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A charge q is kept at the midpoint of the line segment joining two identical charges Q. If the arrangement is to remain in equilibrium, what must be the value of q?

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

Correct answer: -Q/4

Solution

For the system to be in equilibrium, the net force acting on the charge q must be zero. The attractive forces from the two identical charges Q must balance the repulsive force from q, which occurs when q is negative and equal to -Q/4, allowing the forces to counteract each other.

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