Basic Properties of Electric Charge – Class 12 Notes (Electric Charges and Fields)

Electric Charge

Electric charge is a fundamental property associated with elementary particles. It accompanies fundamental particles whenever they exist. Electron, proton and neutron are a few examples of fundamental particles.

According to William Gilbert,

The charge is something possessed by material objects that makes it possible for them to exert electrical forces and to respond to electrical force.

We know that in an atom, electrons revolve around a nucleus which has a positive charge.

Types of Charges

Charges are of two kinds:

(i) negative charge
(ii) positive charge

In an atom, electrons are particles having negative charge. The nucleus consists of protons and neutrons. Protons have positive charge, while neutrons are electrically neutral. Neutrons are electrically neutral and carry zero charge.

The experiments lead to the fundamental results that:

(1) Like charges repel

(2) Unlike charges attract

The electric force between two electrons is the same as the electric force between two protons kept at the same distance apart; in both cases, the force is repulsive.

The electric force between an electron and a proton placed at the same distance apart is attractive in nature.

The assignment of negative charge to the electron and positive charge to the proton is only a convention. It does not mean that the charge on an electron is smaller than that on a proton.

The importance of electric forces lies in the fact that almost all phenomena in our daily life are governed by them. All matter around us consists of atoms or molecules in which electric charges are exactly balanced.

Unit of Charge

• The charge on a system can be measured by comparing it with the charge on a standard body.
• The SI unit of electric charge is the coulomb, written as C.
• One coulomb is the amount of charge flowing through a conductor in 1 second when the current is 1 ampere.
• The charge on an electron is $e = -1.602 \times 10^{-19}\,\text{C}$ and the charge on a proton is equal in magnitude but positive.
• Thus, $1\,\text{C} = 6.25 \times 10^{18}$ elementary charges.

Note: One coulomb is a very large unit of charge. In most electrostatic phenomena, the charges involved are of the order of microcoulomb ($\mu$C) or nanocoulomb (nC).

Conductors and Insulators

There are materials in which electric charges can flow easily, while in other materials charges cannot flow easily. Substances through which electric charges can flow easily are called conductors. All metals like copper, aluminium, etc., are good conductors of electricity.

Substances through which electric charges cannot flow easily are called insulators. Examples of insulating materials are glass, rubber, mica, plastic and dry wood.

The presence or absence of free electrons in a material determines whether it is a conductor or an insulator. Conductors have free electrons which are loosely bound to the nuclei of their atoms. Insulators do not have free electrons; in them, electrons are strongly bound to the nuclei.

It is important to note that:

1. The charge transferred to a conductor gets distributed over its entire surface due to mutual repulsion between like charges.

2. The charge transferred to an insulator remains localised at the point where it is placed.

3. Insulators can be charged, but the charge does not flow freely through them.

Semiconductors form the third class of materials. Their electrical properties lie between those of conductors and insulators. Silicon and germanium are examples of semiconductors.

Note: The electrical conductivity of semiconductors increases with increase in temperature.

Question 1. Why do metals conduct electricity?

Answer. Atoms of metals have outer electrons which are not bound to any one atom. These electrons can move freely throughout the metal lattice and hence conduct electricity.

Methods of Charging

There are three main methods of charging a body:

(i) Charging by Rubbing (Frictional Electricity)

Rubbing, as the term suggests, involves moving two objects back and forth against each other. One of the simplest ways to experience electric charge is by rubbing certain bodies together.

During rubbing, electrons are transferred from one material to another. As a result, one body acquires a positive charge and the other acquires an equal negative charge. These charges remain on the surfaces of the materials until they are discharged.

For example, when a comb is passed through dry hair, the comb becomes charged and can attract small pieces of paper. This happens because the comb either gains or loses electrons during rubbing. The charged comb then exerts electric force on the neutral paper pieces, resulting in attraction.

Many materials, when rubbed, show this effect and attract light objects such as bits of paper, straw, or feathers.

The explanation of charging by rubbing is straightforward. Material bodies normally contain equal numbers of protons and electrons and are electrically neutral. When a glass rod is rubbed with a silk cloth, electrons are transferred from the glass rod to the silk cloth. As a result, the glass rod becomes positively charged and the silk cloth becomes negatively charged.

Charging of a glass rod by rubbing with silk, or a comb by passing it through dry hair, are examples of frictional (static) electricity.

(ii) Charging by Induction (Electrostatic Induction)

The temporary electrification of a conductor when a charged body is brought near it, without physical contact, is called electrostatic induction.

When a charged body is brought near an uncharged conductor, free electrons inside the conductor redistribute themselves due to electrostatic forces. As a result, opposite charges appear on the nearer side and like charges on the farther side.

In this process, there is no transfer of electrons between the charged body and the conductor because there is no physical contact.

Important Note: Charging by induction is possible only for conductors, as free movement of electrons is required.

(iii) Charging by Conduction (Charging by Touch)

Charging by conduction occurs when a charged conductor is brought into direct contact with an uncharged conductor.

Because metals have a large number of free electrons, charge flows from one conductor to the other on contact. After separation, both conductors acquire charges of the same nature.

Note: In charging by conduction, the original charged body always loses some charge.

Comparison of Methods of Charging

Basic Properties of Electric Charge

(i) Additivity of Charges

Electric charges add up like real numbers, i.e., electric charge is a scalar quantity. If a system has n charges $q_1, q_2, q_3, \ldots, q_n$, then the total charge of the system is given by

Electric charge is a scalar quantity, as it adds algebraically and has no direction.

Proper sign must be used while adding charges.

For example, if
$q_1 = +1\,\text{C}$, $q_2 = -2\,\text{C}$ and $q_3 = +4\,\text{C}$, then

(ii) Conservation of Electric Charge

Law of conservation of charge: The algebraic sum of electric charges in an isolated system remains constant.

• The total electric charge of an isolated system remains constant.
• Electric charge can neither be created nor destroyed.
• Electric charge can only be transferred from one body to another.

Note: Conservation of charge is valid in all physical processes, including nuclear reactions.

(iii) Quantization of Electric Charge

• The smallest possible unit of electric charge is the magnitude of charge on an electron,

The elementary charge $e$ is a fundamental constant of nature.

• The charge $Q$ on any body is given by

• Thus, charge always exists in integral multiples of $e$.
• This discrete nature of charge is called quantization of charge.

Important Note: At the macroscopic level, charge appears continuous because $n$ is very large.

Conceptual Note: Fractional charges such as $\pm \frac{e}{3}$ and $\pm \frac{2e}{3}$ exist only on quarks, which are not observed in isolation. Hence, all observable charges are always integral multiples of the elementary charge $e$.

Next Topics to Study in Electric Charges and Fields

Next Topics in Electrostatics

• Electrical and Electrostatic Force
• Coulomb’s Law and Vector Form
• Electric Field and Calculation of Electric Field
• Electric Field Lines

Practice Questions

• Electric Charges and Field Questions and Answers
• Electrostatics Multiple Choice Questions

Electric Charge – Quick Revision (Handwritten Notes)

• Notes
  • Electric Charge , Basic properties of electric charge and Frictional Electricity
  • Electrical and electrostatic force
  • Coulomb's Law (with vector form)
  • Principle Of Superposition
  • Electric Field & Calculation of Electric Field
  • Electric Field Due to Line Charge
  • Electric Field Lines
  • Electric Flux
  • Electric Dipole and Dipole Moment
  • Electric Dipole Field
• Assignment
  • Electrostatics Multiple Choice Questions
  • Electrostatics Problems
  • Electrostatics Questions and answers
  • Electrostatics Important Questions for Class 12