# Homework Set 7

## Magnetic Materials

This question should be fairly easy once we have done Lecture 20

## Faraday's Law and Induced Emf

This tutorial should help you get use to applying Faraday's law to a loop in a magnetic field.

Another tutorial on Faraday's Law. For Part F refer to the notes on the electric field produced by a time varying magnetic field.

## Problem 29.20

Most of the examples we looked at in class concerned flux changing either because the field changed or a loop turned, but the flux can also change if the size of a loop is changed, and then $\frac{d\Phi_{B}}{dt}=B\frac{dA}{dt}=-\mathcal{E}$

## Problem 29.38

Apply the formula for the emf produced by a generator.

## Problem 29.41

You can think of the back emf in the motor as being similar to the emf produced by a generator.

## Problem 29.70

The magnetic field goes from one extreme to another twice every shake, once you have worked out $\frac{dB}{dT}$ use

$\mathcal{E}=-\frac{dB}{dT}NA$

to work out what $N$ should be.