Question

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
and 2a respectively, from the axis of the wire is:
(a)1/2
(b)1\4
(c) 1
(d) 4​

Answer 1

Answer

1

Consider two amperian loops of radius a/2 and 2a as shown in the diagram.

Applying ampere's circuital law for these loops, we get

∮B.dL=μ

0

​

I

enclosed

​

For the smaller loop,