Lenz's law - magnetic fields and Currents

kidi3

Hey Pf..

I am trying to understand Lenz's law, but somehow it doesn't make sense.
In my book there is some checkpoints execise to test wheather you've understood what you read about, one those checkpoints looks like this.

http://snag.gy/NCNLh.jpg


I do understand why the current in situation a is the highest since the magnetic field is increasing will it never become zero, and in situation C will the current decrease until become constant (B=0)=> current =0.
But why should the current in situation B be as high as in situation A.. ?

Doc Al

Take note of the direction of the magnetic field in each half of the loop.

kidi3

It's both in and out.. but wouldn't the dec. part go to 0, and lead the B to become ½A..
I can see B would be as high as A in a short moment,should it not be ½A thereby lead B<A

Doc Al

No.

What matters is the change in the flux through each half of the loop.

In the top half (of B) which way does the field point? Since it's increasing, which way is the change in flux pointing?

Same question for the bottom half. Which way does the field point? Since it's decreasing, which way is the change in flux pointing?

kidi3

Ah.. they all point in the same direction, and since change is equal to the one in situation A, they must be equal in magnitude..

Doc Al

Exactly. The change in flux through each half of B points in the same direction, just like it does in A.