Electric Charges and Fields Class 12 Notes Physics Chapter 1
▶Introduction
The study of static charges is called electrostatics and this complete electrostatic
will be discussed in two chapters. In this chapter, we begin with a discussion of
electric charge, some properties of charged bodies, and the fundamental electric
force between two charged bodies.
▶What is Electric Charge?
Electric Charge is a fundamental property of a matter which is responsible for
electric forces between the bodies. Two electrons placed at a small separation are
found to repeal each other, this repulsive force (Electric force) is only because of
the electric charge on electrons.
When a glass rod is rubbed with silk, the rod acquires one kind of charge and the
silk acquires the second kind of charge. This is true for any pair of objects that are
rubbed to be electrified. Now if the electrified glass rod is brought in contact with
silk, with which it was rubbed, they no longer attract each other.
Types of Electric Charge
There are two types of charges that exist in our nature.
● Positive Charge
● Negative Charge
If any object loses its electrons then they get a positive charge. It is denoted by (+q)
sign. If any object gain electrons from another object then it gets a negative charge.
It is denoted by (-q) sign. The charges were named as positive and negative by the
American scientist Benjamin Franklin. If an object possesses an electric charge, it
is said to be electrified or charged. When it has no charge it is said to be neutral.
▶Basic Properties of Electric Charge
,The important properties and characteristics of electric charge are given below.
(i) Attraction and Repulsion:- Like charges repel each other while unlike charges
attract each other.
(ii) Electric Induction:- When a charged object brings to contact with another
uncharged, it gets the opposite charge of the charged object. It is called charging by
induction.
(iii) Charge is Quantized:- An object that is electrically charged has an excess or
deficiency of some whole number of electrons. Since electrons cannot be divided
into fractions of electrons, it means that the charge of an object is a whole-number
multiple of the charge of an electron. For example, it cannot have a charge equal to
the charge of 0.5 or 1000.5 electrons.
Mathematically q = ± ne, Here n = 1, 2, 3 and e = 1.6 × 10–19 coulomb.
(iv) Electric Charge is Conserved:- According to this property, "An electric charge neither
can be created nor can be destroyed" i.e. total net charge of an isolated system is always
conserved. Thus, when a glass rod is rubbed with silk cloth, both glass rod, and silk cloth
acquire opposite charges in the same quantity. Thus, the total amount of charge remains the
same before rubbing as well as after rubbing.
▶Conductors and Insulators
Some substances easily allow the passage of electricity through them while others
do not. Substances that allow electricity to pass through them easily are called
‘conductors’. They have electrons that are free to move inside the material. Metals,
human and animal bodies, earth, etc. are examples of conductors. Non-metals e.g.,
glass, plastic, and wood are ‘insulators’ because they do not easily allow the
passage of electricity through them.
Most substances are either conductors or insulators. There is a third category called
‘semiconductors’ which are intermediate between conductors and insulators
because they partially allow movement of charges through them.
▶Charging by Induction
, Now as we know that two oppositely charged bodies attract each other. But it also
has been our observation that a charged body attracts a neutral body as well. This is
explained on the basis of charging by induction. In the induction process, two
bodies (at least one body must be charged) are brought very close, but they never
touch each other.
Charging by Induction
Let us examine how a charged body attracts an uncharged body. Imagine a conducting or
partially conducting body (sphere here) is kept on an insulating stand and a charged rod
(positive, for example) is brought very close to it. It will attract electrons to its side and the
farther end of the sphere will become positively charged as it is deficient in electrons.
▶Coulomb’s Law
In 1785 Charles Coulomb (1736-1806) experimentally established the fundamental
law of electric force between two stationary charged particles. He observed that An
electric force between two charged particles has the following properties:
● It is directed along a line joining the two particles and is inversely proportional
to the square of the separation distance r, between them.
● It is proportional to the product of the magnitudes of the charges, |q1| and |q2|,
of the two particles.
● It is attractive if the charges are of opposite sign and repulsive if the charges
have the same sign.
