Magnetic force on one electron

[jsmith613]

Homework Statement

question - see image

Homework Equations

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?

 

[gneill]

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?

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

 

[jsmith613]

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

so density / area = number/m
right?

 

[gneill]

so density / area = number/m
right?

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

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

 

[jsmith613]

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

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

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

therefore answer = D

thanks