Unit II Electrostatics
In this chapter we will discuss on the followings:
• Coulomb's Law
• Electric Field & Electric Flux Density
• Gauss's Law with Application
• Electrostatic Potential, Equipotential Surfaces
• Boundary Conditions for Static Electric Fields
• Capacitance and Capacitors
• Electrostatic Energy
• Laplace's and Poisson's Equations
• Uniqueness of Electrostatic Solutions
• Method of Images
• Solution of Boundary Value Problems in Different Coordinate Systems
Introduction
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 quasi-static.)
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 law is applicable in finding electric field due to any charge distribution, Gauss's law is easier to use when the distribution is symmetrical.
Coulomb's Law
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 idealised model of a particle having an electric charge.
Mathematically, ,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 instead where is called the relative permittivity or the dielectric constant of the medium).
Therefore .......................(2.1)
As shown in the Figure 2.1 let the position vectors of the point charges Q1and Q2 are given by and . Let represent the force on Q1 due to charge Q2.

Fig 2.1: Coulomb's Law
The charges are separated by a distance of . We define the unit vectors as and ..................................(2.2) can be defined as . Similarly the force on Q1 due to charge Q2 can be calculated and if represents this force then we can write
When we have a number of point charges, to determine the force on a particular charge due to all other charges, we apply principle of superposition. If we have N number of charges Q1,Q2,.........QN located respectively at the points represented by the position vectors ,,...... , the force experienced by a charge Q located at is given by, .................................(2.3)
Electric Field
The electric field intensity or the electric field strength at a point is defined as the force per unit charge. That is or, .......................................(2.4)
The electric field intensity E at a point r (observation point) due a point charge Q located at (source point) is given by:
..........................................(2.5)
For a collection of N point charges Q1 ,Q2 ,.........QN located at ,,......, the electric field intensity at point is obtained as
........................................(2.6)
The expression (2.6) can be modified suitably to compute the electric filed due to a continuous distribution of charges.
In figure 2.2 we consider a continuous volume distribution of charge (t) in the region denoted as the source region.