LAWS OF ELECTROSTATICS

LAWS OF ELECTROSTATICS

1. LAWS OF

2. The study of properties of electric charges at rest called The study of properties of electric charges at rest called
ELECTROSTATICS. In an isolated system the total ELECTROSTATICS. In an isolated system the total
charge remains conserved. Law of conservation of charges charge remains conserved. Law of conservation of charges
helps us to study interactive energy and forces. The helps us to study interactive energy and forces. The
objects with same charges repel each other on the other objects with same charges repel each other on the other
hand charges with opposite charges attract one another.hand charges with opposite charges attract one another.

5. Volume charge densityVolume charge density
It is the ratio between small quantity of charge It is the ratio between small quantity of charge
contained in a body and small volume of body. contained in a body and small volume of body.
q = q = PPdvdv
vv
Surface charge densitySurface charge density
It is the ratio between small quantity of charge It is the ratio between small quantity of charge
contained in a body and small surface area of the body.contained in a body and small surface area of the body.
q = dsq = ds
ss
Linear charge densityLinear charge density
It is the ratio between small quantity of charge It is the ratio between small quantity of charge
contained in a body and small length of the body.contained in a body and small length of the body.
q = dlq = dl
ll

7. Coulomb's law states that the electrical force Coulomb's law states that the electrical force
between two charged objects is directly proportional between two charged objects is directly proportional
to the product of the quantity of charge on the to the product of the quantity of charge on the
objects and inversely proportional to the square of objects and inversely proportional to the square of
the separation distance between the two objects. In the separation distance between the two objects. In
equation form, Coulomb's law can be stated asequation form, Coulomb's law can be stated as
The symbol k is a proportionality constant known The symbol k is a proportionality constant known
as the Coulomb's law constant. The SI unit is as the Coulomb's law constant. The SI unit is
newton (N)newton (N)
F = k ( Q1 Q2 ) / r2
The Coulomb's law equation provides an accurate
description of the force between two objects
whenever the objects act as point charges

8. Electric field at any point is given by the force Electric field at any point is given by the force
experienced by unit positive charge (test charge) experienced by unit positive charge (test charge)
kept at that point.kept at that point.
E = F/qE = F/q
E =E =
q/R
2

9. An imaginary line drawn in an electric field such An imaginary line drawn in an electric field such
that any tangent drawn on a point on this curve that any tangent drawn on a point on this curve
gives direction of electric field at that point.gives direction of electric field at that point.
CHARACTERISTICS:CHARACTERISTICS:
They do not intersect each other.They do not intersect each other.
They never form closed loops.They never form closed loops.
They start at positive terminal and end at negative They start at positive terminal and end at negative
terminal.terminal.

12. Two equal and opposite charges separated by a Two equal and opposite charges separated by a
small vector distance may be considered as small vector distance may be considered as
dipole system.dipole system.
aa
The product of charge and distance is called The product of charge and distance is called
dipole moment.dipole moment.

p = q ap = q a
+q -q

13. Electric field due to dipole system on the axial Electric field due to dipole system on the axial
line of the dipole.line of the dipole.
E = 2P/4 E = 2P/4 E E xxxxxx
Electric field at any point on equitorial line due Electric field at any point on equitorial line due
to dipole moment.to dipole moment.
E = P/ 4 E = P/ 4 E E xxxxxx
Torque experienced by dipole system kept in Torque experienced by dipole system kept in
uniform electric field.uniform electric field.
 Potential energyPotential energy

14. Electrostatic potential in electrostatic field is Electrostatic potential in electrostatic field is
nothing but the work done in bringing the unit nothing but the work done in bringing the unit
positive charge from infinity to that point.positive charge from infinity to that point.
 V = W/ qV = W/ q
 V = ErV = Er
 V = - E lV = - E l
 V = q/4 V = q/4 EE r r

15. It states that total number of electric lines of It states that total number of electric lines of
forces (electric flux) through a closed surface forces (electric flux) through a closed surface
is 1/ is 1/ E E times the net charge enclosed in that times the net charge enclosed in that
surface .surface .

= q/ = q/ EE
Electric flux is defined as total no of electric Electric flux is defined as total no of electric
lines of forces passing normally through a lines of forces passing normally through a
surface held inside electric field.surface held inside electric field.

16. It is used to store energy.It is used to store energy.
It is used in digital circuits.It is used in digital circuits.
Tuning of circuits.Tuning of circuits.
It will not allow direct current to pass through It will not allow direct current to pass through
it.it.
As a potential difference is applied across a As a potential difference is applied across a
conductor its charge increases.conductor its charge increases.
q = CVq = CV
Where C is capacitance of conductor.Where C is capacitance of conductor.

17. Every conductor stores charge in small Every conductor stores charge in small
quantity. In order to store large quantity of quantity. In order to store large quantity of
charge capacitor is devised. The capacitance of charge capacitor is devised. The capacitance of
a conductor increases by large amount when an a conductor increases by large amount when an
earthed uncharged conductor is placed near to earthed uncharged conductor is placed near to
it. it.
This constitutes the principle of This constitutes the principle of
capacitor. capacitor.

C = C = EE A/d A/d

18. Resultant capacitance when three capacitors Resultant capacitance when three capacitors
are in series:are in series:
1/C = 1/C ‘ + 1/C ‘‘+ 1/C’’’1/C = 1/C ‘ + 1/C ‘‘+ 1/C’’’
Resultant capacitance when three capacitors Resultant capacitance when three capacitors
are in parallel:are in parallel:
C = C’ + C’’ + C’’’C = C’ + C’’ + C’’’