MC7: Controversial answer (A) (2 Viewers)

[uart]

I finally got a chance to look through the paper today. You know it wouldn't be a Physics HSC exam if there wasn't at least one controversial MC question.

I know that lots of people have posted their answers already and the consensus for MC7 is "B", but technically the correct answer is "A". Don't worry though, I'm sure that the examiners think "B" is correct and will mark it that way, but let me explain why it's really "A".

Lenz's law tells us the direction of the induced current, and the flux due to that induced current. And this pair is most certainly anticlockwise and up, as per answer "B". However Lenz's law also tell us that this induced flux will only partially oppose the externally applied flux (that is, the downward flux through the coil due to the approaching magnet south pole).

So the net flux in the coil, the sum of both the flux due to the externally applied field and the flux due to the induced current, is definitely downward.

Does anyone agree with me?

 

[dan964]

hang on I think I put B
but is the end of the solenoid closest to the magnet
if it the South Pole then B cannot be right. since magnetic field goes from north to South?

(your logic above to get A is flawed as it misunderstands the question)

 

[anomalousdecay]

uart pins are actually a thing.

Any way I don't really agree with you. Magnetic field lines never ever cross each other. Also, it asked for the field lines inside the coil not outside the coil so that cleared up that disambiguation.

And out of curiosity, which one did you put in your exam paper?

 

[uart]

Reread my post.There is absolutely no dispute about the direction of the INDUCED current, it's anticlockwise. I clearly stated that in the op.

The question also asks for the direction of the magnetic field, and this is where the dispute lies.

 

[uart]

Also, it asked for the field lines inside the coil not outside the coil so that cleared up that disambiguation.

Yes I was talking about the magnetic field lines INSIDE the coil, just as the question asked. The direction of the magnetic field inside the coil is downwards. So the correct answer is "A".

And out of curiosity, which one did you put in your exam paper?

I didn't sit the exam, I was just shown a copy of the paper today. TBH I would have been really torn between "A" and "B" if I did have to sit it though. I'd know the correct answer was "A", but I'd also know that the examiners would probably be wanting answer "B".

 

[anomalousdecay]

Reread my post.There is absolutely no dispute about the direction of the INDUCED current, it's anticlockwise. I clearly stated that in the op.

The question also asks for the direction of the magnetic field, and this is where the dispute lies.

The way I see it is that there is a little bit of ambiguity in it as it asks for the magnetic field inside the coil.

However, 99% of students would interpret it to mean the magnetic field due to the INDUCED current. It does not specifically say the magnetic field due to the INDUCED current and hence the confusion you are referring to.

What I can tell you though is that regardless, drawing up the magnetic field due to the magnet in an external manner, the magnetic field points upwards anyway. Its only the magnetic field direction INSIDE the magnet that points downwards.

So what you get actually is lines in the same direction as each other, unless you really wanted to refer to the field lines inside the magnet itself, which would present a different case altogether.

 

[anomalousdecay]

Yes I was talking about the magnetic field lines INSIDE the coil, just as the question asked. The direction of the magnetic field inside the coil is downwards. So the correct answer is "A".

What how did you arrive at that conclusion.

Can you actually draw what you mean? Your reasoning in your original post in this thread is not really a clear reason.

 

[QZP]

He is reasoning that the downward flux from the magnet is greater than the upward flux from the induced current. Hence the overall flux in the solenoid is down. I don't agree with it but whatever floats your boat

 

[anomalousdecay]

Pretty crap drawing but hopefully it makes sense.

Still as I said before, its only internally inside the magnet itself that we see it that way.

So really if you were looking at it that way, then you would have to look at what the question is asking for specifically. That's when it gets a bit dicey.

Hence the reason why I don't agree with it being downwards is that you also have the component outside the magnet itself in the direction upwards.

Its all relative remember. The reason why the coil is used here is that its easier to visualise flux threading through the cross sectional area of the coil.

 

[uart]

What how did you arrive at that conclusion.

Maybe it would help if I explained it in steps.

1. Assume that the magnet starts out far away from the coil, so that initially there is no field inside the coil and no current flowing.

2. The magnet is then moved toward the coil as shown in the question. The field lines from the magnet enter at the magnet's south pole, so the field lines (from the magnet) that enter the coil are directed downwards.

3. If the was no path for current to flow (eg the coil left open) then a voltage would be induced due to the changing magnetic flux, but no current would flow. The field inside the coil in this case would simply be the (downward) field that protrudes from the magnet's south pole.

4. In our case however there is a path for current to flow. So Lenz's law tells us that a current will flow, and in a direction so as to oppose the downward field through the coil as described in 3 above. This current however will only ever produce enough field to partially oppose the impressed (by the magnet) field. This is just how induced currents work.

