Temporary Magnets Become Magnetized Only in External Fields

Steps:

• Define temporary magnet as a soft magnetic material (e.g., soft iron) that gains magnetism in an external field but loses it when removed.
• Examine transformer core: Soft iron core magnetizes cyclically with alternating current's field, acting as temporary magnet during operation.
• Examine compass needle: Permanent magnet (hardened steel) retains magnetism independently, aligning with Earth's field without needing external magnetization.
• Examine iron filings: Soft iron particles temporarily magnetize and align in the applied magnetic field to visualize lines.

Why C is correct:

• Transformer core and iron filings use soft iron that magnetizes only in the presence of an external field, per the definition of temporary (induced) magnetism.

Why the others are wrong:

• A: Includes compass needle, a permanent magnet that retains magnetism without external field.
• B: Includes compass needle (permanent) and excludes iron filings (temporary).
• D: Includes compass needle (permanent) and excludes transformer core (temporary).

Final answer: C