# Biot-savart with electron

1. Mar 5, 2008

### cryptoguy

1. The problem statement, all variables and given/known data
In a model of a hydrogen atom, an electron circles the proton at a distance of 5.29E-11 m with a speed of 2.19E6 m/s. Compute the magnitude of the magnetic field that this motion produces at the location of the proton.

2. Relevant equations
dB = (u/4pi*r^2) I ds x r(roof) (Biot-Savart's law)

3. The attempt at a solution
I did I = e*v/(2*pi*r) where e is electron charge and v is the velocity. I got I = .00105 A. I plug in that into B = uI/(4*pi*r) = 1.99 T but the answer is 12.5 T. What am I doing wrong? Thank you.

2. Mar 5, 2008

### Shooting Star

There's one more 'r' in the denominator. The final expression is

$$\frac{\mu_{0}}{4\pi}$$ $$\frac{ev}{r^2}$$.