Q. An equilateral triangle frame PQR of mass M and side a is kept under the influence of magnetic force due to inward perpendicular magnetic field B and gravitational field as shown in the figure. The magnitude and direction of current in the frame so that the frame remains at rest is

(a) $ \displaystyle I = \frac{2Mg}{aB}$ , anticlockwise

(b) $ \displaystyle I = \frac{2Mg}{aB}$ , clockwise

(c) $ \displaystyle I= \frac{Mg}{aB}$ , anticlockwise

(d) $\displaystyle I= \frac{Mg}{aB}$ , clockwise

Ans: (b)

Sol:

For equilibrium ,

Mg = I l B

Mg = I(a/2)B

I = 2Mg/aB (clockwise a/c to Lenz’ law)