In the previous chapter we have covered the essential mathematical tools needed to study EM
fields. We have already mentioned in the previous chapter that electric charge is a
fundamental property of matter and charge exist in integral multiple of electronic charge.
Electrostatics can be defined as the study of electric charges at rest. Electric fields have their
sources in electric charges.
( Note: Almost all real electric fields vary to some extent with time. However, for many
problems, the field variation is slow and the field may be considered as static. For some other
cases spatial distribution is nearly same as for the static case even though the actual field may
vary with time. Such cases are termed as quasistatic.)
In this chapter we first study two fundamental laws governing the electrostatic fields, viz, (1)
Coulomb's Law and (2) Gauss's Law. Both these law have experimental basis. Coulomb's
to use when the distribution is symmetrical.
law is applicable in finding electric field due to any charge distribution, Gauss's law is easier
Coulomb's Law states that the force between two point charges Q1and Q2 is directly
proportional to the product of the charges and inversely proportional to the square of the
distance between them.
Point charge is a hypothetical charge located at a single point in space. It is an idealized
model of a particle having an electric charge.
, where k is the proportionality constant.
In SI units, Q1 and Q2 are expressed in Coulombs(C) and R is in meters.
Force F is in Newtons (N) and
is called the permittivity of free space.
(We are assuming the charges are in free space. If the charges are any other dielectric
medium, we will use
dielectric constant of the medium).
is called the relative permittivity or the