The
external sources may be known, but often they are unknown or
unspecified; what is specified is the electric field E or the electrostatic
potential V due to the external sources We assume that the charge q
does not significantly affect the sources producing the external field This
is true if q is very small, or the external sources are held fixed by other
unspecified forces 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