# Homework Help: Magnetostatics and Electrostatics

1. Nov 29, 2007

### johanjones190

Here is the problem, I have no idea how to do this!

(In a single straight wire) If the positive charges (density p+) are at rest, and the negative charges (density p-) move at speed v, show that:

p- = -(p+)*(gamma)^2

2. Nov 30, 2007

### Astronuc

Staff Emeritus
Is gamma some kind of ratio?

The traveling charges (enclosed current) induces a magnetic field which in turn imposes a force on moving charges, which then must be offset by an electrostatic attraction.

3. Nov 30, 2007

### johanjones190

Yeah gamma = 1/ sqrt(1-v^2/c^2) where v is velocity and c is the speed of light!

4. Nov 30, 2007

### Marco_84

Try to think that the global electric charge must be conserved, and see how in relativity change the volumes by lorentz contraction. then remember that Q=density times volume and you'll get the answer...

5. Nov 30, 2007

### Astronuc

Staff Emeritus
for get this approach and go with what Marco_84 suggested. I was thinking something entirely different, not realizing that this was about SR.