Even in the extreme case of a superconductor with zero resistance it can only react with enough current to precisely cancel out the external field, never to reverse it! (This is in fact the mechanism by which a superconductor excludes magnetic fields in the Meissner effect and also in magnetic levitation.)

 

[anomalousdecay]

Maybe it would help if I explained it in steps.

1. Assume that the magnet starts out far away from the coil, so that initially there is no field inside the coil and no current flowing.

2. The magnet is moved toward the coil as show in the question. The field lines from the magnet enter at the magnet's south pole, so the field lines (from the magnet) that enter the coil are directed downwards.

3. If the was no path for current to flow (eg the coil left open) then a voltage would be induced due to the changing magnetic flux, but no current would flow. The field inside the coil in this case would simply be the (downward) field that protrudes from the magnet's south pole.

4. In our case however there is a path for current to flow. So Lenz's law tells us that a current will flow, and in a direction so as to oppose the downward field through the coil as described in 3 above. This current however will only ever produce enough field to partially oppose the impressed (by the magnet) field. This is just how induced currents work.

Even in the extreme case of a superconductor with zero resistance it can only react with enough current to precisely cancel out the external field, never to reverse it! (This is in fact the mechanism by which a superconductor excludes magnetic fields in the Meissner effect and also in magnetic levitation btw).

For your 2nd dot point, I get what you mean, however the question says that the magnet goes into the coil. This means the whole magnet is inside the coil. In this case, the flux threading due to the magnet itself is inconclusive as a whole as you have vector components going up down, left and right if you look carefully (similar to how its illustrated in my crappy diagram above).

I do completely agree with your 4th point. And also, superconductors attempt to avoid any magnetic flux threading through them as a whole. So yeah what you said there is true. Of course it can never reverse it, other wise you would be breaking the law of conservation of energy.

 

[dan964]

the magnetic field due to the magnet is up
the magnetic field due to the EMF in the coil is down to oppose the field that caused it.
so it should be down, which B isn't??

 

[dan964]

Reread my post.There is absolutely no dispute about the direction of the INDUCED current, it's anticlockwise. I clearly stated that in the op.

The question also asks for the direction of the magnetic field, and this is where the dispute lies.

if the direction of the induced current is anticlockwise, then the direction of the induced current is down not up.
use the right hand grip rule. fingers curl in the direction of conventional current (anticlockwise) and the thumb points to the north pole (rather than showing the direction). So the top is a North Pole, the bottom a South Pole, and this is consistent with Lenz's Law.

Magnetic field lines run from north to south.
If the question wants direction of the magnetic field due to the induced current in the solenoid, then it is A
If the question wants the direction of the magnetic field due to the magnet then it is B.
It could be the other way around. anyway like most people I put B.

 

[Fizzy_Cyst]

Whilst I agree with what you are saying, it perhaps is not CRYSTAL clear, by using this logic, you could also say that the induced flux will be greater than the flux from the magnet itself (if the magnet is moved REALLY fast), seeing as the velocity of the magnet relative to the coil is not defined and nor is the equation for lenz' law in our syllabus as such, it would seem very unlikely that they meant what you are trying to say.

I am more concerned about Q12, seeing as force is a vector, in a radial field, the direction of force on side PQ of the coil actually changes to ensure that torque is always at maximum, I think the question should say 'magnitude of force' not 'force' as it could be taken as the vertical component of the force, in which case the answer would be D

:\

 

[anomalousdecay]

Whilst I agree with what you are saying, it perhaps is not CRYSTAL clear, by using this logic, you could also say that the induced flux will be greater than the flux from the magnet itself (if the magnet is moved REALLY fast), seeing as the velocity of the magnet relative to the coil is not defined and nor is the equation for lenz' law in our syllabus as such, it would seem very unlikely that they meant what you are trying to say.

Tbh going off my diagram, there's vector components everywhere, and if you looked at it in a lot of complexity as we have done in this thread, then you can't actually know the answer without some quantitave results.

 

[uart]

For your 2nd dot point, I get what you mean, however the question says that the magnet goes into the coil. This means the whole magnet is inside the coil.

Yes the question says that the magnet is moving up into the coil. But this is just to make clear which way the magnet is moving, not an invitation to choose whichever position you want for the magnet to be in when you analyse the field. There is no justification for analysing this problem with the magnet in any position other than one shown in the figure (and of course in the direction of velocity shown in the figure).

Anyway you're opening up a huge tin of worms if you start trying to analyse this with the magnet fully inside the coil, as the direction of the induced voltage actually changes when the magnet passes through the centre of the coil.

 

[anomalousdecay]

Uart I get what you mean now. Its a pretty poorly worded question and I just took it for the conventional meaning that they wanted (HSC way).