HSC Physics Question Thread

[jamonwindeyer]

Yo Swagdaktal you might find this interesting. Assume the opposite of Lenz's law holds, then imagine what would happen if there was a flux change - you would get infinite current ~ infinite energy.
So clearly, you have proved that the opposite of Lenz's law is impossible. Hence Lenz's law must hold.

This is actually the best way of explaining Lenz's Law!! The formal definition for you Swagdaktal would be something like:

Lenz's Law: The direction of an induced current is always such that it opposes whatever change created it.

[jakesilove]

This is actually the best way of explaining Lenz's Law!! The formal definition for you Swagdaktal would be something like:

Lenz's Law: The direction of an induced current is always such that it opposes whatever change created it.

That's exactly how I always recommend describing and, in fact, 'proving', Lenz's law. The easiest analogy is looking at the Magnet falling down the copper tube. The induced current will obvious cause the magnet to experience a force; but will it be up or down? If it is down, then the increased speed will caused increased Eddy currents, which will cause increased speed, which will cause increased eddy currents, and so on and so on until it breaks the speed of light. Obviously, that's not OK (Physicists aren't sure of much, but breaking the universal speed limit? That's a not go). So, the current must OPPOSE the change created! It's a nice, logical argument, and the HSC markers love it if you ever need to explain back emf and lenz's law.

Jake

[Swagadaktal]

Yo Swagdaktal you might find this interesting. Assume the opposite of Lenz's law holds, then imagine what would happen if there was a flux change - you would get infinite current ~ infinite energy.
So clearly, you have proved that the opposite of Lenz's law is impossible. Hence Lenz's law must hold.

OOh yeah - that makes sense.

That's exactly how I always recommend describing and, in fact, 'proving', Lenz's law. The easiest analogy is looking at the Magnet falling down the copper tube. The induced current will obvious cause the magnet to experience a force; but will it be up or down? If it is down, then the increased speed will caused increased Eddy currents, which will cause increased speed, which will cause increased eddy currents, and so on and so on until it breaks the speed of light. Obviously, that's not OK (Physicists aren't sure of much, but breaking the universal speed limit? That's a not go). So, the current must OPPOSE the change created! It's a nice, logical argument, and the HSC markers love it if you ever need to explain back emf and lenz's law.

Jake

We actually did this in class.

Thanks guys

Just to clarify, if in that question there was a north pole facing the loop, then the magnetic field will be going left to right and therefore the field in the loop is going from right to left? And the current would go anticlockwise?

And if it were a north pole facing thing, and it was going away from the loop would the field of the loop be going from left to right (in the same direction of the magnet)?
Or would it still oppose the magnet but just have a weaker magnetic field?

And is my wording correct here?

Thanks heaps guys

[Ahsun]

In one of the first attempts made by JJ Thomson to measure the q/m for cathode ray particles, he accelerated them through a voltage V, using this to calculate the kinetic energy, and hence the speed v, of the particles. He then allowed them to be bent into a circular path of radius r by a uniform magnetic field, B, at 90O.
Show Mathematically that he would have been able to calculate this q/m ratio from the formula: q^m = 2V^B2r2

[jamonwindeyer]

OOh yeah - that makes sense. We actually did this in class.
Thanks guys
Just to clarify, if in that question there was a north pole facing the loop, then the magnetic field will be going left to right and therefore the field in the loop is going from right to left? And the current would go anticlockwise?

Do you mean the question with the bar magnet and the loop?  If so, then yes. The magnetic field of the bar magnet (near the North pole) is going from left to right, and so the induced field must be from right to left. I think "induced by the loop" works a bit better language wise than "in the loop" (though I think it's still okay). And yes, the current would then be anti-clockwise! 

And if it were a north pole facing thing, and it was going away from the loop would the field of the loop be going from left to right (in the same direction of the magnet)?
Or would it still oppose the magnet but just have a weaker magnetic field?
And is my wording correct here?
Thanks heaps guys

Your first analysis is correct. The magnet is moving away, meaning the induced field will act in the same direction as the magnet's magnetic field to try and attract it back to the loop.

Note that if the induced field opposed the magnet, then the magnet would be pushed away faster, thus inducing a stronger field, then the magnet would be pushed away faster, thus inducing a stronger field... You get the idea. Lenz's Law broken! 

I hope that helps! 

[Swagadaktal]

Do you mean the question with the bar magnet and the loop?  If so, then yes. The magnetic field of the bar magnet (near the North pole) is going from left to right, and so the induced field must be from right to left. I think "induced by the loop" works a bit better language wise than "in the loop" (though I think it's still okay). And yes, the current would then be anti-clockwise! 

Your first analysis is correct. The magnet is moving away, meaning the induced field will act in the same direction as the magnet's magnetic field to try and attract it back to the loop.

Note that if the induced field opposed the magnet, then the magnet would be pushed away faster, thus inducing a stronger field, then the magnet would be pushed away faster, thus inducing a stronger field... You get the idea. Lenz's Law broken! 

I hope that helps! 

