What is the difference between a permanent magnet and an electromagnet?

• A. An electromagnet has a constant field; a permanent magnet's field changes with time.
• B. A permanent magnet has a constant magnetic field; an electromagnet's field requires current and can be switched on/off.
• C. Permanent magnets produce fields only when powered by electricity.
• D. Electromagnets and permanent magnets both have constant fields regardless of current.

Answer: (C)