Question

A long, straight, hollow conductor (tube) carrying a current has two sections $A$ and $C$ of unequal cross sections joined by a conical section $B$. 1, 2 and 3 are points on a line parallel to the axis of the conductor. The magnetic fields at 1, 2 and 3 have magnitudes $B_1,B_2$ and $B_3$. Then,

• A. $B_1=B_2=B_3$
• B. $B_1=B_2\ne B_3$
• C. $B_1<B_2<B_3$
• D. $B_2$ cannot be found unless the dimension of the section $B$ are known

Answer 1

The correct option is A $B_1=B_2=B_3$

To find the magnetic field outside a thick conductor, the current may be assumed to flow along the axis.

As points 1, 2 and 3 are equidistant from the axis.
$B_1=B_2=B_3$