Question

A hollow tube is carrying an electric current along its length distributed uniformly over its surface. The magnetic field
(a) increases linearly from the axis to the surface
(b) is constant inside the tube
(c) is zero at the axis
(d) is zero just outside the tube.

Answer 1

Answer:

c

Explanation:

Line integral over loop A gives magnetic field inside tube . But since there is no current flow in to filed will be zero .

Similarly for for loop B

$\int \: B.dl = \mu_0 \: I \\ \\ B(2\pi \: r) = \mu_0I \\ \\ B = \frac{ \mu_0I}{2\pi \: r}$

So it is not zero .

Answer 2

As the current is distributed uniformly on the surface,no current is inside and hence no magnetic field