Magnetic fields out of long wires

[winterbell]

Homework Statement

Assume that I = 5.0A.

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

Homework Equations

Ampere's Law - B = (μ₀I)/(2πr)

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.

 

[TSny]

Hello, winterbell and welcome to PF!

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), ...

Where did the 0.2 cm come from?

Also, should r be in cm or meters?

 

[winterbell]

Hello, winterbell and welcome to PF!
Where did the 0.2 cm come from?

Also, should r be in cm or meters?

r should be in metres - having gone back and checked I realized 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!

Thanks for your help.