This problem comes from Barron's SAT Subject Test Physics book.

A loop of wire and a bar magnet are moving relative to one another. Which motion in the diagrams shown below will NOT induce a current in the loop?

The first four choices are: enter image description here They all experience change in the magnetic field due to the movement of the magnet or the loop. Choice D also has a change in area because the loop is spinning (dashed arrow). Therefore, by process of elimination, Choice E seems to be the answer (and it is, according to the answer key).

I assumed that the dashed arrow here denoted the movement of the loop, since the dashed arrow in choice D didn't seem to represent anything else.

However, there is something I don't quite understand:

There is a change in the magnetic field passing through the loop since the magnet is moving closer. And the area of the loop doesn't seem to change. With these, I decided that there is a change in flux, inducing current in the loop. Yet this was wrong. The answer key just says:

Only choice E demonstrates no change in the amount of magnetic field passing through the loop.

How does the movement of the loop denoted by the dashed arrow affect the flux? Is this something that can "cancel out" with the movement of the magnet so that there is no induced current? Thank you so much in advance.


closed as off-topic by Danu, user36790, ACuriousMind, Gert, mpv Jan 2 '16 at 17:22

This question appears to be off-topic. The users who voted to close gave this specific reason:

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  • $\begingroup$ Please write out where the problem comes from and the description of the diagram. $\endgroup$ – Wang Yun Jan 2 '16 at 12:19
  • $\begingroup$ And what does that mean the dash line arrow ? $\endgroup$ – Wang Yun Jan 2 '16 at 12:21
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    $\begingroup$ It seems to me that this is the type of question we do want, no? $\endgroup$ – Kyle Kanos Jan 2 '16 at 21:24
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    $\begingroup$ @user36790: I voted to reopen because I think it's a good question. It asks about a specific concept (why the moving magnetic field doesn't induce a current in the situation) and shows some rationalization about their thoughts on why they think it should. Seems to me this is okay by the homework policy. $\endgroup$ – Kyle Kanos Jan 4 '16 at 11:02
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    $\begingroup$ Clearly this question in the Barron book has a typo. It is a legit question and it is bad that it was closed. The magnet is shown as moving, so there is change of flux through the loop. There is induced current in the case E too. The magnet should have been drawn without the arrow. $\endgroup$ – 0kcats Oct 29 '16 at 21:53