Demagnetization of bar magnets

Steps:

• Method 1 heats the magnet above its Curie temperature, randomizing atomic magnetic domains and destroying alignment upon cooling.
• Method 2 exposes the magnet to an alternating magnetic field from the coil's current, gradually reducing magnetization as the field strength decreases during slow removal.
• Method 3 applies mechanical shock while aligned east-west, which aligns domains with Earth's magnetic field rather than randomizing them.
• Only methods 1 and 2 effectively disrupt permanent magnetism, as confirmed by standard demagnetization principles.

Why B is correct:

• Heating exceeds Curie temperature (where thermal energy overcomes magnetic domain alignment) and AC fields (from coil) apply hysteresis loop traversal to zero net magnetism.

Why the others are wrong:

• A omits effective AC demagnetization in method 2.
• C includes ineffective hammering, which strengthens rather than removes magnetism.
• D pairs valid heating with invalid hammering.

Final answer: B