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Four point charges –Q, –q, 2q and 2Q are placed, one at each corner of the square. The relation between Q and q for which the potential at the centre of the square is zero is :
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- Q = – q
- Q = – 1/q
- Q = q
- Q = 1/q
Correct Option: A
Let the side length of square be 'a' then potential at centre O is (Given) 
| V = | + | + | + | = 0 | ||||||||||||||||
 |  | | | | | | | |||||||||||||
| √2 | √2 | √2 | √2 | |||||||||||||||||
= – Q – q + 2q + 2Q = 0 = Q + q = 0
Q = – q
