Obtain equation of electric energy of a single charge .

Explanation

The external electric field `vecE` and the corresponding external potential V may vary from point to point.
According to definition of electric potential V at a point P is the work done in bringing a unit positive charge from infinity to the point P (We assume the potential at infinity to be zero.)
Thus, work done in bringing a charge q from infinity to the point P in the external field is W =qV.
This work is stored in the form of potential energy of q ,
`:. U =qV`
If the point P has position vector `vecr` relative to origin, then potential energy at point P
`U(vecr)=qV(vecr)`
Means potential energy in an external field = electric charge `xx` electric potential in external field.