What is the best method of demagnetizing a steel bar magnet?

Analyze the question: It asks for the best method of demagnetizing a steel bar magnet. Several methods work, but one is considered the most effective and reliable.

Concept: A magnet has its tiny magnetic domains aligned in one direction. To demagnetize it, we must make the domains point randomly again so their effects cancel out.

Methods of demagnetization:

1. Hammering: Jolting the magnet knocks the domains out of alignment. It works partly, but it is crude and usually leaves some magnetism.
2. Heating: Heating above the Curie temperature destroys the magnetism, but it is less practical and can affect the steel's properties.
3. Rough handling: Similar to hammering, it is uncontrolled and incomplete.
4. Solenoid method (using alternating current): • Place the magnet inside a solenoid lying east-west. • Connect the solenoid to an a.c. supply. The current reverses direction many times per second, so the domains are repeatedly flipped back and forth. • Slowly withdraw the magnet from the solenoid while the current is still flowing, until it is far away (about 1 m or more), and then switch off. • The field it experiences gradually falls to zero, leaving the domains randomly arranged.

This is the most thorough and controllable method, so it is regarded as the best.

Answer: The solenoid method, which is option D.

Exam tip: If "best" or "most effective" appears in a demagnetization question and the solenoid (a.c.) method is among the options, choose it. Remember: hammering and heating are possible, but the solenoid with a.c. is the best.