Three charges -q, Q and -q are placed at equal distances on a straight line. If the potential energy of the system of three charges is zero, then what is the ratio of Q:q?
Correct answer: D. 1 : 4
- A. 1 : 1
- B. 1 : 2
- C. 1 : 3
- D. 1 : 4
Explanation
To achieve zero potential energy, calculate the potential energy between each pair of charges. Using the formula U = kq1q2/r, the total potential energy is the sum of the energies of all pairs: Utotal = U(-q, Q) + U(Q, -q) + U(-q, -q). Setting this sum to zero and solving for the ratio Q:q, we find 1:4 is necessary to balance the contributions and achieve zero net potential energy. Other options result in non-zero net potential energy, failing to satisfy the condition.
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