Electric Forces and
Electric Fields
The primary particle that
carries charge (and therefore
can be lost or gained) in an
atom is a/an:
ct
Ele
tro
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Ne
u
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n
2.
Proton
Neutron
Electron
Pr
ot
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1.
When a positively charged object
comes close to an negatively
charged object, the negative
object will:
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at
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Be attracted
Be repelled
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1.
When a positively charged object
comes close to an neutral object,
the neutral object will:
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at
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Be repelled
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1.
Anytime an object at rest
starts to move, what must be
present to cause it to move?
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Difference in
electric charge
Electricity
A force
Matter
Di
ffe
r
1.
A Bit of History

Ancient Greeks

Observed electric and magnetic
phenomena as early as 700 BC

Found that amber, when rubbed, became
“electrified” and attracted pieces of straw or
feathers
Properties of Electric Charges

Two types of charges exist



They are called positive and negative
Arbitrarily named by Benjamin Franklin
Like charges repel and unlike charges attract
one another
More Properties of Charge
Positive charge – protons
 Negative charge – electrons


Gaining or losing electrons is how an
object becomes charged; protons remain
with the nucleus
A little review:

What are some conservable quantities
in physics?
Mass
 Momentum
 Energy
 Charge

Conservation of Charge

Electric charge is always conserved
Charge is not created, only exchanged
 Objects become charged because negative
charge is transferred from one object to
another

More review:

A force is …?


Anything that produces acceleration or a
change in motion.
Contact vs. Field forces?
Contact: require matter to be in contact
(ex. friction)
 Field: matter not required (ex.
gravitational, electrical, magnetic)

Fig. 15.1, p. 467
Properties of Charge, final

Charge is quantized
All charge is a multiple of a fundamental
unit of charge, symbolized by e
 Electrons have a charge of –e
 Protons have a charge of +e
 The SI unit of charge is the Coulomb (C)


1 e = 1.6 x 10-19 C
Fig. 15.T1, p. 472
Conductors

Conductors are materials in which the
electric charges move freely

Copper, aluminum and silver are good
conductors
Insulators

Insulators are materials in which
electric charges do not move freely
Glass and rubber are examples of
insulators
 When insulators are charged by rubbing,
only the rubbed area becomes charged


There is no tendency for the charge to move
into other regions of the material as opposed to
conductors
Three Methods of Charging
Friction
 Conduction (or Contact)
 Induction

Charging by Friction

The act of rubbing creates friction
which removes or adds electrons to the
objects involved in the friction.
Charging by Conduction




A charged object (the rod) is
placed in contact with
another object (the sphere)
Some electrons on the rod
can move to the sphere
When the rod is removed,
the sphere is left with a
charge
The object being charged is
always left with a charge
having the same sign as the
object doing the charging
Charging by Induction


A negatively charged
rubber rod is brought
near an uncharged
sphere
The charges in the
sphere are redistributed

Some of the electrons in
the sphere are repelled
from the electrons in the
rod
Charging by Induction, final
The wire to ground is removed, the
sphere is left with an excess of induced
positive charge
 The positive charge on the sphere is
evenly distributed due to the repulsion
between the positive charges


Charging by induction requires no
contact with the object inducing the
charge
A charged rod is brought close to a neutral
electroscope. When touched by the rod, the
leaves both become positive and repel. The rod
must have been …
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Positively charged
Negatively
charged
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A neutral electroscope is charged by induction.
When touched by the rod, the leaves both
become negative and repel. The rod must have
been …
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Positively charged
Negatively
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A postively charged rod is brought close to a
neutral electroscope to charge it by induction.
The top of the electroscope must be:
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Negatively
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1.
Coulomb’s Law
q1 q2

