Electric Force

ELECTRIC FORCE
Electrostatic
 Branch of Physics that deals with the
properties of electricity at rest.
Electrification
 The process of charging a body.
When a body possesses an excess positive charge

or an excess negative charge, it is said to be charged or
electrified.
First Law of Electrostatics
 Like charges REPEL
 Unlike charges ATTRACT

COULOMB’S LAW
The force between two small charged
bodies is directly proportional to the product
of the two charges and inversely proportional
to the square of the distance between them.
In equation form, Coulomb’s Law can be:

where:
kQ1Q2 F = force
F 2
Q1, Q2 = charges
r = distance between the two charges
r k = proportionality constant
= 9 x 109 N m2/C2
Summary of Units:
Conversion of Units:
1 Coulomb = 3 x 109 statcoulomb
1 microcoulomb = 1 x 10-6 coulomb
1 nanocoulomb = 1 x 10-9 coulomb
1 Coulomb = 6.25 x 1018 electrons
Process of Charging
 Charging by Friction
When two objects are rubbed against each
other, charge transfer takes place. One of the
objects loses electrons while the other object
gains electrons. The object that loses electrons
becomes positively charged and the object that
gains electrons becomes negatively charged.
Both the objects get charged due to friction and
this method of charging is commonly known as
electrification by friction.
Process of Charging
 Charging by Conduction
The method of charging an uncharged object by
bringing it close and in contact with a charged object
is known as charging by conduction. The charged
conductor has an unequal number of protons and
electrons, hence when an uncharged conductor is
brought near it, it discharges electrons to stabilize
itself.
A conductor is an object or type of material that
allows the flow of charge (electrical current) in one or
more directions. Materials made of metal are common
electrical conductors.
Process of Charging
 Charging by Induction
The process of charging an uncharged conductor
by bringing it near a charged conductor without
any physical contact is known as charging by
induction.
An electrical insulator is a material in which the
electron does not flow freely or the atom of the
insulator has tightly bound electrons whose
internal electric charges do not flow freely; very
little electric current will flow through it under the
influence of an electric field.
Sample Problems:
1. Suppose that two point charges, each with a
charge of +1.00 Coulomb are separated by a
distance of 1.00 meter. Determine the
magnitude of the electrical force of repulsion
between them?
Sample Problems:
2. Two balloons are charged with an identical
quantity and type of charge: -6.25 nC. They are
held apart at a separation distance of 61.7 cm.
Determine the magnitude of the electrical force
of repulsion between them?
Sample Problems:
3. Two balloons with charges of +3.37 μC and -8.21
μC attract each other with a force of 0.0626
Newtons. Determine the separation distance
between the two balloons.
Assignment (1 whole)
1. A small sphere carrying a charge of +30
stat Coulombs and another sphere
carrying +60 stat Coulomb are 10 cm
apart in vacuum. What is the force on
each sphere?
2. Two charges 12 x 10-9 C and 15 x 10-9 C
are 3 cm apart in vacuum. What is the
force between them?
3. Two balloons are separated 10 cm apart in
air. Calculate the magnitude of charge of
the second balloon if the first one has a
charge of 5 x 10-10 C and is experiencing a
force of 2.7 x 10-7 Newton.
1) Two charges are located on the
positive x-axis of a coordinate
system. Charge q1 = 2.0 x 10-9 C is 2.0
cm from the origin, and charge q2 = -
3.0 x 10-9 C is 4 cm from the origin.
What is the total force exerted by
these two charges on a charge q3 = 5.0
x 10-9 C located at the origin?
2) Four charges are arranged in a small
square with sides of length 2.5 cm.
The two charges in the top right and
bottom left corners are +3.0 x 10-6 C.
The charges in the other two corners
are -3.0 x 10-6 C. What is the net
force exerted on the charge in the
top right corner by the other three
charges?
SEATWORK(1 whole)
1. Calculate the force between two point
charges of 5 x 10-10 C and 6 x 10-10 C
which are 10cm apart in air.
2. What is the distance in meters
between two charges of 250 statC and
50 statC with a 12 Newton force
between them? (1 N = 105 Dyne)
3. Two coins lie 1.5 meters apart on a
table. They carry identical charges.
How large is the charge on each if the
coin experiences a force of 2.0
Newton?

Electric Force

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ELECTRIC FORCE

When a body possesses an excess positive charge

 Like charges REPEL

 Unlike charges ATTRACT

In equation form, Coulomb’s Law can be:

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