# CBSE Class 12 Physics Notes Chapter 1 Electric Charges and Fields

The properties of atoms, matter, and molecules are determined by the magnetic and electric forces present in them.

When an experiment was conducted on electric charges due to frictional electricity, it was found out that conductors assist in the movement of electric charge through them and insulators do not behave in the same manner.

## What are the properties of Electric charge?

An electric charge has three fundamental properties:

• Quantization- This property states that the total charge of a body represents the integral multiple of a basic quantum of charge.
• Additive- This property of electric charges represents the total charge of a body as the algebraic sum all the singular charges acting on the system.
• Conservation- This property states that total charge of a system remains unaffected with time. In other words, when objects get charged due to friction, a transfer of charge from the one object to another occurs. Charges can neither be created nor destroyed.

### Coulomb’s Law

The coulomb’s law states that the mutual electrostatic force existing between two point charges A and B is proportional to their product which is AB and inversely proportional to the square of the distance between them ($r_{AB}$). The equation is

$F_{BA}= force\, on\, B\, due \, to \, A = k(AB)\div r_{AB}^{2}$

Mathematically,

This law consists of constant terms which are also called a constant of proportionality and is represented by ‘k’ and its values are $k=9 * 10^{9} Nm^{2}C^{-2}$ $Nm^{2}C^{-2}$

### Superposition Principles

According to the superposition principle, the property of two charges to repel and attract each other remains unaffected even though there is a presence of third additional charge.

### Properties of Electric Field Lines

Some of the properties of field lines are:

• Field lines show a continuous curves without having any breakage.
• Two line never cross each other
• These electric field lines start on the positive charge and end in the negative charge.

### Electric Dipole

It is a pair of equal or opposite charges A and -B which are separated by distance 2x. The dipole moment vector (let’s assume it as p) has a magnitude 2Ax and is in the direction of the dipole axis from -B to A

### Important Question

1. (a) The Statement needs explanation ‘electric charge of a body is quantized’.

(b) Why can one ignore the quantization of electric charge when dealing with macroscopic i.e., large-scale charges?

1. Explain how the law of conservation of charge works on this phenomenon when charges appear on both the silk cloth and the glass rod when they are rubbed together.?
2. (a)Why electrostatic field line is not a continuous curve and cannot have sudden breaks?
(b) Why two field lines never cross each other at any point?