Even if q is finite, its influence on the external sources
may still be ignored in the situation when very strong sources far away
at infinity produce a finite field E in the region of interest Note again that
we are interested in determining the potential energy of a given charge q
(and later, a system of charges) in the external field; we are not interested
in the potential energy of the sources producing the external electric field The external electric field E and the corresponding external potential
V may vary from point to point By definition, V at a point P is the work
done in bringing a unit positive charge from infinity to the point P