Magnetic force on current-carrying wire

Steps:

• Recall force formula: F = B I L sinθ, where B is magnetic field strength, I is current, L is wire length, θ is angle.
• Evaluate A: Reversing current and poles changes force direction but keeps B and I magnitudes same, so F unchanged.
• Evaluate B: Reversing current reverses force direction but keeps magnitude same.
• Evaluate C: Stronger magnet increases B, directly increasing F.
• Evaluate D: Thicker wire with same I keeps I and B same, so F unchanged.

Why C is correct:

• Force F is directly proportional to B (from F = B I L sinθ); stronger magnet raises B, raising F.

Why the others are wrong:

• A: Reversals preserve B and I magnitudes, so F magnitude stays the same.
• B: Only reverses force direction, not its size.
• D: Same current I means same force, regardless of wire thickness.

Final answer: C