# Axial rotation of a magnetic field detection

1. Feb 24, 2005

### steve stillman

Since the archive didnt have an answer or these keywords, is there any way to detect a magnet spinning on its axis so the magnetic field is rotating axially and lines of flux are rotating around but nowhere in space is the flux level changing? Ive heard rummors over the years. This is not the Homopolor motor which is quite different. Seems like the lines of flux cut a nearby coil but since the level dosnt change there is no induced current. How about the inducing of spin of another nearby magnet due to meshing lines?

A related question is to substitute an electric coil and spin it. Then the magnetic lines do not rotate as the coil rotates and there is the same question of detection but less hope.

Thanks, and Ive learned alot from this site and it runs fast.

Steve Stillman,

2. Feb 25, 2005

### pervect

Staff Emeritus
Here's one way of looking at it:

A moving magnetic field generates an electric field. At the center of the axis, the magnetic field is not moving, but at any point that's not on the center, you'll have a tangential velocity of w*r, and this will generate an electric field in the usual manner.

The secret to detecting this electric field is to not use a coil of constant area. If you take a coil of a fixed area and move it, flux leaves the loop just as fast as it enters, and there is no induced voltage. Another way of looking at it is that the leading part of the loop generates a voltage, but the trailing part of the loop generates an equal and opposite voltage, so there is no net voltage across the loop.

But if you form a circuit loop consisting of one moving wire, and one stationary wire, and whatever wires you need to complete the circuit, you will detect an electric field. The area of the loop will not be constant, and you'll have a net flux through the loop.

If you draw this situation out in detail (one moving wire, one stationary wire), you will have a homopolar generator. Usually one or both of the wires is replaced with a disk.

3. Feb 25, 2005

### steve stillman

Clarification of my question

I should clarify my question. I can detect a magnet presence with a wire cutting the lines, yes, but can I detect a spinning magnet that spins with the axis if its flux the same as the spin axis? And then, is a spinning electrically generated magnetic field any different.

Steve Stillman

4. Feb 26, 2005

### pervect

Staff Emeritus
I'm not sure I understand the question. On the axis of rotation, the velocity of the magnet is zero, and it doesn't matter whether or not the magnet is spinning. As you move some distance 'r' away from the axis of rotation, the magnet (and the magnetic field lines) have some velocity 'v'. These moving magnetic fields create electric fields that are at right angles to both the velocity, and the magnetic field. This means that the electric field points in the direction of increasing 'r', and that the electric field at r=0 is zero, it increases linearly with 'r'.

To generate a net voltage in a loop of wire from the moving magnetic field, you still need to have one wire moving and one wire stationary, otherwise there is no next flux, just as in the case where the wires were moving. One wire must always move, one wire must always be stationary, no matter which viewpoint you look at the system from.

It may be simpler to think of measuring the electric field directly, by measuring the acceleration of a free standing charge. In this case, (as I said before) you will find that the electric field is zero directly on the axis of rotation, and it points radially, increasing as you increase 'r'.

5. Mar 3, 2005

### steve stillman

The question Im asking is not the obvious question. Yes I know that time variant magnetic flux induces a current in a loop of wire or a moving wire cutting lines generates a current......My question, again, is "can I detect a spinning magnet that spins on its magnetic axis"? There were rumors that it was possible talking with my professors way back in college. Ive never seen any discussion of it in any physics book. Some Ive talked too think a magnet drags its lines around with the matter, but an electrically generated magnetic field is unable to do so....but how to detect the flux that moves or dosnt move?
Thanks.
Steve Stillman

6. Jun 1, 2005

### skeptic

People have been arguing, on-and-off, about this for years. Many electrical engineers insist that the magnetic field rotates with the magnet. Physicists take the view that, since the magnetic field lines are merely a mathematical artifice, it is meaningless to talk about their rotation.

7. Jun 1, 2005

### pmb_phy

Sure. I did a derivation right here
http://www.geocities.com/physics_world/em/rotating_magnet.htm

One must use relativity to get the correct answer. This is textbook stuff. Not something that I see debated in the physics literature. But I had to see how things were working, hence the diagrams.

Pete