# Homework Help: Spinning conducting disk in magnetic field

1. Feb 17, 2013

### unscientific

1. The problem statement, all variables and given/known data
I'm supposed to find the time taken for the disk to slow down from ω0 to (1/2)ω0...Here's what I've done:

Since on each side of the disk there is current flowing into the centre, each side experiences a force F, so the net torque on the system = 2Fa. Then I can find the angular acceleration α from that...

2. Feb 17, 2013

### Staff: Mentor

Is there some electron source/drain connected to the disk?

$\alpha = \frac{d\omega}{dt}=-k\omega$
This gives an exponential decay of ω...

3. Feb 17, 2013

### unscientific

hmmm not that I know of, but when i apply the left-hand rule to moving electrons on both sides i end up with current moving towards the centre!

The exponential decay is sort of expected, since the force depends on speed of the disk. The faster the disc is spinning, the higher emf induced and higher current and higher force..

4. Feb 17, 2013

### unscientific

i sorted it out, thanks!

5. Feb 18, 2013

### Staff: Mentor

Sure, but without external connections, the center would aquire a negative charge, and the resulting field would cancel the magnetic force quickly.