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Electrostatics : Electric Charge, Basic properties of electric charge





This page contains notes for class 12 Chapter 1 Electric Charge and Fields. In this page, you will find topics like

  • Introduction to Electrostatics
  • Electric Charge
  • Basic properties of electric charge

In basic properties of electric charge, we will learn that charge is conserved. We will also learn about quantization of charge and additivity of charges.

This chapter lays a foundation for concepts like electric current and magnetism. I would tell you to put extra effort into learning these concepts. These concepts comes in handy while studying electricity and magnetism.


Overview of electrostatics and electricity

Electrostatics involves electric charges namely positive and negative charges, the forces between them which is known as electric force , the field that surrounds them, and their behavior in materials. Coulomb's law is the simple relation that governs electrostatic interactions and the field around the charges is described using the concept of electric field. Coulomb's law is a inverse square law which gives the force between two charges kept at some distance (say r ) apart from each other. Like Coulomb's law, law of gravitation is also a inverse square law but gravitational interactions are only attractive in nature and electrical interactions are attractive as well as repulsive depending on the nature if interacting charges. Charges of same kind repel each other and charges different kinds , i.e. one charge positive and other negative , attract each other. One more thing electric interactions are much more stronger then gravitational interactions and gravitational force are almost negligible in comparison to the forces of electric origin. This is always true when we study the interactions of atomic and subatomic particles. But when we study objects very large in size say a person ,a planet or satellites, the net governing force in this case is gravitational force not electric.

Now coming to the properties of electric charges we know that electric charge is quantized and it also obeys law of conservation means total charge remains conserved. In what we say electrostatic interactions electric charges are at rest in our frame of reference. Now think what happens when we are at rest in our frame of reference and charge under consideration is moving with velocity v with respect to us such a moving charge leads to the origin of magnetism which we will discuss in later section.

Again a question what is electricity. Electricity, deals with stationary and moving electric charges, the actions of force between these charges, and the electric and magnetic fields generated by them. Electrostatics is simply the electricity at rest. Electricity is the backbone of the modern society in which we use various instruments which depends on electric current for their functioning and without it we would not have telephones, television, household appliances and many more gadgets which are now part of our daily life. In electricity we study the motion of electric charges, or electric currents and also the voltages that produce currents and the ways to control currents.

We have learned about what electrostatics is and what we study in it. Now we will discuss why we study electrostatics and where it finds its applications.Electric interactions are of immense importance in chemistry and biology and have many technological applications. Concepts of electricity proved to be of basic importance for studying atomic physics, nuclear physics and solid state physics. It also find importance in studying advanced level physics.

Electrostatics : Introduction

Electrostatics is the study of electric charges at rest. Coulomb's Law explains the Relationship between two or more electric charges. In electrostatics, we do not concern with the movement of charges.

Electrostatics involves electric charges, the forces acting on them, and their behavior in substances.

Electrostatics, as we study today, depends on the nature of electric charges. Nature of charges depends on the models of atom proposed by Ernest Rutherford and Niels Bohr. According to their theories, an atom consists of two types of charges: positively charged protons in a nucleus surrounded by negatively charged electrons. A neutral atom has equal numbers of electrons and protons.

What we study under electrostatics is static electricity. The charges at rest develop due to friction when we rub two insulating bodies against each other.

Some industrial applications of electrostatics are:

  1. In designing electrostatics generators like Van de Graff generator
  2. In electrostatic spraying of paints, powders etc.
  3. In the design of cathode ray tubes for radar, television etc.
  4. Ink-jet printing
  5. Understanding lightning that strikes from the cloud base to the ground.
  6. Adhesive forces of glue associated with surface tension, all are electric in nature

Electric Charge

Electric charge is a fundamental property associated with elementary particles. It accompanies fundamental particles whenever that exists. Electron, proton, neutrons are a few examples of fundamental particles.

According to William Gilbert,

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

Understand the concept of charge

Often times students are unable to visualize electric charge. We know electric charge to be a fundamental property associated with elementary particles. This fundamental property is like characteristics or properties of other substances or objects. For example, you have a fountain pen. If you lift it in your hand you are able to feel its weight. Now we know that,
Weight $W=m \times g$ where m is the mass of pen and g is acceleration due to the gravity of earth.

