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)