# Ampere's law problem help please

1. Mar 10, 2007

### feelau

1. The problem statement, all variables and given/known data
So a long straight wire lies on a horizontal table and carries a current of 1.2x10^-6A. In a vacuum, a proton moves parallel to the wire(opposite the current) with a constant speed of 2.3x10^4 m/s at a distance d above the wire. Determine the value of d. You may ignore the magnetic field due to the Earth.

2. Relevant equations
ampere's law
F(magnetic field)=q(vxB)
Biot savart law?

3. The attempt at a solution
So what I did is have the magnetic field formula for a wire, I then substitute this into the magnetic force equation. But now I don't really know where to go from there. Please help. Thanks very much.

2. Mar 10, 2007

### Staff: Mentor

Hint: Since the proton moves with constant velocity, what must the net force be on it?

3. Mar 11, 2007

### feelau

well net force should be zero but then would it mean F(magnetic)= mg or I am not sure what it would be equal to?

4. Mar 11, 2007

### Staff: Mentor

You got it.

5. Mar 11, 2007

### feelau

oh thanks very much