Magnetic Flux & Current in a Dropped Magnet & Wire Loop

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Homework Help Overview

The problem involves a bar magnet being dropped above a wire loop and examines the induced current in the loop as the magnet falls. The discussion centers around the behavior of magnetic flux and its relationship to induced current, referencing Lenz's law and Faraday's law.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the direction of the induced current as the magnet approaches and passes through the loop, questioning the relationship between magnetic flux and current direction. There is an attempt to visualize the situation and clarify misconceptions regarding the nature of induced emf.

Discussion Status

The discussion is ongoing, with participants examining the changes in magnetic flux as the magnet moves relative to the loop. Some guidance has been offered regarding the nature of induced emf and its dependence on the rate of change of magnetic flux.

Contextual Notes

Participants are working under the constraints of a homework assignment, which may limit the information available for discussion. There is an emphasis on understanding the underlying principles rather than arriving at a definitive solution.

Feodalherren
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Homework Statement


A bar magnet is held above the center of a wire loop lying in the horizontal plane, as shown in the figure below. The south end of the magnet is toward the loop. After the magnet is dropped, what is true of the current in the resistor as viewed from above? (Select all that apply.)
20-mc-figure-11.gif

It is counterclockwise as the magnet falls toward the loop.
It is clockwise as the magnet falls toward the loop.
It is always clockwise.
It is clockwise after the magnet has moved through the loop and moves away from it.
It is first counterclockwise as the magnet approaches the loop and then clockwise after it has passed through the loop.

Homework Equations


Lenz's law and Faraday's Law

The Attempt at a Solution


This is my conceptual picture of what's going on.

Untitled.png

As the magnet is falling in the field lines are going N to S as always. So Just at the moment when the bottom of the South pole of the magnets hits the area inside the circle the field lines are coming toward us (as viewed from the top). As the magnet falls through and the N side is facing us, the field lines are again, coming toward us.
In both cases the current will be induced in such a way that the magnetic flux through the surface is constant. So it seems to me that it should flow clockwise in both cases. This is, however, incorrect. Where am I going wrong?
 
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The induced emf is proportional to the change in (i.e., time derivative of) the magnetic flux, not to the magnetic flux itself.
 
Yes, that is what my pictures are trying to illustrate.
 
So how is the flux changing when the magnet falls into the loop? How is it changing when it falls out of the loop?
 

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