From these observations, Coulomb proposed the following mathematical form for the
electric force between two charges. The magnitude of the electric force F between
charges q1 and q2 separated by a distance r is given by
▶Introduction
The study of static charges is called electrostatics and this complete electrostatic
will be discussed in two chapters. In this chapter, we begin with a discussion of
electric charge, some properties of charged bodies, and the fundamental electric
force between two charged bodies.
▶What is Electric Charge?
Electric Charge is a fundamental property of a matter which is responsible for
electric forces between the bodies. Two electrons placed at a small separation are
found to repeal each other, this repulsive force (Electric force) is only because of
the electric charge on electrons.
When a glass rod is rubbed with silk, the rod acquires one kind of charge and the
silk acquires the second kind of charge. This is true for any pair of objects that are
rubbed to be electrified. Now if the electrified glass rod is brought in contact with
silk, with which it was rubbed, they no longer attract each other.
Types of Electric Charge
There are two types of charges that exist in our nature.
● Positive Charge
● Negative Charge
If any object loses its electrons then they get a positive charge. It is denoted by (+q)
sign. If any object gain electrons from another object then it gets a negative charge.
It is denoted by (-q) sign. The charges were named as positive and negative by the
American scientist Benjamin Franklin. If an object possesses an electric charge, it
is said to be electrified or charged. When it has no charge it is said to be neutral.
▶Basic Properties of Electric Charge
,The important properties and characteristics of electric charge are given below.
(i) Attraction and Repulsion:- Like charges repel each other while unlike charges
attract each other.
(ii) Electric Induction:- When a charged object brings to contact with another
uncharged, it gets the opposite charge of the charged object. It is called charging by
induction.
(iii) Charge is Quantized:- An object that is electrically charged has an excess or
deficiency of some whole number of electrons. Since electrons cannot be divided
into fractions of electrons, it means that the charge of an object is a whole-number
multiple of the charge of an electron. For example, it cannot have a charge equal to
the charge of 0.5 or 1000.5 electrons.
Mathematically q = ± ne, Here n = 1, 2, 3 and e = 1.6 × 10–19 coulomb.
(iv) Electric Charge is Conserved:- According to this property, "An electric charge neither
can be created nor can be destroyed" i.e. total net charge of an isolated system is always
conserved. Thus, when a glass rod is rubbed with silk cloth, both glass rod, and silk cloth
acquire opposite charges in the same quantity. Thus, the total amount of charge remains the
same before rubbing as well as after rubbing.
▶Conductors and Insulators
Some substances easily allow the passage of electricity through them while others
do not. Substances that allow electricity to pass through them easily are called
‘conductors’. They have electrons that are free to move inside the material. Metals,
human and animal bodies, earth, etc. are examples of conductors. Non-metals e.g.,
glass, plastic, and wood are ‘insulators’ because they do not easily allow the
passage of electricity through them.
Most substances are either conductors or insulators. There is a third category called
‘semiconductors’ which are intermediate between conductors and insulators
because they partially allow movement of charges through them.
▶Charging by Induction
, Now as we know that two oppositely charged bodies attract each other. But it also
has been our observation that a charged body attracts a neutral body as well. This is
explained on the basis of charging by induction. In the induction process, two
bodies (at least one body must be charged) are brought very close, but they never
touch each other.
Charging by Induction
Let us examine how a charged body attracts an uncharged body. Imagine a conducting or
partially conducting body (sphere here) is kept on an insulating stand and a charged rod
(positive, for example) is brought very close to it. It will attract electrons to its side and the
farther end of the sphere will become positively charged as it is deficient in electrons.
▶Coulomb’s Law
In 1785 Charles Coulomb (1736-1806) experimentally established the fundamental
law of electric force between two stationary charged particles. He observed that An
electric force between two charged particles has the following properties:
● It is directed along a line joining the two particles and is inversely proportional
to the square of the separation distance r, between them.
● It is proportional to the product of the magnitudes of the charges, |q1| and |q2|,
of the two particles.
● It is attractive if the charges are of opposite sign and repulsive if the charges
have the same sign.
From these observations, Coulomb proposed the following mathematical form for the
electric force between two charges. The magnitude of the electric force F between
charges q1 and q2 separated by a distance r is given by
