Question
Sat April 05, 2014 By: Dinesh Malik

# Two positive point charges +4Q and +Q are separated by distance of 'a' units. What is magnitude and nature and also position of charge such that system is in equilibrium?

Mon April 07, 2014
Let the charge'q' be placed at a point O along the line joining the other two charges.
For a system of charges to be in equilibruim.
net force on all the three charges must be zero.
If charge q is positive in nature,it will experience forces due to other two charges in opposite directions and hence net force on charge q may be 0.But the force on the charge 4Q and Q will not be 0.It is because the force on a charge Q or 4Q due to other two charges will act in same direction.
However if charge q placed is negative,then on a charge Q or 4Q ,forces due to other two charges will act in the opposite direction.Hence the third charge placed must be negative.

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