Electrostatic Potential Energy
Three charges, each +q, are placed at the corners of an isosceles triangle ABC of sides BC and AC, 2. D and E are the mid points of BC and CA. The work done in taking a charge Q from D to E is

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Answer: (d)
- .
- With , the triangle is symmetric about the bisector of angle C, which swaps A with B and so D (mid BC) with E (mid CA).
- Equal charges at the corners give , so .
- So the answer is (d) zero.