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An infinitely long current-carrying wire and a small current-carrying loop are in the plane of the paper as shown. The radius of the loop is a and distance of its centre from the wire is dd>>a . If the loop applies a force F on the wire, then

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By Expert Faculty of Sri Chaitanya
a
F=0
b
Fad2
c
Fad
d
Fa2d3
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detailed solution

Correct option is B

For motion of the loop along  x-direction, P.E. is U(x)=mB
As both m (of loop) and B(of wire at loop) point normally into the plane of paper,
U(x)=mB=πa2iμ0I2πx=μ0iIa22x
P.E. decrease as loop moves closer to the wire. Force on the loop is attractive.
F=dUdx=μ0iIa221x2

 At x =d,F=μ0iI2a22d2( Attraction ) Fa2d2
 


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