# Motional Induction?

1. Mar 29, 2012

### MS La Moreaux

If a permanent magnet is moved in a direction at right angles to the magnet field lines in the magnet, is an emf produced across the magnet?

Mike

2. Mar 29, 2012

### Hassan2

If you mean it is moving in an external field, yes it is induced, just like in a piece of Iron. But if you mean there is no field except for the magnets own field, then in classical electromagnetic no emf is induced because the relative motion of the magnet and its velocity is zero. I don't know what happens whenthe velocity approaches the speed of light.

3. Mar 30, 2012

### MS La Moreaux

Hassan2,

It seems to be the case that a magnet's field is not attached to the magnet. If an electrically conductive disk magnet (with the faces the poles) rotates like a wheel, an emf is developed from the center to the rim. This is a type of homopolar generator known as a DePalma N Machine. My question relates to the linear version of it.

Mike

4. Mar 30, 2012

### Hassan2

If you think the problem is interesting, can you please make a simple drawing of it?

5. Mar 30, 2012

### chrisbaird

Rotational motion and translation motion are different things. If you translate an object (move it in a straight line at a constant speed) then the object it still in an inertial frame and therefore acts (to itself) as if it were at rest. A translating object does not create an EMF in its frame of reference because it is not moving in its frame a reference. A rotating frame, on the other hand, is not inertial and so you cannot put yourself in a rotating object's rest frame and apply the standard equations. Homopolar generators work because the motion is rotational.

6. Mar 30, 2012

### MS La Moreaux

Hassan2,

Sorry, but I do not know how to add a drawing.

7. Mar 30, 2012

### MS La Moreaux

crisbaird,

I am having trouble understanding this. If the N machine is operated as a motor, it will start from a stationary position. Therefore, at the instant of starting, there is no rotary motion and everything is in the same frame of reference.

8. Mar 30, 2012

### chrisbaird

And that is why at the instant of starting, there is no EMF

9. Mar 30, 2012

### MS La Moreaux

Yes, but what causes the disk to rotate? It would seem that the conduction electrons in the disk have a force exerted on them by the magnetic field of the disk even though there is no rotation at the start. How does this differ from the linear version of the motor? Why would there not be movement of the magnet in the linear case?