Two identical metal balls with charges + 2Q and - Q are separated by some distance, and exert a force F on each other. They are joined by a conducting wire, which is then removed. The force between them will now be_________?
Correct answer: D. F/8
- A. F
- B. F/2
- C. F/4
- D. F/8
Explanation
Initially, the magnitude of the force is proportional to |(+2Q)(-Q)| = 2Q². After connection, the total charge is +Q and identical balls share it equally, so each has +Q/2; the new force is proportional to (Q/2)(Q/2) = Q²/4. Therefore, the new force is (Q²/4)/(2Q²) × F = F/8. The likely choice is c, because students may calculate only the final product Q²/4 and miss comparison with the initial product 2Q².
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Coulomb's law gives the electrostatic force between two point charges in terms of their magnitudes, separation and the medium between them. Questions involve attraction and repulsion, the inverse square relationship, superposition of forces and vector direction, which must not be confused with the electric field produced by a charge.
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