Direction of magnetic force on current

Steps:

• Force direction on a current-carrying wire is given by Fleming's left-hand rule: forefinger (magnetic field), middle finger (current), thumb (force).
• Original setup produces downward force.
• Change 1 increases current magnitude but keeps direction, so force remains downward.
• Change 2 increases field strength but keeps direction, so force remains downward.
• Change 3 reverses current, reversing force to upward.
• Change 4 swaps poles, reversing field direction and thus force to upward.

Why A is correct:

• Changes 1 and 2 only affect force magnitude (via F = BIL sinθ), not direction, keeping force downward per Fleming's rule.

Why the others are wrong:

• B includes 3, which reverses force direction.
• C includes 4, which reverses force direction.
• D includes 3 and 4, both of which reverse force direction.

Final answer: A