Yes this does help

Thanks a lot everyone <3

[jamonwindeyer]

In one of the first attempts made by JJ Thomson to measure the q/m for cathode ray particles, he accelerated them through a voltage V, using this to calculate the kinetic energy, and hence the speed v, of the particles. He then allowed them to be bent into a circular path of radius r by a uniform magnetic field, B, at 90O.
Show Mathematically that he would have been able to calculate this q/m ratio from the formula: q^m = 2V^B2r2

Maaaan, these are brutal. Not sure about this one! Let me have a go.

Well the definition of a volt is that it is the potential difference required to give 1 coulomb of charge 1J of kinetic energy. Therefore, V volts will give V joules to 1 coulomb of charge. How many electrons are in 1 coulomb of charge? Well we obtain that by dividing by the charge per electron, so: 1/q. That means, putting that all together, that the kinetic energy per electron is K=Vq:



Doing some algebra:



Okay, so we have a formula for velocity. Now in the second part, the magnetic field provides a centripetal force. Identically to the analysis of the final version of this experiment, we can now equate centripetal and magnetic force (note that I will ignore the sine term in the formula for magnetic force, since the angle is 90 degrees, and sin90 = 1):



Equate the expression from earlier:




Whew, got it!! Aha I hope that makes sense, the top bit might be a little confusing, let me know if that needs any clarification! 

[Ahsun]

Maaaan, these are brutal. Not sure about this one! Let me have a go.

Well the definition of a volt is that it is the potential difference required to give 1 coulomb of charge 1J of kinetic energy. Therefore, V volts will give V joules to 1 coulomb of charge. How many electrons are in 1 coulomb of charge? Well we obtain that by dividing by the charge per electron, so: 1/q. That means, putting that all together, that the kinetic energy per electron is K=Vq:



Doing some algebra:



Okay, so we have a formula for velocity. Now in the second part, the magnetic field provides a centripetal force. Identically to the analysis of the final version of this experiment, we can now equate centripetal and magnetic force (note that I will ignore the sine term in the formula for magnetic force, since the angle is 90 degrees, and sin90 = 1):



Equate the expression from earlier:




Whew, got it!! Aha I hope that makes sense, the top bit might be a little confusing, let me know if that needs any clarification! 

Thanks so much made it so easy to understand thanks so much

[RuiAce]

A note on this

Yo Swagdaktal you might find this interesting. Assume the opposite of Lenz's law holds, then imagine what would happen if there was a flux change - you would get infinite current ~ infinite energy.
So clearly, you have proved that the opposite of Lenz's law is impossible. Hence Lenz's law must hold.

This is actually important. It is a part of the syllabus. An explanation needs to be known as to how Lenz's law reflects the conservation of energy.

[RuiAce]

Thanks so much made it so easy to understand thanks so much

A note on this as well

Any band 6 student will know how to derive

(Disclaimer: I have not been informed that this is a NECESSARY part of the syllabus though)

I think Physics in Focus did give a derivation of it. The short:

[jamonwindeyer]

A note on this

This is actually important. It is a part of the syllabus. An explanation needs to be known as to how Lenz's law reflects the conservation of energy.

Swag is from Victoria, he's just curious (but absolutely) 

A note on this as well
Any band 6 student will know how to derive

(Disclaimer: I have not been informed that this is a NECESSARY part of the syllabus though)
...

Love this. I was asked it as part of an in-school assessment, and I included it as part of a response in either my Trial or my HSC (I forget which off the top of my head). I teach it to everyone I tutor, and understanding it means you understand a solid chunk of the Ideas to Implementation topic, definitely worth knowing this proof properly 

[Ahsun]

What are some applications of some type 1 superconductors? I know which ones are type 1 superconductors but I cant find any applications for any specific one of the type 1 superconductors

[jamonwindeyer]

What are some applications of some type 1 superconductors? I know which ones are type 1 superconductors but I cant find any applications for any specific one of the type 1 superconductors

Hey Ahsun! Hmm, that's tricky, the difference between the two is really just their response to applied magnetic fields (and usually their critical temperatures too, I think). I know that Type 1 superconductors are usually pure metals, and to be honest, I think most applications of superconductors use Type 2 superconductors. This is purely because the alloys and ceramic oxides have higher critical temperatures and so are more practical 

So, in terms of specific applications of Type 1, I'm not sure! It would have to be something that needs a complete Meisner Effect, but I'm not sure about this one  anyone want to tag in here?

[RuiAce]

I thought it really doesn't matter about the type.

But then I did a Google search and this was the first link that popped up.
https://www.physicsforums.com/threads/uses-of-type-i-superconductors.237238/

[jamonwindeyer]

I thought it really doesn't matter about the type.

But then I did a Google search and this was the first link that popped up.
https://www.physicsforums.com/threads/uses-of-type-i-superconductors.237238/

I saw this as well, it looks like it's a tad beyond HSC scope, but for the original poster, it does (can't vouch for accuracy) say that Type 1's are used in SQUID Magnetometers