Magnetics 6 Question 16

16. 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?

(2018 Adv.)

(a) If I1=I2, then B cannot be equal to zero at the origin (0,0,0)

(b) If I1>0 and I2<0, then B can 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 μoI2R

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Solution:

(a) At origin, B=0 due to two wires if I1=I2, 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 of 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)

At centre of ring, B due to wires is along x-axis.

Hence, z-component is only because of ring which B=μ0i2R(k^).



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