On what range of separation between the charges, the Coulomb's force law is found valid?

Explain attraction and repulsion between two charges on the basis of electrostatic lines of force.

When the separation between two charges is increased, the electric potential energy of the system

Is the force of interaction between two points charges affected due to the presence to another charge?

Assume that there is no repulsive force between the electrons in an atom but the force between positive and negative charges is given by Coulomb's law as usual. Under such circumstances, calculate the ground state energy of a He-atom.