Magnetic Force in a Current Carrying Wire

[creemore]

Homework Statement

The current in a wire consists of n electrons per unit volume moving with a velocity v. What is the effect on these electrons of the magnetic field set up by the current itself within the wire? What is the force on one electron at a distance r from the centre of the wire?

Homework Equations

F = QvBsinθ
Magnetic field strength at a distance r from the axis of the wire outside the wire = u₀I/2∏r

The Attempt at a Solution

I know that for part 1, the effect of the magnetic field on the electrons inside the wire will be a radially outward force (taking all electrons to be traveling in straight lines along the wire).

Part 2

F will be equal to QvBsin90 = QvB

Making the subsitution for B I have

F = Qvu₀I/2∏r

I believe now that I have to make subsitution for I. Which would be 4/3∏a³vne

where e is fundamental charge and a is the radius of the wire.

This gives:

F = (3/8)u₀ve²na³r

I know that the answer is:

F = (1/2)u₀v²e²nr
I think I'm making a mistake in calculating my current, but I'm not sure. Help Please?

Thanks

 

[/flûks/]

Hi! This is my first time helping someone out so here goes:

To make the substitution for current I, you should use:
$I=\int J \bullet da$
where J is the current density, equal to nev in this case. After doing a quick integration you should get the proper current which will give the correct answer for the force.