Moving Charges and Magnetism - Result Question 46

47. If a long hollow copper pipe carries a current, then magnetic field is produced

[1999]

(a) inside the pipe only

(b) outside the pipe only

(c) both inside and outside the pipe

(b) no where

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Answer:

Correct Answer: 47. (b)

Solution:

  1. (b) Inside a hollow pipe carrying current, the magnetic field is zero, since according to Ampere’s law, $B_i \cdot 2 \pi r=\mu_0 \times 0 \Rightarrow B_i=0$. But for external points, the current behaves as if it was concentrated at the axis only; so, outside, $B_0=\frac{\mu_0 i}{2 \pi r}$. Thus, the magnetic field is produced outside the pipe only.