Question

# A long, straight wire of radius R carries a current distributed uniformly over its cross-section. Themagnitude of the magnetic field is

A
maximum at the axis of the wire
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B
minimum at the axis of the wire
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C
maximum at the surface of the wire
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D
minimum at the surface of the wire.
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Solution

## The correct options are B minimum at the axis of the wire C maximum at the surface of the wireCurrent enclosed over circular cross section ares πr2 is,I′=I(πr2πR2)=I(rR)2Using Ampere law,∮B×dl=IenclosedμB×2πr=I(πr2πR2)μB=Iμ(2πR2)r ..(1)B is minimum at r=0 axis of the wire.if r>R then,B=I2πr ..(2)B is maximum at r=R at the surface of the wire.where I=current flowing in wire.so option B and C is correct.

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