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Lecture Outline
Chapter 15
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
Units of Chapter 15
Electric Charge
Electrostatic Charging
Electric Force
Electric Field
Conductors and Electric Fields
Gauss’s Law for Electric Fields: A Qualitative
Approach
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15.1 Electric Charge
Electric charge is a
fundamental property of
matter; electric charges may be
positive or negative.
The atom consists of a small
positive nucleus surrounded
by a negative electron cloud.
It’s easier to use a solar
system model.
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15.1 Electric Charge
The Law of Charges or the Charge-Force Law:
Like charges repel; unlike charges attract.
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15.1 Electric Charge
SI unit of charge: the coulomb, C. All charges
are integer multiples of the charge on the
electron:
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15.1 Electric Charge
The Law of Conservation of Charge:
The net charge of an isolated system remains constant.
Aka…charge must be conserved!
It’s easy to think of conservation of charge if using
ionic compounds as the example.
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15.1 Electric Charge
• When you shuffle across a carpeted floor on a dry day,
the carpet acquires a net positive charge.
• A.) Will you have a deficiency or excess of electrons?
Why?
• B.) If the charge the carpet acquired has a magnitude of
2.15 nC, how many electrons were transferred?
15.2 Electrostatic Charging
Conductors transmit
charges readily.
Semiconductors are
intermediate; their
conductivity can depend
on impurities and can be
manipulated by external
voltages.
Insulators do not transmit
charge at all.
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15.2 Electrostatic Charging
An electroscope may be used to determine if an
object is electrically charged. Let’s talk about
how an electroscope works.
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15.2 Electrostatic Charging
• 4 methods of Electrostatic Charging:
– Charging by Friction
– Charging by Conduction (Contact)
– Charging by Induction
– Charge Separation by Polarization
15.2 Electrostatic Charging
Charging by friction:
Certain insulator materials are rubbed
(typically with cloth or fur) and they become
electrically charged by a transfer of charge.
The transfer of charge is due to the frictional
contact between the materials.
Walking across the carpet in the winter.
“Static Cling”
Rubbing a Balloon on your Hair
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15.2 Electrostatic Charging
An electroscope can be
given a net charge by
conduction—when it is
touched with a charged
object, the excess
charges flow freely onto
the electroscope.
“Conduction” refers to
the flow of charge during
a short period of time due
to contact”
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15.2 Electrostatic Charging
An electroscope may also be charged by
induction, if there is a way of grounding it while
charge is being induced. Induction is a method
used to charge without actually touching the
object.
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15.2 Electrostatic Charging
Charge may also be
moved within an
object—without
changing its net
charge—through a
process called
polarization.
Polarization is a
separation of positive
and negative charge.
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15.3 Electric Force
The force exerted by one charged particle on
another is given by: [This is the magnitude of
force between 2 point charges.]
Called Coulomb’s Law.
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15.3 Electric Force
• A rubber comb combed through
dry hair can acquire a net negative
charge. That comb will then attract
small pieces of uncharged paper.
Since the paper has no net charge,
you might expect there to be no
electric force on it. Which charging
mechanism explains this
phenomenon, and how does it
explain it?
– Conduction
– Friction
– Polarization
15.3 Electric Force
• Two point charges of -1.0 nC and +2.0 nC are separated
by a distance of 0.30m. What is the electric force on
each particle?
15.3 Electric Force
• What is the magnitude of the repulsive electrostatic force
between two protons in a nucleus? Take the distance from
the center of these protons to be 3.00 x 10-15 m.
• If the protons were released from rest, what would the
magnitude of the acceleration be? How does this compare
with acceleration due to gravity?
15.3 Electric Force
• A configuration of three charges is shown
in the figure below. What is the net electric
force on q3.
15.4 Electric Field
Definition of the electric field:
The direction of the field is the direction
the force would be on a positive charge.
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15.4 Electric Field
Charges create electric fields, and these fields in turn
exert electric forces on other charges.
Electric field of a point charge:
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15.4 Electric Field
• In the figure below, what quadrant is the electric field at
the origin? The first? Second? Third? Explain.
• Calculate the magnitude and direction of the electric field
at the origin due to these charges.
15.4 Electric Field
• A convenient way of graphically representing
the electric field is by use of electric lines of
force (electric field lines).
• There are rules that dictate field lines.
15.4 Electric Field
Rules for drawing electric field lines:
1. Closer lines mean a stronger field.
2. The field is tangent to the lines at every point.
3. Field lines start on positive charges and end
on negative charges.
4. The number of lines entering or leaving a
charge is proportional to the magnitude of the
charge.
5. Field lines never cross.
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15.4 Electric Field
15.4 Electric Field
Use the electric field line rules to construct a
typical electric field line due to an electric
dipole.
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15.4 Electric Field
15.5 Conductors and Electric Fields
Electric charges are free to move within a
conductor; however, in a static electric field,
electrons are at rest which means there is no
electric field:
The electric field is zero inside a charged conductor.
Excess charges on a conductor will repel each
other, and will wind up being as far apart as
possible.
Any excess charge on an isolated conductor resides
entirely on the surface of the conductor.
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15.5 Conductors and Electric Fields
The electric field at the surface of a charged conductor
is perpendicular to the surface.
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15.5 Conductors and Electric Fields
Excess charge tends to accumulate at sharp points, or
locations of highest curvature, on charged
conductors. As a result, the electric field is greatest at such
locations.
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