Electric Charge

The branch of physics which deals with the study of charges at rest, the force between them, fields and potential due to these charges. It is called Electrostatics or static electricity or frictional electricity.

Electric Charge

The charge is something possessed by a material object that makes it possible for them to exert electrical force and to respond electrical forces. The property of protons and electrons which gives rise to an electric force between them is called electric charge. It is a scalar quantity and its SI units is coulomb (C).

Types of Charges

There are two types of charge

1. Positive charge
2. Negative charge

An object can obtain positive charge by losing electrons while other can attain negative charge by gaining electrons. Charge always resides on the surface of the charged conducting object. An object can be charged by different methods like friction and induction. The proton possesses the positive charge. The electron possesses the negative charge (-e).

e = ±1.6 x 10^-19

Properties of Electric Charge

1. Two like charge repel each other, while two, unlike charge, attract each other.
2. Electrification by friction can be explained on the basis of the transfer of electron. Charges are always distributed on the surface of the conductor.
3. If a car is struck by lightning, persons sitting inside the car are shielded from the electricity and not harmed at all, since, all the charges remain on the outer surface and move to the ground through the lowest metallic part of the car.
4. Charge is invariant (i.e., charge does not change with change in velocity)
5. A charged body attracts lighter neutral body.
6. Electric charge is additive (i.e., the total charge on a body is the algebraic sum of the charge present in different parts of the body). e.g., if a body has different charges as +2q, +4q, + -3q, –q, then total charge on the body is +2q.
7. During any process, the net electric charge of an isolated system remains constant (i.e., conserved). In simple words, the charge can be neither created nor destroyed.
8. Charge on a body can only have some particular values, this property of charge is called quantization of charge. The charge on a body always occurs in an integral multiple of charge on one electron (e).

Charging by induction

In charging by induction, a charged body imparts to another body some charge of opposite nature without any actual contact between them. In this process, the charging body does not lose any charge as it is not in contact with the charged body. Thus, we can say that induction is a process when the charge on a body is rearranged under influence of a nearby charged body (not in contact).

Coulomb’s Law

It states that two stationary point charges attract or repel each other with a force which is directly proportional to the product of the magnitude of charges and inversely proportional to the square of the distance between them.

Consider two point charges q1 and q2 placed at a distance r from each other, then force between them is

F ∝ q1q2/r²

or

F = q1q2/4πε_or² = 9×10⁹×q1q2/r²

where the term 1/4πε_o is called dielectric constant of the medium

and ε_o(=8.85×10-12^–¹²C²/N-m²) is called absolute permittivity of free space.

If there is another medium between the point charges except for air or vacuum, then ε_o is replaced by ε_o,ε_r which is known as relative permittivity of the medium.