Lenz's Law: Direction of Flux & Current

[defetey]

Homework Statement

I don't have the question with me, it was on a test I did. It was something like this though:

A coil is placed vertically on a table. The magnetic field there is also vertical. If the magnetic field is suddenly switched off, in which direction would the current in the solenoid move?

A) Clockwise
B) Counter-clockwise
C) There would be no current
-and I forgot the rest of the choices.

The Attempt at a Solution

Well it would look something like this:

http://img43.imageshack.us/img43/2058/emfb.jpg

So, current is produced when there is a change in the magnetic field, so there is obviously would be current if the field is turned off. And that current's field would try to counter the flux.

But some things aren't clear to me;

1) Since the field is even on all points of the solenoid, which side of the solenoid (inner or outer) would be the one that creates the "counter-"magnetic field against the flux? Would the field be countered from within or from the outside?

2) Which direction would the flux be (that creates the current) if the "field is suddenly switched off"?

 

[stanli121]

Your current would be counterclockwise. Lenz's Law states that this coil will attempt to fight the change in flux that is brought about when the B field is switched off. Since that original B field was pointed up, the current must be CCW to produce a B field in that same direction in an attempt "to fight" the change in flux from some flux to zero.

 

[kerimek]

Lenz's Law states that the direction of an induced current is always such that it opposes the change that caused it. In this case, the change that caused the current is the sudden switching off of the magnetic field. Therefore, the current in the solenoid would move in a direction that creates a magnetic field that opposes the original magnetic field.

To answer your questions:

1) The direction of the induced current would depend on the direction of the original magnetic field. If the original field was directed downwards, then the induced current would flow clockwise, creating a magnetic field that points upwards to oppose the original field. If the original field was directed upwards, then the induced current would flow counterclockwise, creating a magnetic field that points downwards to oppose the original field.

2) The direction of the induced current would also depend on the rate at which the magnetic field is switched off. If the magnetic field is switched off slowly, the induced current would be weaker and the flux would be in the same direction as the original field. If the magnetic field is switched off quickly, the induced current would be stronger and the flux would be in the opposite direction to the original field.