Moving Permanent Magnets Into and Out-of Current Loops

Principle:  currents are induced in loops according to Lenz's Law:  a changing magnetic flux in a loop of wire induces a current in the loop that tends to oppose the change that's occurring.

To apply Lenz's Law, you must do two things: 

  1. Determine the change in magnetic flux that is occurring,
  2. Determine the current flow that would oppose this change.

The animations below show a top view of a rectangular wire loop that contains a galvanometer. The galvanometer enables you to see the direction of current flow. Current flowing into the + terminal, i.e., counterclockwise, will deflect the meter to the right.   

A magnet is pushed into the coil in a time dependent manner:  during the time interval t=2 s to t=8 s the magnet is slowly pushed completely through  the loop.  Observe the direction of current flow to determine how the magnet moves through the coil.  The animation starts over every 10 seconds.   Note:  the magnet is not shown in the animations.

 In each case below, click the "play button" to begin the animation.  Observe how the current in the loop changes in time and answer the associated question.

 Case 1: Suppose the magnet is introduced into the loop from the bottom. (You can also think of  the magnet as being pushed out of the computer monitor toward the user). In this case, as the magnet moves through the loop, which pole of the magnet enters the loop first?
North end.
South end.
This cannot be determined from the information given.

Case 2: Same as Case 1 above (magnet moving out of the screen), but notice that the current direction has flipped. In this case, as the magnet moves through the loop, which pole of the magnet enters the loop first?
North end.
South end.
This cannot be determined from the information given.

Case 3: Suppose now that the magnet is introduced into the loop from the top. (You can also think of  the magnet as being pushed into the computer monitor away the user. In this case, as the magnet moves through the loop, which pole of the magnet enters the loop first?
North end.
South end.
This cannot be determined from the information given.

Case 4: Same as Case 3 (magnet moving into the screen), but notice that the current direction has flipped. In this case, as the magnet moves through the loop, which pole of the magnet enters the loop first?
North end.
South end.
This cannot be determined from the information given.


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