This page contains notes for class 12 Chapter 1 Electric Charge and Fields. In this page, you will find topics like
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.
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.
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,
(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. That is both sets repel each other.
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.
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.
There are three main methods for charging of a body
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|
The temporary electrification of a conductor, when a charged body is brought near it is called electrostatic induction.
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.
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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.
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
$q = q_1 + q_2 + q_3$
$q = (+1) + (-2) + (+4) C = (+3) C $
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