# What force acts against a magnet passing through a coil?

1. Oct 4, 2007

### Zorodius

A magnet will induce EMF when it passes through a conducting coil. Where does that energy come from? Does the coil do work on the magnet somehow? The B field induced in the coil should be parallel to the magnet's motion, so I would think it could not slow the magnet's progress. Does the magnet become de-magnetized to some extent, so that it loses some kind of internal energy to the coil?

2. Oct 4, 2007

### mjsd

the energy come from the movement of the magnet... no movement no emf.

3. Oct 4, 2007

### Zorodius

So the magnet loses kinetic energy - what force causes that to happen?

4. Oct 4, 2007

### Staff: Mentor

Realize that something must be pushing the magnet through the coil--that something is the source of the energy.

5. Oct 4, 2007

### Zorodius

Er, I don't quite follow - if the magnet has some initial velocity that is aimed through the coil, it will move through the coil without anything pushing it.

6. Oct 4, 2007

### Staff: Mentor

The moving magnet will induce a current in the coil that will oppose its motion and slow it down. If you want it to go through the coil without losing speed, you'll have to push it.

7. Oct 4, 2007

### Zorodius

this is what I'm confused about - why does the presence of a current in the coil slow the magnet? The magnet has no net charge, and so no Lorentz force acts on it.

8. Oct 4, 2007

### Staff: Mentor

Look at it this way. The current-carrying coil is itself a magnet. Its magnetic pole opposes the magnetic pole of the moving magnet.

9. Oct 4, 2007

### Staff: Mentor

Neither does a current-carrying wire.

But current-carrying wires do experience magnetic forces, and so do magnets.

In classical electrodynamics, we "explain" a magnet by invoking surface currents along the surface of the magnet, which produce a magnetic field and in turn have forces exerted on them by external magnetic fields. At the quantum level, we explain magnets via the intrinsic magnetic moment of electrons, which is related to their intrinsic angular momentum (also known as "spin").

10. Jun 16, 2008

### Murtnowski

What if the magnet is passes OVER instead of through the coil?

To slow down the magnet the side of the coils nearest the magnet would have to have the same polarity as the magnet when approachs and then change polarity when the coil and magnet are aligned, so the magnet isn't pushed/accelerated when it moves off the coil. This swaping of polarity in the coil would mean the the voltage in the coil would need to change direction, ie. from + to - or - to +.

I always thought that passing a magnet over a coil makes a voltage that starts at zero, ramps up to a peak when they are aligned, and then ramps back down to zero. From what you guys have said the voltage would have to go from one peak and then instantly change direction to another peak of a diffent sign then ramp back down to zero.

Code (Text):
/|                     /\
/ |                    /  \
_/  |   _              _/    \_
|  /
| /
|/
Can someone please explain which it really is.

Last edited by a moderator: Jun 17, 2008
11. Jun 17, 2008

### Staff: Mentor

I'm not clear as to what you mean. I think you are talking about moving a pole of a magnet from one side of the coil to the other as you pass over it (moving perpendicular to the plane of the coil). (If I'm mistaken, let me know.)
What matters is the rate of change of the magnet's flux through the coil. The induced current will oppose that changing flux. When magnet and coil are aligned, the rate of change in flux will already have dropped to zero. Why would you think the polarity will swap? As the magnet approaches the coil, the near side must have the same polarity, and as it leaves the near side must have opposite polarity--but that means the voltage peaks have the same polarity since the "near side" changes as you move from one side to the other.
As I already noted, the peak voltage would occur before the magnet and coil are aligned, return to zero as they reach alignment, then peak again. Nothing takes place "instantly".

I'd say neither is correct: The first because the polarity reverses (with infinite slope!); the second because it only has one peak.