Electric force between two point charges q&Q at rest is F. Now, if a charge -q is placed next to 'q', what will be the (A) force on Q due to q (B)...

, where k is a constant and r is the distance between the two charges. In the given problem, we have the two charges q and Q and the force F between the two of them is known.

When another charge is added to the system, the total force on Q will change, since there will be another force on Q acting from another charge. However, the force from the original charge q on Q will remain unchanged, it will still be determined by the Coulomb's Law and equal F.

So, the answer for A) is F.

If another charge equals -q, and it is placed next the charge q, then the distance between this charge and Q is the same as that between q and Q. Therefore,  the force between this charge and Q will be

, the force equal in magnitude but opposite in direction to the force F between q and Q.

The total force on Q will then be F + (-F) = 0.

B) The total force on Q is 0.