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Three point charges q, 2q and -(x q)/3 are placed at the vertices of an equilateral triangle (each pair separated by the same distance a). For the total electrostatic potential energy of the system to be zero, find the value of x.

1. 1
2. 2
3. 3
4. 4

Correct answer: 2

Solution

Sum of pairwise products: (q)(2q) + (q)(-xq/3) + (2q)(-xq/3) = 2q² - xq²/3 - 2xq²/3 = 2q² - x q². Setting the total to zero: 2q² - x q² = 0 gives x = 2.

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