To obtain the work done per unit test
charge, we should take an infinitesimal test charge
dq, obtain the work done dW in bringing it from
infinity to the point and determine the ratio
dW/dq Also, the external force at every point of the
path is to be equal and opposite to the electrostatic
force on the test charge at that point 2 3  POTENTIAL DUE TO A POINT CHARGE
Consider a point charge Q at the origin (Fig