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Lecture Outline
Chapter 15
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
Chapter 15
Electric Charge, Forces, and
Fields
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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.
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15.1 Electric Charge
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
Charge is conserved:
The net charge of an isolated system
remains constant.
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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.
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15.2 Electrostatic Charging
Charging by friction: This is
the process by which you
get “charged up” walking
across the carpet in the
winter. It is also the process
that creates “static cling” in
your laundry, and makes it
possible for you to rub a
balloon on your hair and
then stick the balloon to the
wall.
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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.
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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.
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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.
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15.3 Electric Force
The force exerted by one charged particle
on another is given by:
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15.3 Electric Force
If there are multiple point charges, the force
vectors must be added to get the net force.
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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
For multiple charges, the total electric field is
found using the superposition principle:
For a configuration of charges, the total, or net, electric
field at any point is the vector sum of the electric fields
due to the individual charges.
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15.4 Electric Field
It is convenient to represent
the electric field using
electric field lines, or lines
of force. These lines are
drawn so the field is tangent
to the line at every point.
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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
Electric field lines of a dipole:
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15.4 Electric Field
Electric field lines due to very large parallel
plates:
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15.4 Electric Field
Electric field lines due to like
charges: (a) equal charges;
(b) unequal charges.
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15.5 Conductors and Electric Fields
Electric charges are free to move within a
conductor; therefore, there cannot be a static
field within the conductor:
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
There cannot be any component of the electric
field parallel to the surface of a conductor;
otherwise charges would move.
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
The force from neighboring charges is less when
the curvature of the surface is large:
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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15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
A surface, called a Gaussian
surface, that completely
surrounds a point charge
intercepts the same number
of field lines regardless of its
shape. For a positive charge,
the lines exit the surface; for
a negative one they enter it.
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15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
If a greater amount of charge is enclosed,
more field lines cross the surface.
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15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
Here, surface 1 surrounds the
positive charge and has lines
exiting it.
Surface 2 surrounds the negative
charge and has lines entering it.
Surface 3 does not enclose any
charge, and the same number of
lines exit as enter.
Surface 4 encloses both charges; as they are
equal in magnitude, the same number of lines exit
the surface as enter it.
© 2010 Pearson Education, Inc.
15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
The underlying physical principle of Gauss’s
law:
The net number of electric field lines passing through
an imaginary closed surface is proportional to the
amount of net charge enclosed within that surface.
This can be used to show
that excess charge on a
conductor must reside on
the surface.
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Summary of Chapter 15
Like charges repel; unlike charges attract.
Charge is conserved.
Electrons move freely inside conductors, but
not inside insulators.
Objects may be charged electrostatically by
friction, conduction, or induction.
Polarization is the separation of positive and
negative charge within an object.
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Summary of Chapter 15
Coulomb’s law:
The electric field, a vector, is the force per
unit charge. Electric fields from multiple
charges add by superposition.
Electric field lines are used to represent the
electric field.
A conductor has zero electric field inside
and has all excess charge on its surface.
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Summary of Chapter 15
The electric field is always perpendicular to
the surface of a conductor.
The charge density and electric field are
greatest on a conductor where the curvature
is largest.
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