The given figure shows two magnetized needles and a magnet P with their magnetic moments, shown with the arrow sign. Which of the following statements are correct in respect of this configuration?
(A) $Q_1$ is in stable equilibrium
(B) $Q_2$ is in unstable equilibrium
(C) $Q_1$ is in unstable equilibrium
(D) $Q_2$ is in stable equilibrium
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (B) only
Correct options: (A) and (B)
Magnetic potential energy: $U=-\vec{m}\cdot\vec{B}=-mB\cos\theta$. Minimum (stable) when $\theta=0$ (parallel), maximum (unstable) when $\theta=\pi$ (antiparallel).
At Q1 the needle's magnetic moment is parallel to the local field $\vec{B}$ → $\theta=0$ → stable.
At Q2 the needle's magnetic moment is antiparallel to the local field $\vec{B}$ → $\theta=\pi$ → unstable.