# Homework Help: Long straight wire: Ampere/Biot Savart

1. Apr 10, 2013

### Roodles01

1. The problem statement, all variables and given/known data
A long straight wire carries a current of 2.0A. An electron moves parallel to it at a distance of 0.01m (I'm assuming id doesn't matter which way it moves) at a velocity, v = 100ms-1
Calculate magnitude & direction of force acting on it.

2. Relevant equations
B = μ0 L / 2∏r

F = q(v X B)

I know the second is the Lorentz force law & that the first relates to Biot-Savart but for the Biot-Savart I also have,
F12 = μ0/4∏ * I1 δl1 * ( I2 δl2 * rhat 12) / r122

So how does one relate to the other, please.

3. The attempt at a solution

Surely it can't be this easy!

B = μ0 L / 2∏r
B = 4∏x10-7 * 2.0 / 2∏(0.01)
B = 3.95x10-8 T

F = q(v X B)
F = (-1.6x10-19)(100x103)(3.95x10-8
F = -6.32x10-22 N

Magnitude 6.32x10-22 N

2. Apr 10, 2013

### rude man

Surely it can!
Except the direction could be towards or away from the wire, depending on the relative direction of v and i and the position of q.

BTW I've never seen L used for current before ...

3. Apr 11, 2013

### Roodles01

L . . . oops!