Download Chapter 16 Notes

Chapter 16
Electric Charge and Electric Field
© 2002, B.J. Lieb
Giancoli, PHYSICS,5/E © 1998. Electronically reproduced
by permission of Pearson Education, Inc., Upper Saddle
River, New Jersey
Structure of the atom (protons, neutrons and electrons)
• Nucleus contains positive charge
•Protons have positive charge, neutrons are neutral
•Mass of proton (and neutron)  1800 x mass of
electron
• Electrons have negative charge and are attracted to
nucleus
• Charge of electron is equal in magnitude to that of
proton
• Normal atom is neutral
• Ion is atom that has gained or lost one or more
electrons
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Conductors and Insulators
Conductor: metal atoms in solids have one or two “free”
electrons which move freely through the material
Insulator: no free electrons so it will not conduct electricity
Static Electricity:
• Rubbing certain materials together can separate electrons
from their atoms
• Removing electrons from a material makes it positive
• In solids, it is always the electrons that move because
they are lighter
• Induced charge: If you bring a + charge near a
conductor, it will attract electrons to it leaving the other
half of the metal positive.
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Coulomb’s Law
Q1Q2
F k 2
r
• k  9.0  109 N·m2 / C2
•Forces are equal in magnitude but opposite in
direction
•This equation gives the magnitude of the
force-you have to figure the direction from the
signs of the charges
•For spherical charges, r is the center to center
distance
•C is the unit of charge-the Coulomb
•Coulomb’s Law applies to charges at rest
• e = 1.60x10-19 C electronic charge (positive)
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Example 1
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Example 2
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The Electric Field
•Graphical representation of electrical forces
•Electrical force “acts at a distance” like
gravity
•Electric field E surrounds every charge
•We investigate the field with a small
positive charge called a “test charge” q
•The magnitude of the field is:
F
E
q
•Units are N/C
•E is a vector –direction of force experienced
by positive test charge
•The magnitude of q is so small that it does
not disturb the charges that cause the field
•To plot the field, move the test charge
around the charges that cause the field
•Since q is positive the field points away
from a + charge and towards a - charge
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Field of a Point Charge
F
E
q
Q1Q2
F k 2
r
q = Q2
Q
Ek 2
r
This is the field created by a point
charge or a spherical charge
distribution Q
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Electric Field is a Vector
•Since test charge is positive, the direction
of the electric field is the direction of the
force felt by a positive charge
•Field thus points toward a negative
charge and away from a positive charge
•If there are two or more charges creating
the field then the field at any point is the
vector sum of the fields created by each of
the charges
•The test charge does not contribute to the
field and it is too weak to cause any of the
charges creating the filed to move.
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Electric Field Lines
• Graphical way of showing the electric field
• You have seen graphical representations of
the earth’s magnetic field-the electric field
maps are similar.
• We also refer to the gravitational field of
the earth.
• Sometimes called lines of force
• Arrow on field line gives direction of force
• The closer together the lines of force are,
the stronger the electric field
• Electric field lines start on positive charges
and end on negative charges
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Electric Field of Point Charges
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Electric Field of Parallel Plates
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Electric Fields and Conductors
•If a conductor is placed in an electric field, the
electrons will rearrange themselves until until the field
inside the conductor is zero
•This is called the static situation
•In the static situation, the field outside the conductor is
perpendicular to the surface of the conductor (if the
field had a component parallel to the surface, it would
cause the electrons to move.)
•The field inside a hollow conductor shell is zero (Fig
16-33)
•If a charge is placed within a spherical conductor shell,
there is a field inside and outside the shell, but not in
the conductor (Fig 16-31)
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Electric Field Examples
Study examples 16-6, 16-7 and 16-8.
Since the electric field is a vector, it is
necessary to calculate the field created by
each charge and then use vector addition to
determine the net field.
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Example 3
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