magnetic field.

According to Lenz's Law, the rotors magnetic field will oppose the change that created it. It doesn't want the North pole to be spinning. So, it gives chase. It spins as well, effectively to 'cancel out' the change.

This is, kind of, like looking at a sideways picture on a piece of paper glued to the table. The paper can't be moved, so YOU turn your head to cancel out the change and look at it the right way up. Same thing above; the rotor spins so it is doing the same thing as the spinning North pole, thus cancelling out the change. Cancelling out the change isn't the appropriate way to think of it as a Physicist, but for getting the intuitive sense of things, it helps!

The rotor wants to stay aligned with the North pole, and so it rotates around to chase it. This seems kind of silly, but it makes sense! The easiest model for this is spinning a permanent magnet below an aluminium disc; the disc will spin around and chase the magnet. Same thing 

The rotor is shaped the way it is to maximise induced current flow, thus maximising the response of the rotor and maximising the efficiency of the induction motor as a whole 

I hope this helps! Let me know if anything needs clearing up 

[FallonXay]

Hey FallonXay! First and foremost, we must remind ourselves that validity refers to how well variables are controlled in an experiment. The independent variable should be altered systematically, the dependent variable should be measured correctly, and everything else should be kept constant  essentially, this amounts to the following question: Does the experiment test what it actually should?

For these experiments it's a little trickier, because the experiments are quantitative, not qualitative. There are no actual measurements made, so we step back a little bit, and just consider in general how well the variables were controlled. Would anything have affected the results of these experiments, and/or were results considered correctly?

My interpretation of this would be to discuss how correctly the results were interpreted. Obviously, some of the Cathode ray experiments suggested that the rays were waves, and other suggested they were particles. The ones suggesting particle behaviour were valid because the results were interpreted correctly, the others were (somewhat) invalid because the results, while controlled, were interpreted incorrectly.

This would be how I would approach it, as well as coming up with variables in the experiments that may not have been controlled correctly (air pressure in tube, voltage, etc.)  in saying that, this question is just a tad strange, I personally doubt it would be asked in the HSC hope this helps!

Ok, makes sense - thanks. However, wouldn't the theories that demonstrated the wave nature of cathode rays still be valid due to the wave-particle duality proposal?

[Aliceyyy98]

This was so clear! Thanks so much!!

Hey there!! For the PN junction in solar cells: In terms of how a PN junction works, check out the post I made right before your post, that should explain it for you! Now, for solar cells specifically:

The PN junction effectively works to ensure that any charge can only flow in the same direction as the electric field. Well, it can flow the other way if you really want it to, but that isn't relevant here. In solar cells, the photoelectric effect causes electrons to be released from the semiconductors in the junction. A special coating maximises the amount of photoelectric emission occurring. These released electrons are then forced to move according to the electric field in the junction: They move in the opposite direction. Remember, the electric field indicates the direction positive charges will flow: Electrons do the opposite.

So what we've established is that these electrons are forced to move in a single direction by the electric field in the depletion zone caused by the PN junction. The junction therefore becomes a current source! Electrons flow out of the junction, and through an external load or battery, and this process continues. Keep in mind that a complete circuit is required for this to continue; the electrons continually flow around the circuit and back into the PN junction ready to be re-emitted.

Note that current will just be in the opposite direction to electron flow, by definition 

Now to the induction squirrel cage rotor. The principle at play here is Lenz's Law, the idea that induced currents will oppose the change that created them. An induction motor is exposed to a rotating magnetic field. Essentially, this means that a North pole (several, but lets simplify) is spinning around the rotor. The changing field induces a current in the rotor, which will cause the rotor to generate its own magnetic field.

According to Lenz's Law, the rotors magnetic field will oppose the change that created it. It doesn't want the North pole to be spinning. So, it gives chase. It spins as well, effectively to 'cancel out' the change.

This is, kind of, like looking at a sideways picture on a piece of paper glued to the table. The paper can't be moved, so YOU turn your head to cancel out the change and look at it the right way up. Same thing above; the rotor spins so it is doing the same thing as the spinning North pole, thus cancelling out the change. Cancelling out the change isn't the appropriate way to think of it as a Physicist, but for getting the intuitive sense of things, it helps!

The rotor wants to stay aligned with the North pole, and so it rotates around to chase it. This seems kind of silly, but it makes sense! The easiest model for this is spinning a permanent magnet below an aluminium disc; the disc will spin around and chase the magnet. Same thing 

The rotor is shaped the way it is to maximise induced current flow, thus maximising the response of the rotor and maximising the efficiency of the induction motor as a whole 

I hope this helps! Let me know if anything needs clearing up 

[Spencerr]

Ok, makes sense - thanks. However, wouldn't the theories that demonstrated the wave nature of cathode rays still be valid due to the wave-particle duality proposal?

Hey there, as Jamon mentioned validity refers to how well the experiment is set out to test what it intends to test.  At the time of the cathode ray debate, the way theory of cathode rays wad proposed because the germans passed a cathode ray in between two electrically charged plates and found that the rays did not deflect. Since charged particles would deflect and waves wouldn't, they concluded that cathode rays were waves. However the experiment was not valid because the experiment had inherent flaws. At that time they did not have the technology to fully pump air out of the vacuum tube therefore when they did the experiment, there were a significant amount of gas in the discharge tube
 These gas molecules were ionized by the cathode rays causing them to be attracted to the electric fields. This neutralized the electric fields once equilibrium was reached and so there was no e field acting on the cathode rays and hence no deflection. This explains why the interpretation of the results were invalid.

