The adjacent figure shows, the cross-section of a long rod with its length perpendicular to the plane of the paper. It carries constant current flowing along its length. `B_(1),B_(2),B_(3)"and"B_(4)` respectively respresent the magnetic fields due to the current in the rod at points `1,2,3"and"4` lying at different separations from the centre O as shown in the figure. which of the following shall hold true?

Options

A. `B_(1) gt B_(2) ne 0`
B. `B_(2) gt B_(3) ne 0`
C. `B_(1) = B_(2)= B_(3)= 0`
D. `B_(3) gt B_(4) ne 0`

Answer

D

Explanation

`B prop r`, Inside, `B prop (1)/(r)`, Outside
`because B_(1)= B_(2)= 0,B_(3) gt B_(4) ne 0`