question_answer

A direct current I flows along the length of an infinitely long straight thin walled pipe, then the magnetic field is

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 edge

Correct Answer: C

Solution :

Required arrangement is shown in figure. According to Amperes circuital law. For an internal point, r < R \[{{B}_{\operatorname{int}ernal}}=\frac{{{\mu }_{0}}(0)}{2\pi r}=0\] For a point on the pipe, r = R \[B=\frac{{{\mu }_{0}}I}{2\pi r}\] For an external point, r < R Therefore, option  is correct.