Electricity and Its Properties 

We study electricity into two parts: 

(1) Static electricity, deals with charges at rest and phenomenon associated with them.

(2) Current electricity, deals with charges in motion and phenomenon associated with them.

● Matter consists of tiny particles called atoms. The atom has a central nucleus of positively charged protons and uncharged neutrons. Protons and electrons have electric charge and neutrons have no electric charge. Protons have positive electric charge and electrons have negative electric charge. The number of electrons is equal to the number of protons and so overall an atom is uncharged.

1.0Static Electricity (Frictional Electricity)

Most of the substances release static electricity when rubbed against another. This is most noticeable when the rubbed substance is a very good insulating material.

● The electricity developed on the surfaces of insulating bodies when rubbed against each other is called frictional electricity or static electricity.

Source of Static Electricity 

When two substances are rubbed together, some electrons are removed from the atoms on the surface of one and transferred to the other. The substance which gains electrons becomes negatively charged and the one which loses electrons becomes positively charged. Thus, the excess or deficiency of electrons makes a substance negative or positive respectively (see figure).

2.0Electric Charge

Electric charge is the property associated with matter due to which it produces and experiences electric and magnetic effects.

List of objects acquiring two kinds of charges on rubbing

Properties of Electric Charge

(1) Electric charge is a scalar quantity. It can be of two types : positive and negative.

(2) Like charges repel each other while unlike charges attract each other. (see figure)

(3) Additivity of charge : The total charge of the body is the algebraic sum of all the individual contributions by various charges present on it, each being taken with its proper sign. This property of charge is called ‘additivity’.

(a) A neutral atom has no net charge because the numbers of electrons and protons are balanced. 

(b) Removing an electron produces net positive charge; the charged atom is called positive ion. 

(c) The addition of an electron produces a net negative charge and a negative ion.

(4) Conservation of electric charge: Electric charge can neither be created nor be destroyed.

Examples :

(a) Rubbing a glass rod on silk cloth results in both becoming charged, one positive and another negative. The total charge remains zero, as it was in the beginning. This means that frictional electricity is the transfer of electric charge from one body to the other, and not the creation of charge.

(b) Conservation of electric charge holds good in all types of reactions either ‘chemical’ or ‘nuclear’ reactions.

(5) Quantisation of charge : Every charge is an integral multiple of a certain smallest amount of charge that exists in nature. This property of charge is called ‘quantisation’.  Q = ± ne

Where, n = 1,2,3........... and e is called ‘elementary charge’, the smallest amount of charge that exists independently in nature.

e = 1.6 × 10^–19 C, ‘e’ is the magnitude of charge of an electron (or proton).

e = 1.6 $\times 10^{-19}C$

Reason : Since, electrons are indivisible, thus, only integral number of electrons can be transferred from one body to another, on rubbing. Hence, the charge bodies will have charges which are integral multiples of the charge on electron (e).

Electric charge of an electron, q = –e 

$=-1.6\times 10^{-19}C$

Electric charge of a proton, q = + e

$=+1.6\times 10^{-19}C$

Unit of Electric Charge

SI unit : Coulomb (C)

CGS unit :  Electrostatic unit (esu); also called stat coulomb (stat C)

$1Coulomb=3\times 10^{9}esu$

Definition of 1 coulomb (in terms of electric current)

One coulomb is defined as the amount of charge that flows through a given cross-section of a wire in one second if there is a steady current of one ampere flowing in the wire.

1 C = 1 A.s           (∵ Q = I × t )

Number of electrons in 1 coulomb

According to quantisation of charge, q = ne or

$n=\frac{Q}{e}=\frac{1C}{1.6\times 10^{-19}}=6.25\times 10^{18}\approx 6\times 10^8$electrons

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