HSC Physics Question Thread

[talitha_h]

Hi guys
How can I explain the Meissner effect? I read different sources and they all say different things, induced current, exclusion of magnetic field. I dont know what should I write in the exam?
Thank you !!

Hope this helps, anyone can correct me if I'm wrong.

The Meissner effect is the phenomenon that a superconductor is able to totally exclude external magnetic fields below its critical temperature, therefore its internal magnetic field is always zero.

When an external magnetic field attempts to enter a superconductor, it induces a perfect eddy current to circulate in the superconductor, as a result of zero resistance. This ‘perfect’ current flows in such a direction that the magnetic field it produces is just as strong, but in the opposite direction to the external magnetic field (Lenz’s Law). This leads to a total cancellation of this external magnetic field and allows none of it to penetrate through the superconductor.
This idea can also be used to explain why a small magnet is able to hover over a piece of superconductor. The perfect flow of induced current in the superconductor will allow it to set up magnetic poles that are strong enough to repel the small magnet forcefully enough to overcome its weight force. Superconductors:  - Have 0 electrical resistance. - Demonstrate the Meissner effect.

[jamonwindeyer]

Hope this helps, anyone can correct me if I'm wrong.

The Meissner effect is the phenomenon that a superconductor is able to totally exclude external magnetic fields below its critical temperature, therefore its internal magnetic field is always zero.

When an external magnetic field attempts to enter a superconductor, it induces a perfect eddy current to circulate in the superconductor, as a result of zero resistance. This ‘perfect’ current flows in such a direction that the magnetic field it produces is just as strong, but in the opposite direction to the external magnetic field (Lenz’s Law). This leads to a total cancellation of this external magnetic field and allows none of it to penetrate through the superconductor.
This idea can also be used to explain why a small magnet is able to hover over a piece of superconductor. The perfect flow of induced current in the superconductor will allow it to set up magnetic poles that are strong enough to repel the small magnet forcefully enough to overcome its weight force. Superconductors:  - Have 0 electrical resistance. - Demonstrate the Meissner effect.

Almost perfect Talitha - I'll just fix one thing if that's okay.

The Meissner Effect cannot be attributed to eddy currents. We know this because we can hold a magnet above a metal, cool it below critical temperature, and then release the magnet, and it will hover. The magnet wasn't moving, so there was no change in flux, so there can't have been eddy currents. Yet, the magnetic flux is still excluded.

I actually think this paragraph from Wikipedia does a fantastic job explaining it:

Any perfect conductor will prevent any change to magnetic flux passing through its surface due to ordinary electromagnetic induction at zero resistance. The Meissner effect is distinct from this: when an ordinary conductor is cooled so that it makes the transition to a superconducting state in the presence of a constant applied magnetic field, the magnetic flux is expelled during the transition. This effect cannot be explained by infinite conductivity. The placement and subsequent levitation of a magnet above an already superconducting material does not demonstrate the Meissner effect, while an initially stationary magnet later being repelled by a superconductor as it is cooled through its critical temperature does.

As to actually explaining why the flux is excluded, that is beyond the HSC syllabus. You just need to know what the Meissner effect is

[Dragomistress]

Hi,
Why is the answer D?

[jamonwindeyer]

Hi,
Why is the answer D?

Hey! Because for a line to be in gradient-form, it needs to be in the form \(y=mx\), not \(y=m\sqrt{x}\). So squaring both sides:



In this form, \(y=T^2\), and \(x=l\), which leaves the gradient as the expression in D

Note: This is a poorly worded question, so I don't blame you for getting a bit confused

[parallaxd]

For the striation patterns part of ideas to implementation syllabus I'm confused as to whether high pressure means a low amount of gas particles or a high amount???

[clovvy]

For the striation patterns part of ideas to implementation syllabus I'm confused as to whether high pressure means a low amount of gas particles or a high amount???

High pressure should definitely mean high amount of gas particles...  Because within a closed system,  the more gas particles present, the pressure will build up for sure...  And generally this is always the case..

Unless my understanding is wrong, then please correct me before I lose marks in HSC...

[jamonwindeyer]

For the striation patterns part of ideas to implementation syllabus I'm confused as to whether high pressure means a low amount of gas particles or a high amount???

High pressure should definitely mean high amount of had particles...  Because within a close system,  the more gas particles present, the pressure will build up for sure...  And generally this is always the case..

Unless my understanding is wrong, then please correct me before I lose marks in HSC...

