# Determine the magnetic field between 2 long straight wires

1. Jan 7, 2005

### joej

determine the magnetic field between 2 long straight wires 2cm apart in terms of the current (I) in one, when the other carries 15A. Assume currents are in the same direction.

okay so if they're in the same direction the field should be up

now I'm guessing I should use (4pi x 10^ -7)/(2pi) (I1 x I2)/(d) = F/l

now from that I'm guessing the length of the wire is no a factor here

so I get ((2 x 10 ^ -7)(I1 x I2))/d = F

now.... where should I go from here?

2. Jan 7, 2005

### Galileo

Use the formula for the magnetic field of a(n) (infinitely) long wire:

$$B=\frac{\mu_0 I}{2\pi r}$$

plus the superposition principle to find the field at a point between the wires.

3. Jan 7, 2005

### Andrew Mason

At the midpoint between the two wires, they are equally strong and in opposite directions. Use the right hand rule to find direction of B for each wire.

$$2\pi rB = \mu_0I$$ for each wire. You can't find the magnetic force between the two wires unless you know their length.

AM