Electrostatics 1 Question 10

10. 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

(1987, 2M)

(a) Q2

(b) Q4

(c) +Q4

(d) +Q2

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Answer:

Correct Answer: 10. (d)

Solution:

  1. Since, q is at the centre of two charges Q and Q, net force on it is zero, whatever the magnitude and sign of charge

Q˙q˙Q˙

on it. For the equilibrium of Q,q should be negative because other charge Q will repel it, so q should attract it. Simultaneously these attractions and repulsions should be equal.

or

14πε0QQr2=14πε0Qq(r/2)2

or with sign

q=Q4



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