Magnetic torque on suspended magnet in fields

Steps:

• Identify conditions in each experiment that produce a magnetic field interacting with the suspended magnet.
• Experiment 1: Current in nearby wire creates field, exerting torque causing movement.
• Experiment 2: Earth's magnetic field or bar magnet nearby induces deflection, leading to swing.
• Experiment 3: Solenoid with current generates field, resulting in rotational movement.
• Conclude movement occurs when net torque from B-field on magnetic dipole is non-zero.

Why A is correct:

• All experiments involve external magnetic fields (from currents or permanent magnets) producing torque τ = m × B on the dipole moment m, causing movement per magnetic dipole law.

Why the others are wrong:

• B excludes 3, but solenoid field in 3 causes torque and movement.
• C excludes 2, but field from nearby magnet or Earth in 2 induces deflection.
• D excludes 1, but wire current in 1 generates field exerting force/torque.

Final answer: A