Magnetism and Conducting Loops: Which Description is Correct?

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Discussion Overview

The discussion revolves around the behavior of a conducting loop in the presence of a bar magnet, specifically addressing the effects of the magnet's poles on the loop's motion and area. Participants explore the implications of induced currents and forces within the loop as the magnet approaches, considering various descriptions of the scenario presented in a physics exam question.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant suggests that the correct answer to the exam question is either C or D, expressing confusion about the involvement of the loop's area change.
  • Another participant explains that the induced current in the loop causes segments on opposite sides to repel each other, leading to an expansion of the loop.
  • A different participant questions the notion of area change, proposing that only volume expansion occurs, similar to the behavior of two parallel current-carrying wires repelling each other.
  • The initial participant responds by clarifying that if the loop is flexible, the repulsion between opposite sides would indeed cause the loop to stretch, thereby increasing its area.

Areas of Agreement / Disagreement

Participants express differing views on whether the area of the loop changes and the nature of the expansion (area vs. volume). There is no consensus on the correct answer to the exam question or the mechanics involved.

Contextual Notes

Participants discuss the effects of induced currents and the forces acting on the loop, but the discussion does not resolve the underlying assumptions about the nature of the loop's expansion or the specific effects of the magnet's poles.

KLscilevothma
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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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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.
 
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 ?
 
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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