2 charges +q and+q are seperated by a distance r.If the seperation is decreased,the work done should 

be done :

a) by an external agent

b) by the electric field

c) against the electric field

d)none of these

 

In the above case if one of charges is replaced by -q , what would be the answer? 

When two charges +q and +q are seperated by a distance r, the electrostatic force of repulsion devloped between them which keep them apart. If the seperation decreased still the charges keep away from each other but to stay at that position, an external force is nedded. Hence in this case work done by an external agent. 

 

When one of charges is replaced by -q  seperated by distance is called electric diple and electrostatic force of attraction devloped between them.

Electric potential due to charge varies inversely as the distance from the charge i.e. 

Work done is given as w = Vq. Therefore, work done also varies inversely as the distance from the charge. Hence, in this case work done by an electric field.