UPSC NDA I 2018 General Ability Solved Paper

Concept: A bar magnet behaves as a magnetic dipole with two equal and opposite poles. When placed in a uniform magnetic field, the forces acting on the two poles are equal in magnitude and opposite in direction. Hence, the net translational force acting on the magnet is zero. Explanation: Let a bar magnet of pole strength $m$ and magnetic length $2l$ be placed in a uniform magnetic field $B$. The north pole experiences a force $F_N = +mB$ in the direction of the field. The south pole experiences a force $F_S = -mB$ opposite to the direction of the field. The net force is the sum of these two forces: $F_{net} = F_N + F_S = mB - mB = 0$. Thus, the bar magnet does not experience any net force; it only experiences a torque $\tau = mB \sin \theta$ that tends to rotate it. This outcome is independent of the length of the magnet or its temperature. Note that in a non-uniform magnetic field, the forces on the two poles would be different, leading to a non-zero net force. Answer: Zero (Option A).