# Homework Help: Purcell 7.31

1. Nov 2, 2012

### NullSpace0

1. The problem statement, all variables and given/known data
There is a picture of two possible set ups for a dynamo-- the only difference I can see is the direction in which the main shaft rotates. He says that it is a "self-excited dc generator" because the rotation of the shaft inside of the single loop coil provides the changing flux which provides the emf which provides the current for the dc circuit.

2. Relevant equations

3. The attempt at a solution
I'm completely lost. First of all, if there is no magnetic field to begin which, how can there be any induced emf? There would be no reason for charge carriers to move unless there IS an external B-field, I would think...

Secondly, how could the direction of rotation influence the emf in such a way that one direction of rotation produces no current while the other direction does?

2. Nov 3, 2012

### TSny

You can assume that at the start there would be a small "stray" magnetic field that would have a vertical component. Then, rotating the disk with an externally applied torque will result in an induced current in the disk and in the nonrotating circular wires connected to the disk by commutators. If the current in the wires produces a magnetic field in the same direction as the initial vertical component of the stray field, the net B field will increase. Hence, more current will flow which will increase B some more which will increase the current even more, etc. This will happen in Purcell's sketch only if the disk rotates in the correct direction. You have to decide which direction is the right one. And check to see if the answer depends on whether the initial stray field had a vertically upward component or a vertically downward component.

Last edited: Nov 3, 2012