# Homework Help: Magnetic fields out of long wires

1. May 16, 2015

### winterbell

1. The problem statement, all variables and given/known data
Assume that I = 5.0A.

What is the magnetic field at point b in the figure? Give your answer as a vector.

2. Relevant equations
Ampere's Law - B = (μ0I)/(2πr)

3. The attempt at a solution
I tried plugging the given numbers into Ampere's Law for point b, which gave me B = (4π*10-7) *5.0 I/(2π*0.2 cm), or a result of 1*10-5 T pointing in the positive direction. I assumed this should be doubled as there are two identical fields, giving me 2*10-5 T, but this is wrong. Any help on where I went wrong would be appreciated.

2. May 16, 2015

### TSny

Hello, winterbell and welcome to PF!

Where did the 0.2 cm come from?

Also, should r be in cm or meters?

3. May 20, 2015

### winterbell

r should be in metres - having gone back and checked I realised that I should actually have had an extra 0 in there and it should have been 0.01 m. I should probably check my values better as I seem to often have problems with this!