You're all good clovvy, high pressure means more particles (meaning more collisions with electrons)

[Mate2425]

Hi guys having real trouble trying to understand the graphs for motors and generators in HSC multiple choice questions e.g HSC 2014 https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-physics.pdf     
is there some sort of standard process or helpful trick i can employ to ensure i am on the right track or know that i have the right sort of answers for these sort of questions?
Thank you!!

[clovvy]

Hi guys having real trouble trying to understand the graphs for motors and generators in HSC multiple choice questions e.g HSC 2014 https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-physics.pdf     
is there some sort of standard process or helpful trick i can employ to ensure i am on the right track or know that i have the right sort of answers for these sort of questions?
Thank you!!

I don't see a graph question as MC but I do see graphs provided for answer, are you refering to Q12 or Q14?
 

[Mate2425]

I don't see a graph question as MC but I do see graphs provided for answer, are you refering to Q14?

Yes thank you that's the sort of questions i am struggling with!

[clovvy]

Yes thank you that's the sort of questions i am struggling with!

I'll try to explain both as best as I can with my limited knowledge anyway..
So for Q12, the force acting on the wire PQ is given by F=BILsinθ (θ is the angle of the wire in relation to the field),  and so in both cases the angle is 90°. As the coil rotates through 360° the current supplied is constant, the length PQ does not change,  and the magnetic field is constant in the case of the parallel field,  and may be considered uniform in the case of the radial field.  So the answer should be C..
From the diagram provided in Q14, when the switch is open the resistance in the outside circuit is infinite, so the potential difference generated produces 0 current (V=IR,  when R is infinity).  However, when the switch is closed a cycle of AC potential difference causes a cycle of current to flow,  hence the answer is C..

This is as best as I can explain it, if anyone sees any incorrect/incomplete info or want to add something to my explanation, that would be appreaciated...

[jamonwindeyer]

Yes thank you that's the sort of questions i am struggling with!

So for Q12, the force acting on the wire PQ is given by F=BILsinθ (θ is the angle of the wire in relation to the field),  and so in both cases the angle is 90°. As the coil rotates through 360° the current supplied is constant, the length PQ does not change,  and the magnetic field is constant in the case of the parallel field,  and may be considered uniform in the case of the radial field.  So the answer should be C..

Love your answer for Q14, but the one for Q12 is just a tiny bit off - Let me help

For Q12, we remember that \(\tau=nBIA\cos{\theta}\), the torque is dependent on the angle of the coil with the field. What the radial field does is eliminates this angle - As you hint at, it is considered to always be \(\theta=90^\circ\). So, the magnitude of the torque for the radial motor is constant. For the parallel field motor, it varies sinusoidally with the \(\cos{\theta}\) term. In both, we need the torque to swap magnitude every half turn.

The graph that matches is actually B - The radial field has constant magnitude of torque (just swapping direction), and the parallel field has sinusoidal variation as required

As for your question on how to tackle graphs, Mate, unfortunately there aren't any shortcuts. You just have to know the content. Use process of elimination to knock out obviously incorrect answers to give yourself the best chances of a correct answer. But the fact you are struggling with these is normal - They are tough questions

[Bruh01]

Hi, could someone explain the effect of the magnetic field on the proton (as in whether it goes clockwise or anti) and how the resultant vector points to the perpendicular direction. Thanks

[clovvy]

Love your answer for Q14, but the one for Q12 is just a tiny bit off - Let me help

For Q12, we remember that \(\tau=nBIA\cos{\theta}\), the torque is dependent on the angle of the coil with the field. What the radial field does is eliminates this angle - As you hint at, it is considered to always be \(\theta=90^\circ\). So, the magnitude of the torque for the radial motor is constant. For the parallel field motor, it varies sinusoidally with the \(\cos{\theta}\) term. In both, we need the torque to swap magnitude every half turn.

The graph that matches is actually B - The radial field has constant magnitude of torque (just swapping direction), and the parallel field has sinusoidal variation as required

As for your question on how to tackle graphs, Mate, unfortunately there aren't any shortcuts. You just have to know the content. Use process of elimination to knock out obviously incorrect answers to give yourself the best chances of a correct answer. But the fact you are struggling with these is normal - They are tough questions

Oddly enough BOSTES answers confirms it to be C...  Unless it is one of those years where they actually screw up the answer..  After I look at the graphs more closely I am sort of leaning towards your answer but idk why BOSTES pick C as the correct answer

[jamonwindeyer]

Oddly enough BOSTES answers confirms it to be C...  Unless it is one of those years where they actually screw up the answer..  After I look at the graphs more closely I am sort of leaning towards your answer but idk why BOSTES pick C as the correct answer

Ahh no you were right! I did TORQUE, the question wanted FORCE, I didn’t read the question properly sorry!!