Answer: halves.
- A halves
- B doubles
- C becomes four times
- D is unchanged
Correct answer: A. halves
Explanation: T ∝ 1/√B, so 4× field gives T/2 - it halves.
Magnetic field lines form closed loops: outside the bar magnet they run from the N pole to the S pole; inside, they continue from S back to N, so the lines never start or end anywhere.
Concept context
Bar magnets, Earth's magnetism, and how different materials respond to an external magnetic field - diamagnetic, paramagnetic, and ferromagnetic behaviour.