If a long hollow copper pipe carriers a direct current, the magnetic field associated with the current will be:

Options

A. only inside the pipe
B. only outside the pipe
C. both inside and outside the pipe
D. nether inside nor outside the pipe

Answer

B

Explanation

According to Ampere's law
`ointvec(B).vec(dl)=mu_(0)l`
Only for outside pint, path of line integration will enclose the current. Hence, `B=0` inside.