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 center and minimum at the edge

Answer

C

Explanation

Required arrangement is according to ampere circuital law `B_("outside") =(mu_(0))/(4pi)(2I)/(r )`

for an internal point `r lt R`
`B_("internal")=(mu_(0)(0))/(2pi r)=0`
For a point on pipe r=R
`B=(mu_(0)I)/(2pi R)`
for an external point `r gtR`
`B_("external") =(mu_(0)I)/(2pi r)`
Hence magnetic field is zero only inside the pipe at any point
moving coil galvanometer voltmeter and ammeter