Coulomb's Laws Of Electrostatics :
First Law : Like charges repel each other and unlike charges attract each other. In other words, bodies having same charges repel each other and bodies oppositely charged attract each other.
The first law tells only about the nature of force i.e., repulsive or attractive. It does not tell about the magnitude of force. The magnitude of the force between two charged bodies is given by the second law.
Second Law : The force of attraction or repulsion between two charges is directly proportional to the product of magnitude of the two charges and inversely proportional to the square of distance between them.
F = K × Q1Q2/d2
Where K is a constant of proportionality and its value depends upon the medium in which the charges are placed and the system of units used. In SI units force is measured in newton, charge in coulomb, distance in meter and the value of K is given as:
K = 1/4πϵoϵr (in S.I. system)
where
ϵo = Absolute Permittivity of vacuum or prematurity of free space
ϵr = Relative Permittivity of the medium w.r.t. vacuum in which the charges are placed.
The value of ϵo = 8.854 x 10-12 farad/metre and the value of ϵr is different for different media, for air ϵr = 1. The value of 1/4πϵo = 9 x 109
F = K × Q1Q2/d2
F = (1/4πϵoϵr) × Q1Q2/d2
F = Q1Q2/4πϵoϵrd2 (in N)
Units. In the formula, F = Q1Q2/4πϵoϵrd2
F is measured in newton, Q1,Q2 in coulomb, d in metre and ϵo in farad/metre.
Unit Charge :
(i) S.I.System : By unit charge, we mean one coulomb charge. By coulomb's second law,
F = Q1Q2/4πϵoϵrd2 newton
F = 9 x 109 newton
and Q1, Q2 = Q
ϵr = 1
ϵo = 8.854 x 10-12
d = 1m
Put everything in above relation,
9 x 109 = ( Q × Q ) / 4π × 8.854 x 10-12 ×1 ×1
Q2 = ± 1 coulomb
Hence unit charge or one coulomb charge in S.I.system can be defined as:-
A unit charge or One coulomb is that much charge which when placed at a distance of one meter from an equal and similar charge in air, is repelled with a force of 9 x 109 newton from it.
(ii) In C.G.S System : Unit of charge is stat coulomb
1 coulomb = 3 x 109 stat coulomb
First Law : Like charges repel each other and unlike charges attract each other. In other words, bodies having same charges repel each other and bodies oppositely charged attract each other.
The first law tells only about the nature of force i.e., repulsive or attractive. It does not tell about the magnitude of force. The magnitude of the force between two charged bodies is given by the second law.
Second Law : The force of attraction or repulsion between two charges is directly proportional to the product of magnitude of the two charges and inversely proportional to the square of distance between them.
If two point charges Q1
and Q2 have distance d between them, then force F between the charges can be mathematically expressed as :
|  |
F = K × Q1Q2/d2
Where K is a constant of proportionality and its value depends upon the medium in which the charges are placed and the system of units used. In SI units force is measured in newton, charge in coulomb, distance in meter and the value of K is given as:
K = 1/4πϵoϵr (in S.I. system)
where
ϵo = Absolute Permittivity of vacuum or prematurity of free space
ϵr = Relative Permittivity of the medium w.r.t. vacuum in which the charges are placed.
The value of ϵo = 8.854 x 10-12 farad/metre and the value of ϵr is different for different media, for air ϵr = 1. The value of 1/4πϵo = 9 x 109
F = K × Q1Q2/d2
F = (1/4πϵoϵr) × Q1Q2/d2
F = Q1Q2/4πϵoϵrd2 (in N)
Units. In the formula, F = Q1Q2/4πϵoϵrd2
F is measured in newton, Q1,Q2 in coulomb, d in metre and ϵo in farad/metre.
Unit Charge :
(i) S.I.System : By unit charge, we mean one coulomb charge. By coulomb's second law,
F = Q1Q2/4πϵoϵrd2 newton
F = 9 x 109 newton
and Q1, Q2 = Q
ϵr = 1
ϵo = 8.854 x 10-12
d = 1m
Put everything in above relation,
9 x 109 = ( Q × Q ) / 4π × 8.854 x 10-12 ×1 ×1
Q2 = ± 1 coulomb
Hence unit charge or one coulomb charge in S.I.system can be defined as:-
A unit charge or One coulomb is that much charge which when placed at a distance of one meter from an equal and similar charge in air, is repelled with a force of 9 x 109 newton from it.
(ii) In C.G.S System : Unit of charge is stat coulomb
1 coulomb = 3 x 109 stat coulomb
