A coaxial cable made up of two conductors. The inner conductor is solid and is of radius `R_1` and the outer conductor is hollow of inner radius `R_2` and outer radius `R_3`. The space between the conductors is filled with air. The inner and outer conductors are carrying currents of equal magnitudes and in opposite directions. Then the variation of magnetic field with distance from the axis is best plotted as

Options

A.
B.
C.
D.

Answer

C

Explanation

(c)
From Ampere's law, the feild at the axis is zero. From `x=0`
to `R_1`, the field increases linearly as the current enclsed
increase.
From `x=R_1 to R_2` and from `x=R_2 to R_3`, the field decrease
hyperbolically but with different slopes as the media are different.
Hence, the required graph is (c).