Class 12th Physics Chapter 1

1. Chapter-1
Electric Charges &
Fields

2. Introduction
• Electricity is a branch of physics which deals
with presence & flow of charge in any material
body. This branch is further divided into two
main categories:-
• Static electricity or Electrostatics
• Current electricity

3. Electrostatics
Electro means electric Static means rest
Thus Electrostatics is a branch of physics which study
the forces, fields & potentials arising from static
charges.
This branch is also known as Static electricity or
Frictional electricity.
Static because here we study charges at rest , &
Frictional because the main cause of production of
charges is friction i.e. when two insulating bodies are
rubbed against each other they led to transfer of
electrons from one body to other body.

4. • Some common examples of static electricity are:-
• Lightning in sky
• Bits of paper get attracted towards comb when it is
rubbed with any silk piece
• Sensation of electric shock while opening the door
& many more are there.

5. • What is electric charge?
• It is an inherent property possessed by material bodies due to which
they are able to exert & respond to any type of electrical force on any
other object. There are two types of charges:-
a) Positive charge (+) b) Negative charge (-)
• Both charges have same magnitude of 1.6×10-19C but they differ in sign
& this property which differentiates both charges is known as polarity
of charges.
• Electric charge is a scalar quantity & is measured in C (Coulomb) which
is named after a very renowned physicist Charles Augustin de Coulomb.
• Properties of electric charge:-
• Like (same) charges always repel each other & unlike (different) charges
attract.

6. Charged bodies always attract neutral bodies.
Body having equal number of positive & negative charges is said to be neutral
body.
Conservation of charge:- Charge can neither be created nor be destroyed
OR The total charge of an isolated system always remain constant.
Quantization of charge:- It is a property by virtue of which charge on body is an
integral multiple of basic unit of charge i.e. electron/ proton represented by e.
q = ne where n is any integer & e is basic unit having value = 1.6×10-19C .No body
can possess any fractional charge.
Additivity of charge:- it is the property by which total charge of system is obtained
by simply adding algebraically all charges present on the system. If a system is
having n charges then total charge is given as q = q1 + q2 + q3 + ………….+ qn.
Proper signs have to be used while adding like is a system is having charges +q , -
3q ,+ 5q & -7q then total charge of system is = +q-3q+5q- 7q = -4q.

7. • Methods of transfer of charge are:-
• By Rubbing:- It is a method of transfer of charge between two insulating
bodies when they are rubbed against each other. Hence charge transferred
in this is very small in amount. Also it is called as charge transferred due to
friction.
Different materials possess different number of electrons. If two same nature
bodies are rubbed against each other then no transfer of charge takes place
whereas if two different nature bodies are rubbed then there is transfer of
charge & the one who is gaining electrons acquire negative charge & the
other one who is losing electrons acquire positive charge.
(a) (b)

8. 2) By Conduction:- It is a process of transfer of charge between two
bodies when they are in actual contact with each other.
The body having more number of electrons will transfer its charge to
another body having less charge when they are brought in actual
contact with each other i.e. charging by Touching.
Thus in this way when among the two bodies when one body is earth
then that sharing of charge with earth is called as Grounding.

9. 3) By Induction:- This phenomenon of charging an
uncharged body by bringing a charged body near it
without making any direct contact between two bodies.

10. • Hence materials are classified into two categories
• Conductors :- Those material which can conduct
charge from one end to other. Examples are metals,
human bodies, earth etc.
• Insulators:- those material which can’t conduct
electricity i.e. they are poor conductors of electricity.
Examples are glass, rubber, wool, plastic etc.
However, in case of insulator when an electric field is
applied, induced charges appear on their surface for
some time i.e. why they are also called as dielectrics.
Hence dielectrics are defined as insulating materials
which transmit electric effects without conducting.
• In case of conductor when some charge is
transferred then it distributes readily over its entire
surface whereas in case of insulator if some charge is
put then it stays at the same place.

11. Coulomb’s Law

12. Vector form of Coulomb’s Law

13. Superposition principle
• It states that force on any charge due to a number
of charges is vector sum of all forces on that
charge due to other charges taken one at a time.
The individual forces are unaffected due to
presence of other charges.

15. • It is a vector quantity whose direction is always from positive charge to negative
charge.
• Its SI unit is N/C & cgs unit is Dyne/stat coulomb & its dimensional formula is
[MLT-3A-1].
•
A(+Q) B(+q)
• Hence using superposition principle we can also calculate the value of total
electric field acting on a point when a number of charges are present.

16. Electric Field Lines
• It is an imaginary path either straight or curved of an electric field such
that tangent to it will give the direction of electric field at that point.
They are always continuous curves but they don’t form closed loops.
• In case of single positive charge they are always directed radially
outwards whereas for single negative charge they are always directed
radially inwards.
• Field lines of different
charges are shown

18. Electric Dipole

21. Electric Flux
• It is a measure of electric field lines crossing this area. It is a scalar
quantity . Its unit is Nm/C2 or V-m & its dimensional formula is
[ML3T-3A-1].
• There are two types of flux one is inward flux which is taken to be
positive & other is inward flux which is taken to be negative.

22. Continuous charge distribution
• Charges on surface of system are located so close together that
they are supposed to be continuous distribution of charges. In a
system the continuous distribution is along
• Some line
• Over a surface
• Throughout a volume

25. Gauss’s Law
• It states that total electric flux over any closed
surface enclosing a volume in vacuum is 1/ε0
times the total charge enclosed by the
surface.

26. Applications of Gauss’s Law
• Field due to an infinitely long straight uniformly charged wire :-Consider an
infinite line which has linear charge density λ. Using Gauss’s law let us calculate
the value of electric field over a distance ‘r’ from the line charge.

27. Electric field intensity due to
a thin infinite sheet of charge

28. • Electric field intensity due to a uniformly
charged spherical shell:- Consider a thin
spherical shell of radius R & centre O. Let
a charge Q be distributed uniformly over
the surface of shell. Now we have to
calculate the value of field intensity at
three different regions of the shell.

29. Numericals for practice
• Two bodies A & B carry charges -3µC & -0.44µC . How many electrons should be
transferred from A to B so that they acquire equal charge?
• Force of attraction between two point charges placed at a distance ‘d’ is F. What
distance apart should they be kept in same medium, so that force between
them is 2F?
• Three point charges are placed at the following points on x-axis: 3µC at x=0, -
4µC at x= 50cm & -5µC at x= 120cm. Calculate the force on -4µC charge.
• An infinite line charge produces a field of 19 x 10-4 N/C at a distance of 5cm.
Calculate linear charge density.
• A plastic rod of length 2.2m & radius 3.6mm carries a negative charge of 3.8 x
10-7 C spread uniformly over its surface. What is the electric field near mid point
of rod, at a point on its surface?
• A particle of mass 10-4Kg & charge 5µC thrown at a speed of 20m/s against a
uniform electric field of strength 2 x 105 N/C. How much distance will it travel
before coming to rest momentarily?
• Two charges +20µC & -20µC are held 1cm apart. Calculate electric field at a
point on the equatorial line at a a distance of 50cm from the centre of dipole.
• A dipole consisting of an electron & proton separated by a distance of 4 x 10-10m
is situated in an electric field of intensity 3 x 105 N/C at an angle of 30° with the
field. Calculate the dipole moment & the torque acting on it.