# Induction in Toroidal Coil

1. Nov 25, 2007

### cscott

Say I've inserted a north pole of a bar magnet into a toroidal coil. When the bar is moving there will be an induced EMF in the coil which will lead to a magnetic field that opposes the initial magnetic field. From what I understand the B field in the toroidal coil is circular inside the coil. I don't see how to pick the B field in this coil so that it opposes the initial field due to the bar magnetic. Aren't they orthogonal to each other?

2. Nov 26, 2007

### marcusl

Is this a homework problem? Can you describe the geometry a little better? (i.e., how do you insert the north pole into the toroid?)

3. Nov 27, 2007

### cscott

It was part of a lab I did a while ago.

The north pole goes straight through the loop, like this:

[] <-N---S :tongue2:

4. Nov 27, 2007

### Staff: Mentor

The orthogonal components do not interact. The only interaction is from the components of the bar magnet field that pierce the surface areas of the coils. Remember what the field near the end of a bar magnet looks like -- it is like a fountain, right? The lines of magnetic field are bending out and around, to return to the opposite pole.

5. Nov 27, 2007

### cscott

Doesn't the same amount of lines go "left" as "right" when one pictures in the end of the north pole, so why is there an effect?

6. Nov 28, 2007

### Staff: Mentor

Hmmm. Good point. I think that you're right. In a very symmetrical situation, there would be no net induction in the overall coil. If you think of a simplified geometry with just two turns, linked in series, with room to push the bar magnet through between them, and you measured the net voltage between the ends, I believe that the net would be zero as the bar magnet passed through between them.

7. Feb 2, 2010

### carbonchain

I have been thinking over toroidal fields and from what I have found, the magnetic field in a toroid is limited to the space enclosed by the loops of the wire.

I am actually interested of the magnetic field that is on the surface of the toroid, and googling around I got to here..

So it seems to me, that when you move the bar magnet through the middle of the toroid, the induces EMF will create a magnetic field, but this magnetic field will be inside the toroid only, and it would not oppose the field that generated it on a first place.

What you guys think?

8. Jul 28, 2010

### nspoken

There is a weak field along the symmetry axis of a toroidal coil. If you shrink the toroidal cross-section to zero you are left with a loop (or loops) of wire about the symmetry axis.