Two charges -Q. and -4Q. are a distance l apart. These two charges are free to move but do not because there is a third charge nearby. What must be the magnitude of the third charge and its placement in order for the first two to be in equilibrium?

Question

anonymous · Answer

The third charge is going to sit somewhere on a line joining the first two charges
Just use coulombs law to calculate the forces and require the total force on each of the negative charges to be zero.

There are two possible solutions.