Question

The magnetic field 0.02 m from a wire is 0.1 T. What is the magnitude of the magnetic field 0.01 m from the same wire?

0.01 T
0.05 T
0.1 T
0.2 T

Answer 1

B for a long straight wire varies as 1/r. Halving r doubles B:

B2 = B1*(r1/r2) = 0.1*(0.02/0.01) = 0.2 T.

Answer: 0.2 T.