# Homework Help: Induced Emf, Contributing factors

1. Jul 30, 2008

### K3nt70

1. The problem statement, all variables and given/known data
A generator is constructed by rotating a coil of N turns in a magnetic field B at a frequency f. The internal resistance of the coil is R and the cross sectional area of the coil is A.
Which of the following statements are true? (Give ALL correct answers, i.e., B, AC, BCD...)

A) The maximum induced EMF occurs when the coil is rotated about an axis parallel to the magnetic field lines.
B) The average induced EMF doubles if the area A is doubled.
C) The average induced EMF doubles if the frequency f is doubled.
D) The average induced EMF doubles if the resistance R is doubled.
E) The maximum induced EMF occurs when the coil is rotated about the axis of the coil.
F) The average induced EMF doubles if the magnetic field B is doubled.

2. Relevant equations

$$|\epsilon| = I R$$
$$|\epsilon| = NA\frac{\Delta B}{\Delta t}$$

3. The attempt at a solution
Well, im fairly sure A is false, B is true, E is true and F is true. I think D is false, and i cant find any expressions in my book relating frequency to emf. I don't really know the relationship between frequency and induced emf.

2. Jul 30, 2008

### Defennder

The frequency tells you the rate of change of area of the coil in the magnetic field. You can start by imagining the coil rotating from its maximum exposed area to the B-field to zero flux of the B-field passing through the coil. You are given the angular frequency of rotation, so you can write an expression for the time taken for it to go from max B flux to zero B flux through the coil. From this just make use of your second equation to come up with a rough expression for emf induced throughout that quarter cycle of rotation.