# Electromagnetic Induction

1. Feb 15, 2013

### Roodles01

EDIT! Hmmm, sorry.
What is the difference between the origin of the emf in a small current loop when;
a) a magnet is stationary & the coil moves,
b) the coil is stationary & the magnet moves

This was the original message, now realised to be incorrect.
A current is generated when a magnet & coil come close to one another, but is there any difference between;
a) when a magnet is stationary & the coil moves,
b) when the coil is stationary & the magnet moves.

Thank you.

Last edited: Feb 15, 2013
2. Feb 15, 2013

### Simon Bridge

Um... nothing.
I think the only way to tell the difference is if one or the other is accelerating.

3. Feb 15, 2013

### CompuChip

That's exactly why it's called EMF. In the old days, electricity and magnetism were seen as two completely different things. It is due to a bunch of 19th century physicists - notably Farady and Maxwell - that we found out that they are really the same thing.

As Simon remarks, the idea that the laws of physics should look the same to observers at constant speed relative to each other (because each of them can claim that he is at rest and the other is not) imposes quite heavy (mathematical) restrictions, which eventually lead to Maxwell's laws.

4. Feb 15, 2013

### dydxforsn

5. Feb 15, 2013

### Roodles01

Is it different mathematically, though?
I'm under the impression that it can be shown to be different in terms of line integrals, but not qite sure how.

Last edited: Feb 15, 2013
6. Feb 15, 2013

### arydberg

7. Feb 15, 2013

### MS La Moreaux

I believe that when the coil moves, motional emf is induced in it. When the magnet moves, because its field is not uniform, its magnitude changes with time at the position of the coil. This is accompanied by an electric field which results in an emf in the coil, as given by the Maxwell-Faraday Law, which is one of Maxwell's equations. If the magnet had a uniform field, its motion would not result in a change in magnitude at the position of the coil and there would be no emf. But, if instead, the coil moved in this uniform magnetic field, an emf would be induced in it.

8. Feb 15, 2013

### marcusl

There is no difference, as Simon says.
This is incorrect.

9. Feb 15, 2013

### Simon Bridge

You can still induce a current in a uniform B field by changing the area of the loop perpendicular to the magnet ... you can do this with a stationary loop and a moving magnet by changing the angle of the magnet and the other way by changing the angle of the loop. It is how modern electricity generators typically work. The magnitude of B everywhere on, around, or within, the loop need not change.

Last edited: Feb 15, 2013
10. Feb 15, 2013

### marcusl

True, coil rotation in a uniform field will induce an emf. But it still doesn't matter whether it is the magnet or the coil that moves, contrary to what MS La Moreaux stated.

11. Feb 15, 2013

### MS La Moreaux

A stationary coil in a constant magnetic field experiences no emf. The motion of the magnet is irrelevant. It only becomes relevant if the motion of the magnet results in a changing magnetic field.

12. Feb 15, 2013

### Simon Bridge

That's what I said ;)

The statement of belief in post #7 was that if the magnitude of the B field does not change then there is no induced emf. You asserted that this was incorrect and I sought to support your assertion by providing an example. Sorry for the confusion.

Last edited: Feb 15, 2013
13. Feb 15, 2013

### arydberg

All that is required for an emf is that the wires of the coil "cut" the lines of the magnetic field.

The field can be uniform as it is ( almost) in a d'arsendal meter movement.

14. Feb 15, 2013

### marcusl

This again is slightly off the mark. It is not the field at the loop that counts, but the integrated flux across the loop area. An emf is produced in response to time-varying flux.
Likewise sorry if I misunderstood. You and I are on the same page.

15. Feb 16, 2013

### Roodles01

Thank you all for the input.

As with many things which seem trivial in the definition there are always different perspectives & appreciate the constructive arguments here.

16. Feb 16, 2013

### arydberg

So could someone explain how a radio speaker works where the field is radial. The coil is perpendicular to the field and a current causes a force perpendicular to both Where is the chance in flux per unit time?

17. Feb 16, 2013

### Simon Bridge

Reality is not a matter of opinion, and perspective is a matter of geometry. To see someone elses POV, you just need ruler and compass.
I don't think that description is correct for the speakers I know about - but I may be misunderstanding you - do you have a reference?

Anyway:
The change in flux comes from the signal - as you know.
Under the speaker cone is an electromagnet in which the current varies with the signal, varying the magnetic field-strength, and thus the flux.
http://electronics.howstuffworks.com/speaker6.htm

18. Feb 16, 2013

### arydberg

Your source is overly simplistic. The magnet is at the back of the speaker but the lines of magnetic flux are conducted through soft iron to a Cylindrical gap. A voice coil exists within this gap. The voice coil moves perpendicular to both the radial field and the direction of the coil.

These same voice coil movements were used in the early 8 inch floppy disks.

19. Feb 16, 2013

### Simon Bridge

20. Feb 16, 2013

### arydberg

Take apart a speaker.

21. Feb 16, 2013

### marcusl

We have been discussing induced EMF's, which are described by Faraday's law. This is a new topic and belongs in a new thread, because loudspeaker operation depends on the Lorentz force $$\vec{F}=q\vec{v}\times\vec{B}$$
It describes how the force is orthogonal to both current I=qv and field B.

22. Feb 16, 2013

### MS La Moreaux

Let us shed a little light on the subject by considering a case simpler than a coil, namely the Faraday Paradox. This employs two disks, say of the same size. One is made of copper and the other is a magnet with its faces the poles. These disks are arranged face to face, close but not touching. Each is mounted on an axle like a wheel and the axles are colinear. If the copper disk is spun while the magnet is stationary, a non-electrostatic emf appears between the copper disk's center and rim. If the magnet is spun and the copper disk remains stationary, there is no emf in the copper disk.

23. Feb 16, 2013

### arydberg

There seems to be two effects here. One where a coil is put into or out of a magnetic field and a EMF is produced.

But the loudspeaker has a radial magnetic field in the shape of a cylinder and a EMF is produced when the coil is moved up or down . The field inside the coil is the same before and after the movement.

24. Feb 16, 2013

### Simon Bridge

Found one - it looks just like the link I gave you above.
There is a cylindrical magnet, with a coil on a diaphragm. Input wires go to the coil. The field near the coil is not radial.
Note - the speaker back sticks nicely to my fridge.

Please provide a clear reference of what you are trying to describe.
Sounds like you are thinking of a microphone - you speak into a diaphragm, which vibrates, moving a coil back and forth next to a magnet, which induces an emf, producing the signal.

A speaker goes the other way - a varying current is supplied directly to the coil. No need to induce an emf to make it work.
This changes the magnetic field of the coil, changing the amount that it is attracted to the magnet. The field inside the coil changes with the input signal.

Last edited: Feb 16, 2013
25. Feb 16, 2013

### arydberg

You have to take it apart. Cut the voice coil wires. Use a razor blade and cut the cone loose, Cut around the spider and lift out the cone & voice coil assembly and you will see what i mean.