# Homework Help: Mutual inductance

1. Jun 17, 2013

### sapz

1. The problem statement, all variables and given/known data
We have a toroidal coil, and an infinite wire that runs through it on its symmetry axis. (See picture)

We need to find the mutual inductance between the coil and the wire in two ways:

1. By running a current through the wire, and finding the induced voltage in the toroidal coil.
2. By running a current through the toroidal coil, and finding the induced voltage in the wire

2. Relevant equations

3. The attempt at a solution

So, I manage when trying to solve (1): If there's a current in the wire, there's a magnetic field in the Theta direction, through the coil, and we can find L.

However, if I try to solve it through (2): If I run a current through the coil, then the magnetic field would exist only inside the coil! So how can I find the flux on the wire, if it's outside the coil?... It would just be zero all the time, wouldnt it?

Any help would be appreciated!

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• ###### toroidal.jpg
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2. Jun 18, 2013

### zephyr5050

I'm not sure if this is part of the problem, but it appears to me that the toroidal solenoid is not complete - that is, there's a gap in the wiring. If this is truly part of the problem, then there would be a field which "comes out" of the gap and thus a field on the wire.

If the picture is just misleading and the toroid is supposed to be complete, then my only offering is that saying there is no magnetic field outside the solenoid is not technically correct. That is only the case in an ideal system (in the same world where all cows are spherical). In the real world there will be a field outside of the solenoid, albeit it will be very small and decay rapidly.

I don't have an answer to your problem but hopefully those points might be insightful enough for you.

3. Jun 19, 2013

### sapz

Ive got it! You take the infinite wire, and close it in infinity.
That way you have a closed loop that the flux through it is the flux through the toroid, and therefore there's a current and we can get L.

4. Jun 20, 2013

### Basic_Physics

The magnetic field that the toroidal coil creates is identical to the magnetic field that one finds around a wire with current in it. Can one exists without the other?