# Strange Problem

1. Dec 2, 2004

### ambuj123

Hello
Well i am having this strange problem in electromagnetic induction can someone help me here?

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2. Dec 2, 2004

### Andrew Mason

It is not entirely clear to me how the diagram relates to the description but this is what it looks like: There is a wheel of radius R in a horizontal plane free to rotate about the z axis, the z axis being in the direction of $\hat k$. The magnetic field is $\vec B = -B_0\hat k$ and extends over a 'doughnut' area from r to R. When the magnetic field collapses, Faraday's law states that there is an electro-motive force or voltage created in a closed loop which encloses the magnetic field. This emf causes the charges to move. Since the charges are 'pasted' to the rim of the wheel, the emf provides angular acceleration to the wheel.

To find the emf you would have to use Faraday's law:
$V\Delta t = -N\Delta BA$ where N = 1 (no. of windings) and A is the area of the magnetic field enclosed by the wheel rim. The $\Delta t$ term should fall out when you work out the angular impulse $T\Delta t$ in terms of $V\Delta t$.

AM