# Electromagnetism issues need clearing up

1. Nov 27, 2006

In electromagnetism, when there is an emf is induced in a secondary coil from a current flowing in a nearby primary coil (such as in a transformer), then how is the mutual inductance, number of turns of each coil, the current in the primary coil, and the emf in the secondary coil related?

From the reading that I have been doing, I think that for a single coil:
$$B = \frac{u_0NI}{2 \pi r}$$
Where the $u_0$ is the permeability of free space (I don't know how to get the proper symbol), $N$ is the number of turns, and $B$ is the magnetic field strength.

From what I have gathered from the reading, is:
$$E = -\frac{d \psi}{dt} = -L \frac{dI}{dt}$$
Where I have used $E$ for emf, and psi for the flux (I don't know how to get the convention symbol), and $L$ is the inductance of the secondary coil, however, I am unsure as to how the mutual inductance (M) is related to the inductance (L).

I think the problem is that when people speak of inductance, I am unsure what they mean the inductance of.

The reason why I am asking is that I have a question which uses this idea, but I am so close to getting it, that I would rather not post the problem in the homework section.

Last edited: Nov 27, 2006
2. Nov 27, 2006

### Andrew Mason

L, self inductance, is the ratio of induced emf in an inductor (coil) to rate of change of current in that inductor: Emf = L dI/dt

Mutual inductance relates to two coils in which a change of current in one coil induces an emf in the other. Emf = MdI/dt where Emf is the emf in the other coil.

AM

3. Nov 27, 2006

### cesiumfrog

$\phi$?