Spinning conducting disk in magnetic field

[unscientific]

Homework Statement

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...

 

[mfb]

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

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

 

[unscientific]

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

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

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..

 

[unscientific]

i sorted it out, thanks!

 

[mfb]

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!

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