# Rotating magnet inside coil = flux change?

1. Jan 17, 2012

### lluke9

Apparently, that's how a lot of car speedometers work... but I don't understand how a rotating magnet inside a coil (or any conductive material, really) can induce a current in the coil.

I can understand how pushing a magnet in and out through a coil can induce current, because
Flux = Area * Magnetic field strength * sin θ, and the magnetic field acting upon the coil is becoming stronger.

But I don't see how spinning a magnet inside a coil is going to do anything, much less create eddy currents to oppose the spinning and somehow create a torque.

Could someone refer me to a video or an image of some kind? I've tried to figure this out but nothing really comes to mind.

2. Jan 17, 2012

### Staff: Mentor

Do it like this -- put half the coil on one side of the magnet and the other half on the other side. You can space the two half-coils arbitrarily close together, if you use a short magnet. As the magnet turns, its flux goes one way through the coil, and then the opposite way as it turns to 180 degrees... Make sense?