# Static magnetic field , rotating magnet

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.

## Answers and Replies

Simon Bridge
Science Advisor
Homework Helper
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.
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.

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

What you are talking about is well know as the Faraday paradox.
http://en.wikipedia.org/wiki/Faraday_paradox

Classically, rotating a cylindrical magnet on it's axis does not change the magnetic field - it is as if the magnetic flux creates it's own preferred reference frame. That would be all she wrote except that turning both magnet and disk together, same direction, same speed, still generates an EMF.
Also see:
https://www.physicsforums.com/showthread.php?t=691967#post4394872

Basically it means that you cannot always get away with a simplistic understanding of how fields behave.