Chapter 26. Electric Charges and Forces

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Chapter 26. Electric Charges and Forces
The electric force is one of the fundamental
forces of nature. Controlled electricity is the
cornerstone of our modern, technological
society. Chapter Goal To developa basic
understanding of electric phenomena in terms of
charges, forces, and fields.
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Chapter 26. Electric Charges and Forces

• Topics
• Developing a Charge Model
• Charge
• Insulators and Conductors
• Coulombs Law
• The Field Model

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Stop to think 26.1 page 792Stop to think
26.2 page 795Stop to think 26.3 page
799Stop to think 26.4 page 805Stop to think
26.5 page 808Stop to think 26.6 page 810
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Charge Model, Part I
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Charges

• Quantization of Electric
• Charges
• The electric charge, q, is said to be
  quantized
• q is the standard symbol used for charge as a
  variable
• Electric charge exists as discrete packets
• q Ne
• N is an integer
• e is the fundamental unit of charge
• e 1.6 x 10-19 C
• Electron q -e
• Proton q e

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Rank in order, from most positive to most
negative, the charges qa to qe of these five
systems.

1. qa qb gt qe gt qc gt qd
2. qa gt qe gt qd gt qc gt qb
3. qe gt qa gt qd gt qb gt qc
4. qd gt qc gt qe gt qa qb
5. qd gt qc gt qe gt qa gt qb

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Conductors Electrical conductors are materials in
which some of the electrons are free
electrons Free electrons are not bound to the
atoms These electrons can move relatively freely
through the Material Examples of good conductors
include copper, aluminum and silver When a good
conductor is charged in a small region,
the charge readily distributes itself over the
entire surface of the material
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Insulators Electrical insulators are materials
in which all of the electrons are bound to
atoms These electrons can not move relatively
freely through the material Examples of good
insulators include glass, rubber and wood When a
good insulator is charged in a small region,
the charge is unable to move to other regions of
the material
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Charging an electroscope
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Charge Polarization
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Charge Polarization
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The Electric Dipole
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The Electric Dipole
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Charging by Induction, Step 1
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Charging by Induction, Step 2
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Charging by Induction, Step 3
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Metal spheres A and B are initially neutral and
are touching. A positively charge rod is brought
near A, but not touching. Is A now positive,
Negative or neutral. If keeping rod near the A,
separate A and B, are they still neutral or
charged?
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Coulombs Law Charles Coulomb measured the
magnitudes of electric forces between two small
charged spheres He found the force depended on
the charges and the distance between them
In SI units K 8.99 109 N m2/C2. K 1/4pe0 ,e
0 8.85x10 -12 C2 /Nm2
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Charges A and B exert repulsive forces on each
other. qA 4qB. Which statement is true?

1. FA on B gt FB on A
2. FA on B lt FB on A
3. FA on B FB on A

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Vector Nature of Electrical Forces Electrical
forces obey Newtons Third Law The force on q1
is equal in magnitude and opposite in direction
to the force on q2 F21 -F12 With like signs
for the charges, the product q1q2 is positive and
the force is repulsive
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Example 26.4 The point of zero force.Two
positively charged particles q1 and q2 3q1 are
10 cm apart. Where(other than at infinity) could
a third charge q3 be placed so as to experience
no net force.From the figure, you can see At
point A, above the axis, and at B, outside the
charges, cannot possibly add to zero. However, at
point C on the x-axis between the charges, the
two forces are oppositely directed
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Important Concepts
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The Electric Field

• We begin our investigation of electric fields by
  postulating a field model that describes how
  charges interact
• Some charges, which we will call the source
  charges,    alter the space around them by
  creating an electric field.
• A separate charge in the electric field
  experiences a force    exerted by the field.
• Suppose probe charge q experiences an electric
  force
• Fon q due to other charges.

The units of the electric field are N/C. The
magnitude E of the electric field is called the
electric field strength.
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Important Concepts
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An electron is placed at the position marked by
the dot. The force on the electron is

1. to the right.
2. to the left.
3. zero.
4. Theres not enough information to tell.

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Rank in order, from largest to smallest, the
electric field strengths E1 to E4 at points 1 to
4.

1. E2 gt E4 gt E1 gt E3
2. E1 E2 gt E3 E4
3. E2 gt E1 E4 gt E3
4. E2 gt E1 gt E4 gt E3
5. E1 gt E2 gt E3 gt E4