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,
-
-
1
-
4
-
1/4
Solution
B.
1
b) Consider two amperian loops of radius a/2 and 2a as shown in the diagram.
For the smaller loop,
B1 acts at a distance a/2 from the axis of the wire.
Similarly, for bigger amperian loop,
B' x
(2a) = 
Total current enclosed by amperian loop 2.
, at a distance 2a from the axis of the wire.Therefore,
Ratio of
