Question

If an electric wire is allowed to produce a magnetic field no larger than that of the Earth (0.50×10−4T) at a distance of 22 cm from the wire, what is the maximum current the wire can carry?

Answer 1

The magnetic field at a distance r from long straight wire is

B = u0 I / 2 pi r
where u0 = 4pi x 10^-7 T m/A

0.50 x 10^-4 = 4 pi x 10^-7 I / 2 pi x 0.22m
I = 0.50 x 10^-4* 0.22m / 2x10^-7

I = 55 A