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

Asked in JEE Main 2007 · Thick wires and cables

• (1) the magnetic field at all points inside the pipe is the same, but not zero
• (2) the magnetic field is zero only on the axis of the pipe
• (3) the magnetic field is different at different points inside the pipe
• (4) the magnetic field at any point inside the pipe is zero

Step-by-step solution

Why the other options are wrong

• (1) Treats the pipe like a solid wire that has current inside.
• (2) Uses only the symmetry argument on the axis; Ampere's law gives zero at every radius inside.
• (3) Assumes the field varies with radius as inside a solid wire.

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