# Magnetic force on one electron

1. Apr 5, 2012

### jsmith613

1. The problem statement, all variables and given/known data
question - see image

2. Relevant equations

3. The attempt at a solution

so I use the equation F = BIl
F = 0.2 * 0.4 * 8 = 0.64
Surely I just divide it by the number of electrons (8*1028) to get the answer
BUT this does not give the correct answer.

The correct answer is D???

File size:
16.2 KB
Views:
79
2. Apr 5, 2012

### Staff: Mentor

Ah. That's not the number of free electrons, that's the density of free electrons.

3. Apr 5, 2012

### jsmith613

so density / area = number/m
right?

Last edited: Apr 5, 2012
4. Apr 5, 2012

### Staff: Mentor

$\rho = \frac{n}{volume}$

If you calculate the volume of the wire you can find the total n.

5. Apr 5, 2012

### jsmith613

so n = density * volume
n = 8 * 1028 * 0.4 * 10-6

therefore answer = D

thanks