JEE Main 2017 · 9 Apr · Shift 1 · Physics Q19

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A uniform magnetic field B of 0.3 T is along the positive Z-direction. A rectangular loop (abcd) of sides 10 cm × 5 cm carries a current I of 12 A. Out of the following different orientations which one corresponds to stable equilibrium ?

Loop in the YZ plane

Rectangular loop abcd with current I, axes X, Y, Z and B along Z: the loop lies in the YZ plane, with a at the origin, b on the Y-axis and d on the Z-axis.

Loop in the XZ plane

Rectangular loop abcd with current I, axes X, Y, Z and B along Z: the loop lies in the XZ plane, with a at the origin, b along X and d on the Z-axis.

Loop in the XY plane, current counterclockwise seen from +Z

Rectangular loop abcd with current I, axes X, Y, Z and B along Z: the loop lies in the XY plane with a at the origin, b along X, d along Y; the current runs from d to a and from b to c (counterclockwise seen from +Z).

Loop in the XY plane, current clockwise seen from +Z

Rectangular loop abcd with current I, axes X, Y, Z and B along Z: the loop lies in the XY plane with a at the origin, b along X, d along Y; the current runs from a to d and from c to b (clockwise seen from +Z).

Answer and worked solution

Official answer: C

Stable equilibrium needs the magnetic moment along $\vec B$ (+Z), i.e. the loop in the XY plane with the current counterclockwise seen from +Z. In (3) the current runs a → b → c → d (along +X, then +Y), which gives a moment along +Z.

Topics:Torque on a current loop Magnetic dipole and bar magnet

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JEE Main 2017 · 9 Apr · Shift 1, Physics question 19