# Static magnetic field , rotating magnet

1. Feb 15, 2014

### Mr.Bomzh

Hi, I would like to clear some doubts over the field of a rotating magnet. I understand that in a typical alternator or generator , brushed or brushless , the current in the wires is induced because the opposite poles of either a permanent or electromagnet pass by the coils and the magnetic force repeatedly changes direction and also strength which then results in a changing current or AC. Normally I read that it doesnt matter whether you rotate the coils and keep the magnet stationary or rotate the magnet and keep the coils stationary , a current is induced both ways. But in a homopolar generator current is induced only when spinning the conductor disc , if one keeps the disc static and rotates the permanent magnet no current is induced, so it leads me to conclude that the field of a permanent magnet is static whether the magnet is stationary or rotating doesnt make a difference is that correct? Also maybe this " non induction" is related to the way the machine is set up and the geometry of the parts , because we use permanent magnets in ordinary generators of lower output power and they work in terms of inducing current even though their field is stationary. I assume it is because of the layout of the device because in an oridnary generator you have coils that experience both poles of the magnet passing by one by one , while in a homopolar desing you have a disc which is next to a rotating magnet but since the field is stationary one can assume as if the magnet isn't rotating and just being next to the disc so no current induced , correct? Thank you.

2. Feb 21, 2014

### Simon Bridge

Usually a generator uses a moving coil rather than a moving magnet - in that case, it does not matter which pole the wires pass closest to nor do they have to alternate from one to the other. All that is needed is a changing magnetic field.

You can change the magnetic field by moving the wire or by moving the magnet or both.

Nope - if the permanent magnet changes position, the magnetic field in space also changes.