Now start thinking about mass. What is mass? It is nothing but a characteristic that the pen in question has. Can you see this mass? No, it is the material of the pen what you are able to see which might be plastic or metallic.

Again does the mass of the pen change when you move it to the moon or another planet. The answer is again no. The mass of the pen does not change. It is the wight of the pen that change because acceleration due to gravity g is different for both moon and earth. So mass is nothing but the fundamental property associated with different objects. We can now say that any object in the universe can have any number of characteristics or properties.

Now coming back to the question of charge let us try to understand with the example of an electron. For ease let us assume electron is static or is at rest. The electron is a particle and has certain characteristics or properties. Two properties of electrons are
1. It has mass
2. It has a negative charge.
Like the electron, we have another fundamental particle called a proton. Protons also have properties like
1. It has mass
2. It has a positive charge
So, the negative charge on the electron and a positive charge on a proton is a property of both electron and proton. We call a charge positive or negative only by convention to distinguish between two types of charges. We should never mistake negative and positive charges with additive or subtractive sign.

Where does charge come from?

The electric charge on a body comes as a result of transferring of electrons from one body to another. This way one body has an excess and the other a deficiency of electrons. Electrons are very small particles that have a negative charge. Sir J.J. Thomson discovered electrons.

To give a body an excess negative charge, we may add some electrons. And to give an excess of positive charge, we may remove the electrons from the body.

It is important to note here that the “Charge” of a body refers to its excess charge only. The excess charge is always a very small fraction of the total positive or negative charge in the body as a whole.
Electric Charge is a property of the material of a given body like mass in mechanics.
Note:- symbol 'Q' or 'q' represents the electric charge.

We know that in an atom electrons revolve around a nucleus which has a positive charge. Electric charge is the property responsible for electric forces acting between nucleus and electrons in an atom. This electric force between the nucleus and electrons bind the atom together.
Charges are of two kinds
(i) negative charge
(ii) positive charge
In an atom electron are particles having a negative charge? The nucleus consists of protons and neutrons. In a nucleus of an atom, protons have a positive charge and neutrons are neutral. The experiments lead to the fundamental results that

(1) like charges repel,

like charges repel each other

(2) unlike charges attract

unlike charges attract each other

The electric force between two electrons is the same as the electric force between two protons kept at the same distance apart. That is both sets repel each other.
The electric force between an electron and proton placed at the same distance apart is not repulsive but attractive in nature.
Assignment of a negative charge on the electron and a positive charge on a proton is only a convention. It does not mean that charge on an electron is less than that on the proton. Importance of electric forces is that it encompasses almost every field associated with our life. It is important because 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.
  • SI unit of charge is Coulomb written as C.
  • Coulomb is the charge flowing through the wire in 1 second if the electric current in it is 1A.
  • Charge on electron is $e=-1.602 \times 10^{ -19} C$ and charge on proton is positive of this value.

Conductors and Insulators

There is a category of 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 are called insulators. Few examples of insulating materials are glass, rubber, mica, plastic, dry wood etc.

Presence or absence of free electrons in a material makes it a conductor or insulator. Conductors have free electrons which are loosely held by nuclei of their atoms. Insulators do not have free electrons. In insulators, electrons are strongly held by nuclei of their atoms.

It is important to note that

1. The charge transferred to a conductor gets distributed over the entire surface.

2. The charge transferred to an insulator stays at the same place.

Semiconductors are the third class of materials. Electrical properties of semiconductors are somewhat between insulators and conductors. Silicon and germanium are examples of semiconductors.

Question 1. Why do metals conduct electricity?

Answer. Atoms of metals have outer electrons which are not tied to any one atom. These electrons can move freely within the structure of a metal when an electric current is applied. That is why metals conduct electricity.

What happens when a body is charged?

Case 1: Body has a positive charge
When the body has a positive charge, it means electrons are somehow removed from the body. This results also in a decrease of mass of the body. The decrease in the mass of the body equals the total mass of electrons removed from the body.

Case 2: Body has a negative charge
When the body has a negative charge, it means electrons are somehow added to the body. This results in an increase of mass of the body. Increase in the mass of the body equals the total mass of electrons added to the body.