Referring to the wave particle duality, I believe that is a separate concept that comes up much later in quanta to quarks with de broglie. It's beyond what you need to know in i2i as its a core component of the elective q2q.

As for the other experiments to showboat they were particles you could argue they are valid due to correct experimental procedure, controlled variables  etc. One thing I would like to mention that is beyond hsc is that the paddle wheel experiment which showed the cathode rays had momentum is actually invalid. This is because the rotation of the paddle wheels are cause air currents flowing in a certain direction due to the heating produced by the cathode rays. So there was no momentum pushing the wheel but air currents. But this is beyond the syllabus.

I hope this has helped

[FallonXay]

Hey there, as Jamon mentioned validity refers to how well the experiment is set out to test what it intends to test.  At the time of the cathode ray debate, the way theory of cathode rays wad proposed because the germans passed a cathode ray in between two electrically charged plates and found that the rays did not deflect. Since charged particles would deflect and waves wouldn't, they concluded that cathode rays were waves. However the experiment was not valid because the experiment had inherent flaws. At that time they did not have the technology to fully pump air out of the vacuum tube therefore when they did the experiment, there were a significant amount of gas in the discharge tube
 These gas molecules were ionized by the cathode rays causing them to be attracted to the electric fields. This neutralized the electric fields once equilibrium was reached and so there was no e field acting on the cathode rays and hence no deflection. This explains why the interpretation of the results were invalid.

Referring to the wave particle duality, I believe that is a separate concept that comes up much later in quanta to quarks with de broglie. It's beyond what you need to know in i2i as its a core component of the elective q2q.

As for the other experiments to showboat they were particles you could argue they are valid due to correct experimental procedure, controlled variables  etc. One thing I would like to mention that is beyond hsc is that the paddle wheel experiment which showed the cathode rays had momentum is actually invalid. This is because the rotation of the paddle wheels are cause air currents flowing in a certain direction due to the heating produced by the cathode rays. So there was no momentum pushing the wheel but air currents. But this is beyond the syllabus.

I hope this has helped

Yeah, definitely helps, thanks!

Just one thing: Doesn't Einstein's use the wave/particle duality to explain the photoelectric effect in Idea to Implementation?

[Spencerr]

Your right he does but that's after Planck introduced the idea of quantified energy in black bodies so it's still part of quantum theory which is still much later.

[FallonXay]

Your right he does but that's after Planck introduced the idea of quantified energy in black bodies so it's still part of quantum theory which is still much later.

ok, Cheers 

[Aliceyyy98]

Hi！

Could someone clarify the g-forces during launch and the physics behind slingshot effect for me!

Thankyou~

[ATWalk]

Hey,

I'm struggling with these multiple choice questions from a 2013 Sydney Boys trial and would love an explanation on how they got these answers.

The answers:
14 - D (Wouldn't a back emf be induced because of the relative motion of the coil?)
16 - B

Thank you! 

[Spencerr]

Hey,

I'm struggling with these multiple choice questions from a 2013 Sydney Boys trial and would love an explanation on how they got these answers.

The answers:
14 - D (Wouldn't a back emf be induced because of the relative motion of the coil?)
16 - B

Thank you! 

Hey there for 14. Note that it's a generator and not a motor. Back emf only occurs in motors due to the relative motion between the rotor and the b field. However in generators although this same relative motion exists the back emf in this case is actually the generated electricity. Whereas the back emf in motors opposed the supply voltage. The back emf in generators is the output so technically there is no back emf.

For the next question note how it days the ammeter voltmeter are connected in series. Remember back to when you did prac in class, you would always have to connect the voltmeter in parallel because the voltmeter has a significant amount of resistance. This explains why there is no current reading and the light bulb does not work as the large resistance nullifies the current flow.

[jamonwindeyer]

Hi！

Could someone clarify the g-forces during launch and the physics behind slingshot effect for me!

Thankyou~

Hey there Alice!!

Okay, so G forces. Basically, G forces are a way to measure the forces on astronaut in terms of multiples of the regular gravitational force of 1G. If we were travelling upwards with an acceleration of 9.8 metres per second per second, then we would experience a reaction force of 1G, as well as the regular gravitational force of 1G, meaning a total of 2G's. Astronauts accelerating upwards can experience G forces in excess of 5-6G's (may want to confirm that number, going off memory) 

Excessive or prolonged G force can injure or kill, however, we need a lot of acceleration to launch a rocket. So, it becomes a balance between accelerating the rocket, without exposing the astronauts to excess G forces. Acceleration is very controlled throughout the launch, and the moments of separation (when the fuel tanks are separated from the rocket and return to earth) also offer a break for the astronauts from the G forces 

Slingshot effect is much easier to picture with an example. Imagine you are riding a push bike down a road. You want to speed up, so you grab on to the back of one of the cars. The car drags you along, and then you let go, and you've sped up with absolutely no extra pedalling, and the car barely knew you were there!

In the slingshot effect, you are the probe, and the car is the planet. The probe is dragged along by the planet's gravitational field. Effectively, some of the momentum of the planet is transferred to the probe. However, since the planets mass is massively larger than the probe's, the massive speed increase of the probe has almost no effect on the motion of the planet