Question

A long, straight wire of radius a carries a current distributed uniformly over its cross-section. The ratio of the magnetic fields due to the wire at distance a/3 and 2a, respectively from the axis of the wire is :

(1) 2/3

(2) 3/2

(3) 1/2

(4) 2

Answer 1

Answer is (1) 2/3

Let current density be J. 

∴ Applying Ampere's law.

Answer 2

The correct answer is (1) 2/3