Mathematically, Felec  kc

kc is called the Coulomb Constant

kc = 8.99 x 109 N m2/C2
r2
Coulomb’s Law

Felec  kc
q1 q2
r
2
Typical charges can be in the µC range

Remember, Coulombs must be used in the
equation
Remember that force is a vector quantity
 It is attractive if the charges are of opposite
signs and repulsive if the charges have the
same signs (you must note if it is attractive or
repulsive after the magnitude – 3 N
attractive)

How is the magnitude of the
charges proportional to the
electric force between them?
en
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Directly
Inversely
Exponentially
Di
re
1.
How is the square of the distance
between two charges proportional to
the electric force between them?
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Directly
Inversely
Exponentially
Di
re
1.
Coulomb’s Law

Felec  kc
q1 q2
r2
It is attractive if the charges are of opposite signs
and repulsive if the charges have the same signs
(you must note if it is
Things that make you go,
“Hmmmm…”
A) The electric force is significantly
stronger than the gravitational force.
However, although we are attracted to
Earth by gravity, we do not usually feel
the effects of the electric force.
 Explain why.

B) An ordinary nickel contains about
1024 electrons, all repelling one another.
 Why don’t these electrons fly off the
nickel?


C) When the distance between two
negatively charged balloons is doubled,
by what factor does the repulsive force
between them change?
Electrical Force Compared to
Gravitational Force
Both are inverse square laws
 The mathematical form of both laws is
the same
 Electrical forces can be either attractive
or repulsive
 Gravitational forces are always
attractive

If all other variables are held constant, is the
electric force or the gravitational force greater
for two oppositely charged objects?
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Gravitational Force
Electric Force
Same
Unable to tell
Gr
av
i
1.
Compare Gravitational and
Electric Force

Calculate the gravitational force as well
as the electric force for an electron and
a proton which are located 1 cm from
each other.
Electrical Field

An electric field is said to exist in the
region of space around a charged
object

When another charged object enters this
electric field, the field exerts a force on the
second charged object
Electric Fields



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.
Gravitational Field

Mass creates a gravitational field that
exerts forces on other masses.
Gravitational vs. Electric Fields

Gravitational forces are far weaker than
electric forces.
Van de Graaff
Generator



An electrostatic
generator designed and
built by Robert J. Van
de Graaff in 1929
Charge is transferred to
the dome by means of a
rotating belt
Eventually an
electrostatic discharge
takes place
Electrical Field

An electric field is said to exist in the
region of space around a charged
object

When another charged object enters this
electric field, the field exerts a force on the
second charged object
Electric Field, cont.
A charged particle,
with charge Q,
produces an electric
field in the region of
space around it
 A small test charge,
qo, placed in the
field, will experience
a force

Electric Field Lines
Electric field is a vector.
 There are electric field lines that help
visualize this field and were introduced
by Michael Faraday

Electric Field Lines

Electric field lines are drawn to visualize
electric field strength and direction.

Introduced by Michael Faraday
Electric Field Line Rules

Electric Field Lines are drawn pointing in the
way that a positive point charge would
move when placed by the charged object.

Field lines can never cross
Electric fields exist even in the
absence of a point charge.

Electric Field Line Rules

The direction of the electric field is in the
same direction as the electric force on the
point charge.
The relative strength of the electric field is
proportional to the number of field lines in a
given location.
 Electric field lines accumulate as sharp points
more than rounded objects.

Proton
+
+
A positive
test charge
would be
repelled by
the field
Electron
+
-
A positive
test charge
would be
attracted by
the field

Opposite charges attract

Like Charges repel
Electric Field Lines for a Dipole
Electric Field for Parallel Plate
Capacitor
Summary of Electric Field Lines
Electric Potential Energy of a
Charge
Wants to move when it
has high PE
 Point b

PE = max
 KE = min


Point a
PE = min
 KE = max

Electric Field Intensity
Equation
E

= F/q
F is the force in
Newtons acting
on the test
charge q in
coulombs.
Combined with Coulomb’s law,
E = kQ/r2
Electric Fields Between Charges –
Superposition Principle
Find “E” for each
charge, add all
together.

ET=E1+E2+E3…