Charges of +3 µC and –5 µC are 2 mm from each other. The –5 µC charge is replaced with a +5 µC charge.

By Coulomb’s law, the force magnitude depends on the product of the charges’ absolute values and their separation. Changing –5 µC to +5 µC leaves the magnitudes, 3 µC and 5 µC, and the 2 mm separation unchanged, so the force is the same strength. The original opposite charges attract, while the new like charges repel, so the force reverses direction. The correct choice is “same force, but in the opposite direction.”

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