NCERT Book Class 12 Physics Chapter 2 Electrostatic Potential and Capacitance

Chapter Two

ELECTROSTATIC
POTENTIAL AND
CAPACITANCE

2.1 INTRODUCTION
In Chapters 5 and 7 (Class XI), the notion of potential energy was
introduced. When an external force does work in taking a body from a
point to another against a force like spring force or gravitational force,
that work gets stored as potential energy of the body. When the external
force is removed, the body moves, gaining kinetic energy and losing
an equal amount of potential energy. The sum of kinetic and
potential energies is thus conserved. Forces of this kind are called
conservative forces. Spring force and gravitational force are examples of
conservative forces.
Coulomb force between two (stationary) charges is also a conservative
force. This is not surprising, since both have inverse-square dependence
on distance and differ mainly in the proportionality constants – the
masses in the gravitational law are replaced by charges in Coulomb’s
law. Thus, like the potential energy of a mass in a gravitational
field, we can define electrostatic potential energy of a charge in an
electrostatic field.
Consider an electrostatic field E due to some charge configuration.
First, for simplicity, consider the field E due to a charge Q placed at the
origin. Now, imagine that we bring a test charge q from a point R to a
point P against the repulsive force on it due to the charge Q. With reference