MC question about magnetism

  • #1
This question appeared in the physics exam yesterday and I'm not sure what is the correct answer.

The figure shows a light and flexible conducting loop C freely hung on a smooth horizontal rail. A bar magnet PQ approaches the loop from the right. Which of the following descriptions about this process is correct ?
A. if P is an N-pole, the loop will be repelled ot the left and its area will increase slightly.
B. If P is an N-ple, the loop will be attracted to the right and its area will decrease slightly.
C. No matter whether P is an N-pole or a S-pole, the loop will be repelled to the left and its are will decrease slightly.
D. No matter whether P is an N-pole or a S-pole, the loop will be repelled to the left and its area will increase slightly.

I think the correct answer is either C or D but I don't know why the change in area of the loop is involved in this process.

Edit:typo
 

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Answers and Replies

  • #2
Doc Al
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Originally posted by KLscilevothma
I think the correct answer is either C or D but I don't know why the change in area of the loop is involved in this process.
Think of two current-carrying wires side by side. They exert forces on each other, depending on the orientation of the currents. Now consider that loop with its induced current. Segments of the loop on opposite sides carry current in opposite directions, so they repel each other. So the flexible loop expands.
 
  • #3
Um.. but what I can imagine is volume expansion of the loop but the area doesn't change, just like the case when 2 current carring wires are put side by side, they repel each other if the currect direction is opposite. Am I missing something here ?
 
  • #4
Doc Al
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Originally posted by KLscilevothma
Um.. but what I can imagine is volume expansion of the loop but the area doesn't change, just like the case when 2 current carring wires are put side by side, they repel each other if the currect direction is opposite. Am I missing something here ?
I don't know what you mean by "volume" expansion of the loop. If two current-carrying wires repel, they will spread apart if free to do so. For the loop, opposite sides repel. Since the loop is flexible, the sides will stretch open, increasing the loop area.
 

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