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1. ELECTRIC CHARGE AND
ELECTRIC FIELD.

2. Electric Charge and Electric
Field

3. Learning objectives
*Properties of electric charges
*Coulomb’s law
*Electric field
*Electric field of continuous charge distribution
*Electric field lines
*Motion of charged particles in a uniform electric field

4. Electric Charge and
Its Conservation
• Positive charge – possessed
by protons
• Negative charge –
possessed by electrons
• Charges of same sign repel
• Charges of opposite signs
attract

5. Insulators and Conductors
Conductors: Materials in which some of the electrons are free electrons that
are not bound to individual atoms and can move relatively freely through the
material. Most metals are conductors.
Insulators: are materials in which electrons are bound to individual atoms
and cannot move freely through the material.
Most insulators are non-metals.

6. Conductor:
Charge flows freely
Metals
Insulator:
Almost no charge flows
Most other materials
Semiconductors:
allow movement of
charge in some cases
but not others.

7. Electric Charge in the Atom
Atom is electrically neutral.
Rubbing charges objects by moving electrons
from one to the other.

8. Charging by Induction
• Electric Charges are at rest when the electric field within a conductor
is zero.
• The electric field is always perpendicular to the surface of
a conductor – if it were not, the charges would move along the
surface.

9. Coulomb’s Law
Experiment shows that the electric force
between two charges is proportional to the
product of the charges and inversely
proportional to the distance between them.

10. Coulombs Law states that the electric force exerted by a
point charge q1 on a second charge q2 is r^12.
Where r is the distance between two charges and r^12 is a
unit vector directed form q1 toward q2.
Coulomb’s Law

11. Properties of electric force
between two stationary charge
particles: The electric force..
• is inversely proportional to square of the
separation between particles and directed along the
line joining them
• is proportional to the product of the charges q1
and q2 on the two particles
• is attractive if charges are of opposite sign and
repulsive if the charges are of the same sign
• Is a conservative force

12. Coulomb’s Law equation
• An equation giving the magnitude of electric
force between two point charges
• (Point charges defined as a particle of zero
size that carries an electric charge)
q1 q 2
r 2
Fe k e
Where ke is called the Coulomb constant and ke =
8.9875 x 109 Nm2C-2 (S.I units) or ke = 1/
4πЄ0 and
Є0 = permittivity of free space
= 8.8542 x 10-12 C2N-1m-2

13. Coulomb’s law:
This equation gives the magnitude of
the force between two charges.

14. Coulomb’s Law
The force is along the line connecting the
charges, and is attractive if the charges are
opposite, and repulsive if they are the same.
The direction of the static
electric force one point
charge exerts on another
is always along the line
joining the two
charges, and depends on
whether the charges have
the same sign as in (a)
and (b), or opposite signs
(c).

15. Coulomb’s Law
Unit of charge: coulomb, C
The proportionality constant in Coulomb’s
law is then:
Charges produced by rubbing are
typically around a microcoulomb:

16. Coulomb’s Law
Electric charge is quantized in units
of the electron charge.

17. Coulomb’s Law
The proportionality constant k can also be
written in terms of , the permittivity of free
space:

18. Two point charges separated by a distance r exert a force on
each other that is given by Coulomb’s law. The force F21
exerted by q2 on q1 is equal in magnitude and opposite in
direction to the force F12 exerted by q1 on q2. When the
charges are of the same sign, the force is repulsive.
Electric Force is a vector

19. When the charges are of opposite signs, the
force is attractive.

20. F e
12
r 2
q1q 2 r
ˆ
k
Where, r
ˆ is a unit vector directed from q1 to q2.
Since the force obeys Newton’s third law, then
F12 = - F21

22. Question 1
• The electron and proton of a hydrogen
atom are separated by a distance of
10-11
approximately 5.3 x m. Find the
magnitude of the electric force.

23. Solution 1
e
e
q1 q 2
F k
11
r 2
9
5.3x10
19
)2
(1.6x10
Fe 8.99x10 x
Fe = 8.2 x 10-8 N

24. •Two kinds of electric charge – positive and
negative.
• Charge is conserved.
• Charge on electron:
e = 1.602 x 10-19 C.
• Conductors: electrons free to move.
• Insulators: nonconductors.
Summary

25. • Charge is quantized in units of e.
•Objects can be charged by conduction or
induction.
• Coulomb’s law:
Summary