this is a summary of the topic electrostatics for class 12 and diploma students

1. Branch of Physics in which study of electric
charges at rest is done

2.  Basic property of matter responsible for all
electric and magnetic phenomena.
 We can observe electric charge in daily life for
e.g. Rub two glass rods with silk cloth if they
are brought close will repel each other.
 Other examples Combing drying hair with
comb, Woolen Cloths in plastic sheets will
attract each other.

3.  Charge is of two types: Positive and Negative.
 Like charges repel and unlike charges attract.
 Charges can be added algebraically.
 Charge is independent of velocity of body
 Charge is Quantized i.e. Q = ±ne where e =
1.6 X 10-19 C
 Charge can never be created or destroyed, it
is always conserved.

4.  The force between two charges is
 Directly proportional to product of magnitude
of two charges.
 Inversely proportional to square of distance
between them.

5.  Si Unit of Charge: Coulomb
 CGS Unit of Charge: Stat Coulomb
 One Coulomb is that much charge which
when placed 1meter apart from equal and
similar charge in air repels with a force of 9 X
109 N.
 1 Coulomb = 3 X 109 Stat Coulomb

6.  Basically it means with how much ease
vacuum/air and medium allows the charge to
move through it.
 It affects the force between two charges.
 Absolute permittivity, εо for vacuum/air is
minimum = 8.854 X 10-12 Farad/m
 Relative Permittivity εr = ε/ εо

7.  An electric field is region of space in which
electric force can be felt or space upto which
presence of charge can be felt.
 Like if you light a candle it gives light in small
area, a bulb glows and its light can illuminate
a room and so on

8.  It is defined as path traced by test charge (Unit
positive charge) when placed in Electric Field.
 Properties of Electric Lines of Force are important
from exam point of view

9.  Electric Lines of Force do not pass through
empty space. For eg inside a hollow sphere,
inside a hollow cylinder.
 The charge cannot exist in hollow space
therefore electric lines of force also do not
exist in hollow space

10.  Total number of electric lines of that cut through
given surface area held perpendicular to the
direction of electric force.
 Φ = E. A

12.  Potential is that electrical condition which
determines direction of flow of charge.
 Potential means ability to do electrical work.
 Work done in bringing a unit positive charge
from infinity to that point.

14.  V = Q/4πr εr εо
 Magnitude of Charge
 Size: Inversely proportional to radius of
sphere
 Neighboring Charges
 Permittivity, εr

15.  Define Electric flux and properties of electric
Lines of Force.
 Two plates are 1cm apart and are having a
potential difference of 10 V. Find electric field
between plates.
 Charge of 4μC is placed in Electric Field
Intensity 50 X 105 N/C. What is magnitude of
force acting on square?
 Electric Lines of Force from a negative charge
are_______
 The dielectric constant of Insulator cannot
be_______. Explain