Thread: charge probs...

please HELP

problem 1:Two identical point charge Q are kept at a distance r from each other. A third point charge q is placed on the line joining the above two charges such that all the three charges are in equilibrium. What is the magnitude, sign and the position of the third charge ?

problem 2:Three point charges q,2q and 8q are to be placed on a 9 cm long straight line.Find the positions where the charges should be placed such that the potential energy of this system is minimum. in this situation, what is the electric field at charge q due to the other two particles?

What have you tried? Where are you stuck?

any ideas about the first one???i think we have to use the coulombs law in vector form and keep the net force 0.also,the third charge must be kept somewhere in between the two identical charges for equilibrium to happen
pls help!

ok please help me do the 1st problem...
i can figure out that the charge q should be opposite the charge of Q and that it must be positioned in the mid point of the line joining the two charges. but cant figure out the magnitude of the charge q.

please reply fast!