Three point charges 4q, Q and q lie on a straight line at positions x = 0, x = L/2 and x = L respectively. For the net force on the charge q to be zero, the value of Q must be:

-q
-2q
-q/2
4q

Correct answer: -q

Solution

Force on q from 4q at distance L is k(4q)(q)/L². Force from Q at distance L/2 is kQq/(L/2)² = 4kQq/L². For balance these must cancel: 4q + 4Q = 0, so Q = -q (attractive, opposite sign).

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