Q. A current I flows along the length of an infinitely long, straight and thin-walled pipe. Then

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The magnetic field at all points inside the pipe is

The magnetic field at any point inside the pipe is
zero

The magnetic field is zero only on the axis of the pipe

The magnetic field is different at different points inside the pipe

Solution:

Applying Ampere's law to any closed path inside the pipe we find no current is enclosed. Hence .

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