# Magnetic interaction between moving and stationary charge

1. Apr 25, 2015

### whatisreality

1. The problem statement, all variables and given/known data
A metal sphere, with charge q1=-2.80μC is held still and off the ground on an insulating rod. Another smaller metal sphere with charge q2=-7.80μC is projected towards q1. When the two spheres are 0.80m apart, q2 moves at 22ms-1.
Assume the spheres are point charges, neglect gravity. What is the magnitude of the magnetic force of q2 on q1 when the spheres are 0.5m apart?

2. Relevant equations
F=qvB, I think...

3. The attempt at a solution
I'm a bit confused about why the magnetic force on the stationary sphere isn't zero. It isn't moving. There may be current in it as electrons are repelled by the approaching sphere, but if we're treating both as point charges this becomes irrelevant, doesn't it? So there's no reason it would feel magnetic force.

Even assuming it does feel a magnetic force, I have no idea which equation to use to find its magnitude. I don't know v or B at 0.5m apart!

2. Apr 25, 2015

### Staff: Mentor

In other words, the force is zero.

Weird sort of problem. You can still calculate the total force, which includes the electric component.

(I deleted your other thread as it is fully redundant to this one)