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# Introduction

Many times we observe a spark or hear a crackle when we take off our synthetic clothes or sweater, particularly in dry weather. This is almost inevitable with garments like a polyester saree. Sometimes, we also experience an unwanted electric shock when we touch something. Another common example of electric discharge is the lightning that we see in the sky during thunderstorms.

All of those phenomenon happens due to electric discharging or electric charge. In this topic we discus about the electric charge, and the forces due to charges.

Electrostatics deals with the study of forces, fields and potentials arising from static charges.

1. Electric Charge

It is the basic property of matter, which is caused by some elementary particles that governs how any particle is affected by electromagnetic field or electric and magnetic field. The S.I. unit of electric charge is coulomb ( C ).

There are two types of electric charge,

1. Positive Charge: they are produced on the removal of electron from a neutral body.
2. Negative Charge: they are produced on the addition of electron to a neutral body.

Charge of proton = 1.6 x 10-19 C

Charge of electron = -1.6 x 10-19 C

Properties of Charge

1. Conservation of Charge: In an isolated system the total charge of a system is always conserved. It means, charge can never be created nor be destroyed. It can only be transferred from one body to another.
• Additive Property of Charge: Total charge of a system is the algebraic sum of the individual charges present in the system.

Example- let us consider a system containing n no of charges then the total charge of the system is,

F= Q1 + Q2 + Q3+ Q4………….+ Qn

• Quantisation of Charge: The total charge of a body is the integral multiple of the elementary charge particle (e = 1.6 x 10-19 C)

i.e, Q = ne

Coulomb’s Law

According to Coulomb’s law the force between two charged particles is directly proportional to the product of charges and inversely proportional to the square of distance between them. The direction of force is along the line joining the two charges, i.e,

F={1/4πε0}{q1q2/r3}r

Principle of Superposition,

If the system contains “n” number of interacting charge particle then, the total force/net force is the vector sum of all the forces. Now if there are “n” number of charges present in a system, then the net force on q1 due to all other charges can be written as,

F= F1 + F2 + F3+ F4………….+ Fn

Continuous Charge Distribution

• Linear Charge Distribution: The total charge per unit length of a charged body is called liner charge density or linear charge distribution. It is denoted by λ. And its S.I. unit is Coulomb per meter (C.m-1).

λ=dq/dl

• Surface charge distribution: The total charge per unit area of a charged body is called surface charge density or surface charge distribution. It is denoted by σ. And the S.I. unit is Coulomb per meter square (C.m-2).

σ=dq/ds

• Volume Charge distribution: The total charge per unit volume of a charged body is called volume charge density or volume charge distribution. It is denoted by σ. And the S.I. unit is Coulomb per meter cube (C.m-3).

Ρ=dq/dv

Electric Field

It is a region around a charge particle, within which an electrostatic force would is experienced by another charge particle.

Electric field intensity: It is the strength of electric field at any point. In other word it is force per unit charge. It is a vector quantity and its direction is along the direction of force. Its S.I. unit is N/C.

So,

E=F/q

E={1/4πε0}{q/r3}r

Electric Field lines:

• It is an imaginary smooth curve, which originates from the charged particle in presence of an electric field, which can cause motion of charge particle or it can exert an electrostatic force on it
• Dependence of electric field strength on the radial distance from the charge. From this we can say that higher the density of line higher the strength of electric field.

Electric field strength at any point, is a vector quantity whose magnitude is measured by the number of electric field lines passing through per unit cross-section area around that point.

Properties of Electric Field Lines:

• The electric field start from positive charge, and it goes towards the negative charge
• Electric field lines never intersect each other
• The electric field lines do not form any close loop

Electric Flux

The total no of electric field lines passing through a surface normally is called electric flux. It is denoted by φ. The S.I unit of electric flux is volt.metre

Δφ= E.ΔS

Electric Dipole

Two equal and opposite charges separated by a finite distance in a system is called electric dipole. The net charge of electric dipole is always zero, but the electric field is non-zero.

Dipole moment is the strength of electric dipole. Ifthe separation between two charge +q & -q, is 2l, then dipole moment can be written as,

P = q.2l

Dipole Moment in a uniform external field

If a dipole is placed in a uniform electric field E, then the net force acting on the dipole is zero but the torque is non zero, which is equal to acts on the dipole, where θ is the angle between , and , In vector form it is represented by

τ=ρ x E

Gauss’s Theorem

According to Gauss law, the total electric flux through a closed surface is equal to times the net charge enclose by the surface.

Φ=Q/ε0

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Categories: General