Methods of charging

There are three main methods for charging of a body

(i) Charging by rubbing and Fractional electricity:-

Rubbing as the term suggest is moving two things back and forth against each other. The simplest way to experience electric charge is to rub certain bodies among each other. Rubbing or friction makes electrons move. This gives one material a positive charge and the other a negative charge. The charges stay on the surfaces of the materials until they can flow or they discharge.
If we pass a comb through hairs, comb becomes charged and can attract small pieces of paper. This is because the comb might have lost its electrons or acquired some electrons when we rub it with hairs. Now, this comb is a charged body. The net charge on the comb interacts with the net charge on small pieces of paper which results in attraction. Many such solid materials are known which on rubbing attract light objects like a light feather, bits of papers, straw etc.

Positive charge Negative charge
Glass rod Silk Cloth
Woolen cloth or cat skin Amber, ebonite, rubber rod
Woolen carpet Rubber shoe soles
Woolen coat Plastic seat

Explanation of appearance of electric charge on rubbing is simple. Material bodies consist of a large number of electrons and protons in equal number and hence is in neutral in their normal state. But when a glass rod is rubbed with a silk cloth, electrons are transferred from glass rod to silk cloth. The glass rod becomes positively charged and the silk cloth becomes negatively charged as it receives extra electrons from the glass rod. In this case rod after rubbing, comb after passing through dry hairs becomes electrified and these are the example of frictional electricity.

(ii) Charging by induction (Electrostatic Induction):-

The temporary electrification of a conductor, when a charged body is brought near it is called electrostatic induction.

Charging by induction (Electrostatic Induction)

Electrostatic Induction :- Electrical charges in the conductor are redistributed
When a body is charged this way there is no transfer of electrons from one body to other. This happens because there is no physical contact taking place between charging body and conductor being charged.
If a charged body is brought near an uncharged body, then the neutral body becomes oppositely charged. By induction method, we can charge any type of material body.

For further reading you can visit this very good article
https://www.school-for-champions.com/science/static_induction.htm

(iii) By conduction (by touch without rubbing):-

Because of having excess free electrons in metals they can be charged by conduction. When we bring two conductors, one charged and other uncharged in contact, the same type of charge will appear on both the conductors.

Basic properties of electric charge

(i) Additivity of charges

Charges adds up like real numbers i. e., they are Scalars more clearly if any system has n number of charges $q_1$, $q_2$, $q_3$, qn then total charge of the system is $q = q_1 + q+2 + q_3 + ................ q_n$.
Proper sign have to be used while adding the charges for example if
$q_1 = +1C$, $q_2 = -2C$ and $q_3 = +4C$
then total charge of the system is
$q = q_1 + q_2 + q_3$
or
$q = (+1) + (-2) + (+4) C = (+3) C $

(ii) conservation of charge :

Charge is conserved Charge of an isolated system is conserved. Charge can not be created or destroyed but charged particles can be created or destroyed.

(iii) Quantization of charge

All free charges are integral multiples of a unit of charge ‘e’, where $e = -1.602 \times 10^{-19} C$ i. e., charge on an electron or proton. Thus charge q on a body is always denoted by
q = one where n = any integer positive or negative

Solved Example

These are some solved examples related to electric charge and properties of electric charge given in this page.
Question 1 Is there any transfer of mass when electrons are transferred from one substance to another?

Question 2 (NCERT) A polythene piece rubbed with wool is found to have a negative charge of 3.2 × 10–7 C.
(a) Estimate the number of electrons transferred (from which to which?)
(b) Is there a transfer of mass from wool to polythene?

Question 3 When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge.

Question 4 (a) Explain the meaning of the statement ‘electric charge of a body is quantized’.
(b) Why can one ignore quantization of electric charge when dealing with macroscopic i.e., large scale charge?

Problem solving tips

  • Firstly read your problem carefully. Reading and understanding the problem is the first step towards solving it.
  • Once you have read and understood the problem then try to analyze it using your previous knowledge of the topic.
  • Comprehensive knowledge of the topic is required and it is prerequisite for solving any problem.
  • Now look for the known and unknown quantities.
  • Try to find the formula that can be applied to the problem based on your known and unknown quantities.
  • Now apply the formula, perform the desired calculation and obtain the results.
  • Try to interpret your results.


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