A direct current l flow along the length of an infinitely long striaght thin walled pipe then the magnetic field is

Options

A. uniform throughout the pipe but not zero
B. zero only along the axis of the pipe
C. zero at any point inside the pipe
D. maximum at the centre and minimum at the edges.

Answer

C

Explanation

According to Ampere's circuital law
For an internal point `r lt R`
`Bxx2pi r =mu_(0)(0)` or `B_("in")=0`
For an external point `r gt R`
`Bxx2pir=mu_(0)I` or `B_("out")=(mu_(0)I)/(2pir)`