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:
- (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.
