Chapter 7
Electric Force ;
Electric Field and
Electric Potential
Facts About Electric Charges
Facts About Electric Charges
Experiments proved the following facts :
(1) There are two types of electric charges :
Positive and Negative charges .
(2) Like charges repel each other and Unlike
charges attract each other .
(3) The electric charge is quantized :
The Millikan Oil-Drop Experiment proved
that : q = n e
Where n is an integer and e is the
electron charge .
Coulomb’s Law
Coulomb’s Law allows the calculation
of electrostatic attraction or repulsion.
Coulomb’s Law
Coulomb’s Law states that :
The magnitude of the electric force between
two POINT CHARGES is directly proportional
to the product of the charges and inversely
proportional to the square of the distance
between them .
POINT CHARGES : mean charges whose
dimensions are very small compared to their
separation .
Coulomb’s Law
Mathematically :
F  q1 q2 / r2
To convert from proportionality to
equality we multiply by a constant k
called : Coulomb’s Constant on the
honor of Coulomb .
F = k q1 q2 / r2
Coulomb’s Constant k
Coulomb’s Constant is not an absolute
constant ; it depends on :
(1) The medium through which the charges are
immersed
(2) The system of units in which the force F ;
the charge q and the distance r are
expressed .
In SI units with vacuum ( or air ) as the
medium it takes the value approximately :
k = 9 x 109 N.m2/C2
The Electric Field E
The electric field at a point is defined as
the electric force per unit charge at that
point . i.e
E = Fe / qo
or Fe = qo E
Where : Fe is the electric force acting on
a test charge qo placed at that point .
The electric field is a vector quantity and
its SI unit is : N/C
The Electric Field E
Relative Directions of Fe and E
Calculation of electric field
(1) Electric field due to point charges :
The magnitude of the electric field E at
a point p at a distance r from a point
charge q is given by : E = k q / r2
The direction of the field is the same as
the direction of the electric force acting
on a positive unit charge placed at that
point .
Electric Field Lines
Electric Field Lines
Properties of Electric Field Lines
(1) Electric field lines are imaginary lines
representing the paths of a unit positive
charge in the field .
(2) Electric field lines never intersect .
(3) The number of electric field lines crossing
a unit area perpendicular to it represents
the electric field strength
(4) Electric field lines are close to each other
where the field is strong and far from each
other where the field is weak .
Electric Potential
The electric potential at a point at a distance r
from a point charge q is given by :
V(r) = k q / r
The SI unit of V is volt .