- The Greeks discovered electricity around 2600 years ago by observing forces between charged pieces of amber and other materials. This led to the terms "electricity" and "electron". - There are two types of electric charge: positive and negative. Opposite charges attract, while like charges repel based on Coulomb's Law. - Atoms contain protons which have a positive charge, electrons which have a negative charge, and neutrons which have no charge. The attraction between protons and electrons is called electrostatic force.

1. ELECTRIC FORCES

2. EVIDENCE FOR ELECTRIC FORCES:
• The discovery of electricity is generally
credited to the Greeks and is thought to have
occurred around 2600 years ago.
• People observed electric charges and the forces
between them. Many of their observations made
use of a material called amber.
• The Greek word for amber, ‘elektron’,is the
origin of the terms electricity and electron.

3. • Amber is the dried, hardened sap
from certain trees. Small creatures
or bits of plant material are often
preserved inside.
• When a plastic or amber rod is rubbed
with fur, the rod acquires an electric
charge.
• A charged rod attracts small bits
of paper and other objects.

4. ELECTRIC CHARGE
TWO TYPES OF ELECTRIC CHARGE:
• Unlike charges attract, i.e. a positive charge and a
negative charge attract each other.
EXPERIMENT DEMONSTRATES:
• POSITIVE CHARGE
• NEGATIVE CHARGE
• Like charge repel i.e. two positive charges or two negative
charges and repel each other.

5. STRUCTURE OF AN
ATOM All objects are composed of atoms.
Size of an atom is 0.1nm
Size of nucleus in an atom is 0.1pm
𝑚𝑝 = 𝑚𝑛 = 1.67×10−27
𝑘𝑔
A proton has a positive charge, and the electron a
negative charge. The neutron carries no charge.
Electron and the nucleus attract each other. This attraction
is called electrostatic force, the force that holds the
electron in orbit.

6. Charge is conserved. The total charge on an object
is the sum of all the individual charges carried by
the object.
The total charge can be positive, negative, or zero.
Charge can move from place to place, and from one
object to another, but the total charge of the
universe does not change.
This important observation brings us to a very
fundamental law in physics: the law of conservation
of electric charge. This law states that the net
charge of an isolated system remains constant.

7. THE UNIT OF CHARGE
• Charge comes in quantized units. All protons carry the same amount of charge +e, all electrons carry a
charge –e.
Q = ne
where n = 1, 2, 3, ……….. and e = 1.6x10-19 C , is the elementary charge.
• The SI standard unit of charges is the Coulomb, which is represented by the symbol C. The charge on a
single electron is,
electron charge = -e = -1.6x10-19 C
And the charge carried by a single proton is
proton charge = +e = +1.6x10-19 C
CHARGE QUANTISATION

8. CONDUCTORS AND INSULATORS
Insulators do not allow electric charge to move easily through them.
In conductors, the outermost electrons (or the valence electron) in atoms are loosely bound.
CHARGING BY FRICTION Charging by Conduction
Charging By Induction

9. ELECTRIC FORCES AND COULOMB’S LAW
𝐹 = 𝑘
𝑞0𝑞1
𝑟2
Where ε0 is called the permittivity of space,
ε0 = 8.85 x 10-12 C2/N · m2
K =
1
4πε0
’ in free
space
k = 8.99 x 109 N·m2/C2

10. SUPERPOSITION OF ELECTRIC CHARGES
Suppose two particles of charge q1 and q3 are separated by a distance as shown in the
figure, what is the force on a third charge q2?
According to the superposition principle, the resultant force on q3 is given by:
F = F12 + F13

11. Example : The figure below shows three small charges, q1 = –2μC, q2 =5μC, q3 =3μC, located along the
positive x-axis. What is the (i) direction, and (ii) magnitude of the resultant force exerted by these two
charges on q2?
Solution:
(i) The force, F12 K exerted by Q1 on Q2 is attractive because these two charges have opposite signs. Similarly,
the force F32 K exerted by Q3 on Q2 is repulsive because both these two charges have the same sign.
(ii) The magnitudes of these forces are given by Coulomb’s Law:
F12 = k
q1q2
r12
2 = (9.0x109)
(2.0 10−6 )(5.0 10−6 )
(0.03)2
= 100 N

12. F32 = k
q3q2
r32
2
= (9.0x109)
(3.0 10−6 )(5.0 10−6 )
(0.03)2
= 150 N
The magnitude of the resultant force is thus 150+100=50 N. This resultant force points in the –
x direction.