question_answer

A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B', at radial distances \[\frac{a}{2}\] and 2a respectively, from the axis of the wire is:

A) \[\frac{1}{4}\]

B) \[\frac{1}{2}\]    

C) 1

D) 4