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Question

Two infinitely long straight wires lie in the xy-plane along the lines x=±R. The wire located at x=+R carries a constant current I1 and the wire located at x=R carries a constant current I2. A circular loop of radius R is suspended with its centre at (0,0,3R) and in a plane parallel to the xy-plane. This loop carries a constant current I in the clockwise direction as seen from above the loop. The current in the wire is taken to be positive if it is in the +^j direction. Which of the following statements regarding the magnetic field B is (are) true ?

A
If I1=I2, then B cannot be equal to zero at the origin (0,0,0)
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B
If I1>0 and I2<0, then B can be equal to zero at the origin (0,0,0)
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C
If I1<0 and I2>0, then B can be equal to zero at the origin (0,0,0)
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D
If I1=I2, then the z-component of the magnetic field at the centre of the loop is (μ0I2R)
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Solution

The correct options are
A If I1=I2, then B cannot be equal to zero at the origin (0,0,0)
C If I1>0 and I2<0, then B can be equal to zero at the origin (0,0,0)
D If I1=I2, then the z-component of the magnetic field at the centre of the loop is (μ0I2R)
(A) At origin, B=0 due to two wires if I1=I2 (they cancel each other)
hence (Bnet) at origin is equal to B due to ring, which is non-zero.
(B) If I1>0 and I2<0,B at origin due to wires will be along +^k direction and B due to ring is along ^k direction and hence B can be zero at origin.
(C) If I1<0 and I2>0,B at origin due to wires is along ^k and also along ^k due to ring, hence B cannot be zero.
(D) (ref. image 2) At centre of ring, B due to wires is along x-axis,
hence z-component is only because of ring which B=μ0I2R(^K)

828458_904035_ans_aa16529624bc4375931697254263551c.png

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