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A charge ‘q’ is placed at the centre of the line joining two equal charges ‘Q’. The system of the three charges will be in equilibrium if ‘q’ is equal to

1. Q/2
2. -Q/4
3. Q/4
4. -Q/2

Correct answer: -Q/4

Solution

For the system to be in equilibrium, the net force on the charge 'q' at the center must be zero. The two charges 'Q' exert forces on 'q' in opposite directions. To balance these forces, 'q' must have a magnitude of Q/4 and an opposite sign to 'Q', i.e., q = -Q/4.

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