A hollow cylindrical conductor (inner radius = a, outer radius = b)carries a current i uniformly spread over its cross section.Which graph below correctly gives B as a function of the distance r from the center of the cylinder?
A)I
B)II
C)III
D)IV

Question

A hollow cylindrical conductor (inner radius = a, outer radius = b)carries a current i uniformly spread over its cross section.Which graph below correctly gives B as a function of the distance r from the center of the cylinder? 
A)I 
B)II 
C)III 
D)IV

Quizplus · Accepted Answer

For a cylindrical conductor with uniform current, the magnetic field inside is zero, and outside is given by:
$B = \frac{\mu _0 i}{2\pi r}$ where r is the distance from the center of the cylinder. 
This implies that B is inversely proportional to r. Hence, graph III, which is a decreasing function, is the correct choice.

A Hollow Cylindrical Conductor (Inner Radius = A, Outer Radius