Let P and Q be two points situated at a very small distance dr from each other in the electric field \(\vec E\) of source charge +q lying at O Fig. The points P and Q and so close that the electric field \(\vec E\) between P and Q is almost uniform as shown in Fig.
Let potential difference between P and Q = dV
By definition,
Potential difference between P and Q = Work done in moving a unit test charge from P to Q.
If a small positive test charge q0 is moved from point P to Q, then