A charge q interacts with this field and experiences
a force F given by
F  =  q  E   = q Q ˆr   / (4pe0) r 2
(4 2)
As pointed out in the Chapter 1, the field E is not just
an artefact but has a physical role It can  convey  energy
and momentum and is not established instantaneously
but takes finite time to propagate The concept of a field
was specially stressed by Faraday  and was incorporated
by Maxwell in his unification of electricity and magnetism