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CHAPTER 20, SECTION 1 – ELECTRIC CHARGE
Main Idea
Like charges repel, and unlike charges attract
EVIDENCE OF CHARGE
Electrostatics is the study of electric charges
that can be collected and held in one place.
Scientist have determined that there are two
types of electric charge
 Benjamin Franklin named them positive and negative charges
 Two objects with like charges always repel each other.
 Two objects with unlike charges always attract each other.
If you rub your hair with a balloon, it will stand on
end because of the electrostatic force between
your hair and the oppositely charged balloon.
A MICROSCOPIC VIEW OF CHARGE
A MICROSCOPIC VIEW OF CHARGE
All materials contain light, negatively
charged particles called electrons. In
addition, each atom has a massive, positively
charged nucleus, containing protons.
For a neutral object, the amount of positive
charge exactly balances the amount of
negative charge.
With the addition of energy, the outer
electrons can be removed from atoms.
An atom missing electrons has an overall
positive charge, and consequently, any
matter made of these electron-deficient
atoms is positively charged.
The freed electrons can remain unattached
or become attached to other atoms,
resulting in negatively charged particles.
A MICROSCOPIC VIEW OF CHARGE
Electric charge carriers are electrons
rather than protons because the
electrons:
Have a lighter mass
Are located far from the nucleus
Are loosely bound to the nucleus
A MICROSCOPIC VIEW OF CHARGE
From a microscopic viewpoint, acquiring charge is
a process of transferring electrons.
If two neutral objects are rubbed together, each
can become charged.
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A MICROSCOPIC VIEW OF CHARGE
For instance, when rubber shoes are rubbed on a wool
rug, electrons are removed from the atoms in the wool,
and transferred to the shoes.
The extra electrons on the rubber result in a net
negative charge. The electrons missing from the wool
result in a new positive charge.
Charge is conserved. This is, individual charges never
are created or destroyed, they just move around.
HAVE YOU EVER EXPERIENCED AN
ELECTRIC CHARGE?
Ever been shocked by a door knob or car
door handle?
Had the saran wrap stick to itself?
Had clothes stick to each other when
getting them out of the dryer (static
cling)?
Gone down a slide and had your hair stick
up?
These are all examples of electric charge.
CONDUCTORS AND INSULATORS
A material through which a charge will not move easily is
called an electric insulator.
A material that allows charges to move about easily is
called an electric conductor.
Metals are good conductors because at least one
electron on each atom can be removed easily. These
electrons move freely throughout the piece of metal.
CHAPTER 20, SECTION 2 – ELECTROSTATIC FORCE
Main Idea
Forces between charged particles are
mathematically related to charge and
distance.
CONDUCTORS AND INSULATORS
Under certain conditions, charges move through air as if
it were a conductor
The spark that jumps between your finger and a
doorknob after you have rubbed your feet on a carpet
discharges you. In other words, you have become neutral
because the excess charges have left you.
Similarly, lightning discharges a thundercloud. In both
of these cases, air became a conductor for a brief
moment. Excess charges in the cloud and on the ground
are great enough to remove electrons from the molecules
in the air.
FORCES ON CHARGED OBJECTS
The electrostatic force can be demonstrated by
suspending a charged rod so that it turns easily.
The results of these experiments and the actions of the
charged rods can be summarized in the following way:
 There are two kinds of electric charge: positive and negative
 Charges exert forces on other charges at a distance.
 The force is stronger when the charges are closer together.
 Like charges repel; opposite charges attract.
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ELECTRIC FORCE
When two charged objects are brought close together,
they may experience motion either toward or away
from each other.
The closer the two objects are to each other, the
stronger the force between them.
The greater the magnitude of the charges the
stronger the force between them
COULOMB’S LAW
qa  qb
r2
The electric force, like all other forces, is a
vector quantity.
COULOMB’S LAW
FK
qa  qb
r2
Used to collect soot in smokestacks, there by
reducing air pollution
Tiny paint droplets, charged by induction, can be used
to paint automobiles and other objects uniformly
Photocopy machines use static electricity to place
black toner on a page to make a copy
FK
qa  qb
r2
The standard unit for Charge is called the Coulomb (C).
The symbol for charge is q.
One coulomb is the charge of 6.24 x 1018 electrons or
protons.
The charge on a single electron is -1.60 x 10-19 C.
The magnitude of the charge of an electron is called
the elementary charge.
The charge on a single proton is +1.60 x 10-19 C.
EXAMPLE – COULOMB’S LAW
•Coulomb’s Law is used to calculate the force between 2
charged objects
Electric force  Coulomb constant 
There are many applications of electric forces on
particles
COULOMB’S LAW
According to Coulomb’s Law, the magnitude of the
force between two point charges (qa and qb) a
distance r apart can be written as follows.
FK
APPLICATIONS OF ELECTROSTATIC
FORCES
(charge 1)(charge 2)
(distance) 2
•Coulomb’s Constant, Kc = 8.99 X 109 N.m2/C2
•qa and qb = Charge on object 1 and 2
•r = distance between the objects.
•F = Electric Force
What is the magnitude of the electric force between a proton and an electron in a hydrogen atom?
The two particles are separated by a distance of 5.3 x 10-11 m on average.
 Given:
 qa = -1.60 x 10-19 C.
 qb = 1.60 x 10-19 C.
 r = 5.3 x 10-11 m
 Unknown:
 F=?
 The equation:
F k
Ans. 8.2 x
qa  qb
r2
10-8 N
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