# Looking for opinions on poorly marked exam question.

1. Mar 24, 2015

### uart

Please note that I am not asking how to do this question. It is not homework. :)

This is part of an externally set exam that some of my students sat, and with which I am disappointed with the "officially" correct solution.

To me the solution is reasonably straight forward, however many people seem to disagree with me, in particular those who wrote the "official" answers by which this was marked.

I won't give it away, but the "officially" correct answer (which as you may have guessed, I believe is wrong) seems to display a poor understanding of Lenz's law.

Can you spot the correct answer and also the other one, which (if given by a student) you might well understand how they made that mistake, but would not expect anyone who really understands this stuff to make.

What do you think is the correct answer?

2. Mar 24, 2015

### ZapperZ

Staff Emeritus
I vote (B).

Zz.

3. Mar 24, 2015

### uart

Hi Zapper. It is my interpretation of the question that it is not asking for the direction of magnetic field due solely to the induced current. This is essentially the nub of my disagreement, I believe we should be looking at the net magnetic field.

Would that make a difference to your vote?

4. Mar 24, 2015

### ZapperZ

Staff Emeritus
I know what you are trying to get at, that the coil is trying to maintain the "status quo". However, based on the option given (zero field is not available), that's the best answer out of all 4.

Also note that it depends on how fast the magnet is moving. If the magnet is moving very slow, the induced current will be very small, and the magnetic field in the coil will be predominantly from the magnet. If the magnet is moving very fast, the induced current will be large, and this might result (depending on where the magnet is) zero net field in the coil. So even with your scenario, the answer will be uncertain.

I certainly do not expect the students at that level to be as sophisticated to consider such a thing, especially given the answer options.

Zz.

5. Mar 24, 2015

### uart

Now with that statement I do agree. :)

Think a bit more about the speed of the magnet being a deciding factor however. Remember that no matter how fast the magnet moves it can never cause an induced current so large at to actually reverse the direction of flux that induced it in the first place. In part, inductance is responsible for limiting the current despite an increasingly fast motion of the magnet.

Last edited: Mar 24, 2015
6. Mar 24, 2015

### ZapperZ

Staff Emeritus
Certainly if the question had asked for the direction of the induced magnetic field, we won't be having this conversation. And I think that is what the question was asking for, assuming that that is the correct answer.

I definitely agree with you that the question could be better constructed. I'm guessing that when you brought this up, it fell on deaf ears?

Zz.

7. Mar 24, 2015

### uart

Yep. :)

8. Mar 24, 2015

### ZapperZ

Staff Emeritus
This is a common lab exercise. If you have a Hall probe, you could simply demonstrate this by repeating the experiment. Stick the Hall probe in the tube, and do the measurement. Record a video of it, put it on YouTube, and voila! You've made your point that they can't ignore!

:)

Zz.

9. Mar 24, 2015

### uart

I understand the desire to take it as only the field due to the current, but where the question asks "and the direction of the magnetic field inside" I really can't see any other interpretation to meaning the net field.

To be honest, if we were to look at various analogous questions in other fields there wouldn't be even the slightest dispute.

Example 1. (Mechanics)

With my left hand I push my pen towards the right so that it slides across the desk.

1. What is the direction of the frictional force applied to the pen by its contact with the desk? (Ans. Towards the left.)
2. In what direction did the pen slide. (Ans. Surely not towards the left!)

Can you see what I mean, to say it slide toward the left (due to frictional forces) would really be quiet an analogous error. To say this would be to assume the the left directed frictional force is stronger than the original force.

Example 2. (Chemistry)

H2, N2, and NH3(g) are in chemical equilibrium in a constant volume reaction vessel at a given temperature. Call this equilibrium point A.

An additional two moles of H2(g) are injected and the system allowed to re-establish equilibrium, again at constant temperature and pressure. Call this equilibrium point B.

1. After the additional H2 is added, in which direction does the reaction proceed to establish the new equilibrium. (Ans. Consumption of H2 and production of NH3)
2. Is there more H2 or less H2 present at equilibrium point B compared with equilibrium point A. (Ans. Surely not less).

Again, to answer the above as "less H2" would be making the false assumption that the change in equilibrium due to Le Chatelier's principle could outweigh the original change in concentration that caused it. It can't.

Last edited: Mar 24, 2015