Question

which statement best compares a permanent magnet and an electromagnet?
a permanent magnet has a north pole and a south pole, but an electromagnet only has a south pole.
a permanent magnet required an external source of energy, but an electromagnet produces its own energy.
a permanent magnet has a fixed magnetic field strength but the magnetic field strength of an electromagnet can be changed.
the magnetic lines from a permanent magnet emerge from the north pole, but they emerge from the south pole of an electromagnet.

Explanation:

• Analyze Option 1: Both permanent magnets and electromagnets have a north and a south pole. So this option is incorrect.
• Analyze Option 2: A permanent magnet does not require an external energy source to maintain its magnetism, while an electromagnet requires an electric current (external energy source) to produce a magnetic field. So this option is incorrect.
• Analyze Option 3: A permanent magnet has a fixed magnetic field strength determined by its material and structure. An electromagnet's magnetic field strength can be changed by varying the current flowing through its coil. This statement is correct.
• Analyze Option 4: Magnetic field lines emerge from the north pole and enter the south pole for both permanent magnets and electromagnets. So this option is incorrect.

Answer:

C. A permanent magnet has a fixed magnetic field strength but the magnetic field strength of an electromagnet can be changed.