# Chapter 16 1 Chapter 16 Electric Forces by ert554898

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Chapter 16: Electric Forces and
Fields

•Electric Charge
•Conductors & Insulators
•Coulomb’s Law
•Electric Field
•Motion of a Point Charge in a Uniform E-field
•Conductors in Electrostatic Equilibrium
•Gauss’s Law
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§16.1 Electric Charge
Experiments involving the rubbing of plastic, glass and wood
rods with wool, silk, and other materials cause the material to
inherit a property that allows for long-range attractive and
repulsive forces.

Examples:
•Unrolling plastic rap
•Running a comb through hair
•Rubbing rubber/plastic/glass rods with fur and silk
•Walking on carpet with slippers
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A charged object always
attracts an uncharged
macroscopic object

Pithballs charged by being
touched by same object
always repel

Pithballs can be charged
such that they are attracted
to each other.
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Results
•Rubbing causes objects to be charged.
•Charged object attracts neutral object
•Sometimes two charged objects attract, sometimes they repel.
•Objects charged the same way always repel.

There are two kinds of electric charge: positive and negative
(a 3rd type of charge that is attracted to both positive and
negative has never been found).
Like charges repel, opposite charges attract.
Choice of what is a positive charge and a negative charge is
arbitrary (glass rubbed with silk is defined as positive
charging).
Two-charge model is consistent with experiments…
The Charge Model

Slide 20-8
Visualizing Charge

• Charges on an insulator do not move.
• Charges on a conductor adjust until
there is no net force on any charge.
We call this electrostatic
equilibrium.

Slide 20-9
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§16.2 Conductors and Insulators

A conductor is made of material that allows electric charge to
move through it easily.
Examples include copper, gold, and silver.

An insulator is made of material that does not allow electric
charge to move through it easily.

Examples include wood, plastic, and rubber.
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A metallic sphere is initially neutral. A negatively charged
rod is brought in contact with a connecting rod, which is
also touching the metallic sphere. What type of rod should
be used such that the metallic sphere is charged?

1.a conducting rod
2.a insulating rod
3.Either kind of rod will charge the sphere.
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Suppose a metal rod is initially neutral and is near a
neutral hanging pith ball. A negatively charged rod is
brought near the left end of the metal rod, but doesn’t
touch it. Why is the pith ball attracted to the metal rod?

1. The metal rod becomes positively charged.
2. The metal rod becomes negatively charged.
3. The metal rod becomes polarized.
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A negatively charged glass rod is brought near a spherical
conductor, which is initially neutral. The spherical
conductor is initially touching another spherical conductor
which is also initially neutral. While the glass rod is near
the conductor, the two conductors are separated. The
glass rod is then removed. What are the charged states of
the two conductors?
1. Left is positively
charged, right is
negatively charged.
2. Right is positively
charged, left is
negatively charged.
3. Left is positively
charged, right is
neutral.
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An electroscope is positively charged by touching it
with a positive glass rod. The electroscope leaves
spread apart and the glass rod is removed. Then a
negatively charged plastic rod is brought close to the
top of the electroscope, but it doesn’t touch. What
happens to the leaves?

A. The leaves spread further apart.
B. The leaves get closer together.
C. One leaf moves higher, the other lower.
D. The leaves don’t move.
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The elementary unit of charge is e = 1.60210-19 C.

The charge on the electron is -1e.
The charge on the proton is +1e.
The charge on the neutron is 0e.
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§16.3 Coulomb’s Law
The magnitude of the force                  k q1 q2
between two point charges is:         F
r2
where q1 and q2 are the charges and r is the separation
between the two charges.

k  8.99 10 Nm /C
9      2   2

1
where k            and  0  8.85 10 12 C 2 /Nm 2
4 0
and 0 is called the permittivity of free space.
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The electric force is directed between the centers of the two
point charges.

F12       F21
q1                        q2     Attractive force
between q1 and q2.
r

q1              q2
Repulsive force      F12                         F21
between q1 and q2.
r
The electric force is an example of a long-range or field
force, just like the force of gravity.
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