Coulomb’s Law
PAUL VICTOR TAMURIA
Science Teacher
Bangbo Witthayakhom School
Electric Charge
• Electric charge is a property of
tiny particles in atoms.
• The unit of electric charge is
the coulomb (C).
• A quantity of charge should
always be identified with a
positive or a negative sign.
Electric forces
• Electric forces are created between all electric charges.
• Because there are two kinds of charge (positive and negative) the
electrical force between charges can attract or repel.
Electric current
•Current is the movement of electric charge
through a substance.
Charge that flows
Current (coulombs)
(amps) I=q
t
Time (sec)
Calculate current
•Two coulombs of charge pass through a wire
in five seconds.
•Calculate the current in the wire.
Conductors and insulators
•All materials contain
electrons.
•The electrons are what carry
the current in a conductor.
•The electrons in insulators
are not free to move—they
are tightly bound inside
atoms.
Conductors and insulators
▪A semiconductor has a few free electrons and atoms
with bound electrons that act as insulators.
Conductors and insulators
•When two neutral objects are
rubbed together, charge is
transferred from one to the
other and the objects become
oppositely charged.
•This is called charging by
friction.
•Objects charged by this
method will attract each
other.
Coulomb's Law
•Coulomb’s law relates the force between two
single charges separated by a distance.
Constant
9 x109 N.m2/C2
Force
(N) F = K q1 q2 Charges (C)
r2
Distance (m)
Coulomb's Law
•The force between two
charges gets stronger as
the charges move closer
together.
•The force also gets
stronger if the amount
of charge becomes
larger.
Coulomb's Law
•The force between two
charges is directed
along the line
connecting their
centers.
•Electric forces always
occur in pairs according
to Newton’s third law,
like all forces.
Coulomb's Law
•The force between
charges is directly
proportional to the
magnitude, or amount, of
each charge.
•Doubling one charge
doubles the force.
•Doubling both charges
quadruples the force.
Coulomb's Law
The force between charges is
inversely proportional to the
square of the distance between
them.
Doubling the distance reduces the
force by a factor of 22 = (4),
decreasing the force to one-fourth
its original value (1/4).
This relationship is called an
inverse square law because force
and distance follow an inverse
square relationship.
Coulomb's Law
1.) Why does electrical force between a pair of charged objects decrease if they
are moved farther apart?
2.) By how much does the electric force between a pair of charged bodies
decrease when their separation is…
(a) …doubled? (b) …tripled?
3.) By what factor does the force between two charged bodies increase if the
separating distance between them is…
(a) …reduced to ½ of its original? (b) …reduced to ¼
its original?
Coulomb's Law
• Double one of the charges
– force doubles
• Change sign of one of the
charges
– force changes direction
• Change sign of both charges
– force stays the same
• Double the distance between
charges
– force four times weaker
• Double both charges
– force four times stronger
Calculating force
• Two balls are each given a static electric charge of one ten-
thousandth (0.0001) of a coulomb.
• Calculate the force between the charges when they are
separated by one-tenth (0.1) of a meter.
• Compare the force with the weight of an average 70 kg
person.
Solutions
1. You are asked to calculate the force and compare it to a
person’s weight.
2. You are given the charges and separation, and the mass of the
person.
3. Use Coulomb’s law, F= -Kq1q2/R2, for the electric force and F=mg
for the weight.
4. Solve:
F = (9×109 N•m2/C2)(0.0001C)(.0001C) ÷ (0.1 m)2 = 9,000 N
The weight of a 70 kg person: F = mg = (70 kg)(9.8 N/kg) = 686 N
The force between the charges is 13.1 times the weight of an
average person (9,000 ÷ 686).
Calculating force
Two positive charges of 6.0 x 10-6 C
are separated by 0.50 m. What is the
magnitude of the force between the
charges? Is this force repulsive or
attractive?
Calculating force
A negative charge of 2.0 x 10-4 C
and a positive charge of 8.0 x 10-4
C are separated by 0.30 m. What
is the magnitude of the force
between the charges? Is this force
repulsive or attractive?
Electric Fields
• An electric field is a region around a charged object in
which a stationary charged object experiences an electric
force.
• An electric field is in all directions (3D).
• Direction of the electric field is the direction in which a
force would act on a positive charge.
Fields and forces
The concept of a field is used to describe any quantity that has
a value for all points in space.
You can think of the field as the way forces are transmitted
between objects.
Charge creates an electric field that creates forces on other
charges.
Fields and forces
•Mass creates a gravitational field that exerts
forces on other masses.
Fields and forces
•Gravitational forces are far weaker than electric
forces.
Drawing the electric field
Electric fields and electric force
• On the Earth’s surface, the gravitational field creates 9.8 N of
force on each kilogram of mass.
• With gravity, the strength of the field is in newtons per
kilogram (N/kg) because the field describes the amount of
force per kilogram of mass.
Electric fields and electric force
• With the electric field, the strength is in newtons per coulomb
(N/C).
• The electric field describes the amount of force per coulomb of
charge.
Coulomb’s Law vs. Law of Universal Gravitation
