Transcript Electric Forces and Electric Fields

Welcome to Physics 112
• Instructor: Mike Talbot
• Web Page :
delta.edu/mttalbot/physics112
Electric Forces and
Electric Fields
Chapter 15
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3
Introduction
• Greek contributions (700 BC)
•Friction generates
electricity
•Rub a piece of amber
with wool
61
Introduction
• Charged particles
– What are they?
– What colors are they?
Introduction
• Coulomb’s Law
– What does it say?
Introduction
• Electric fields
– What is a gravitational
field?
– What is an electric field?
Properties of Electric
Charges
• An electroscope may be used to
demonstrate the existence of
electrostatic forces.
• It detects and identifies charges
produced by such things as:
• Plastic combs
• Glass and silk
• Hard rubber and wool
• Balloons
62, 15.1
Properties of Electric
Charges
• Walking across a carpet in
the winter can also
generate static electricity.
Properties of Electric
Charges
• Dust accumulates on a TV
screen because of the large
static charge on the glass.
Properties of Electric
Charges
• What should you do if a
power line falls on your car
while you are in it?
Properties of Electric
Charges
• How many kinds of charge
exist?
Properties of Electric
Charges
• How many kinds of charge
exist?
– Two
Properties of Electric
Charges
• Two kinds of charge (+ and -)
– Named by Benjamin
Franklin
Properties of Electric
Charges
• Law of Charges
155
Properties of Electric
Charges
• Law of Charges
• At least two particles are
involved
•Like charges repel,
opposites attract.
•Motion may result
Properties of Electric
Charges
• Law of Charges
• At least two particles are
involved
•Like charges repel,
opposites attract.
•Motion may result
•Planetary model of the
atom
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Properties of Electric
Charges
• Are atoms neutral?
Properties of Electric
Charges
• What is meant by the
conservation of charge?
Properties of Electric
Charges
• What is meant by the
quantization of charge (e)
Properties of Electric
Charges
• Demonstrating the
quantization of charge (e)
– The Millikan Oil-Drop
Experiment
•Measured the elementary
charge on an electron
15.21
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Insulators and
Conductors
• Materials may be classified
by their ability to conduct
electricity.
Insulators and
Conductors
• Materials may be classified
by their ability to conduct
electricity.
– Conductors (many free electrons)
– Insulators (few free electrons)
Insulators and
Conductors
• Materials may be classified
by their ability to conduct
electricity.
– Conductors (many free electrons)
– Insulators (few free electrons)
– Semiconductors
Insulators and
Conductors
• What are some examples of
good conductors?
Insulators and
Conductors
• What are some examples of
good insulators?
Insulators and
Conductors
• What is an example of a
semiconductor?
• Where are semiconductors
used?
225
Charging by Friction
Charging by conduction
15.3
Charging by induction
15.4
Grounding
127
Insulators and
Conductors
• Polarization of charge
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Coulomb’s Law
• An electric force has three
properties:
15.6
Coulomb’s Law
• An electric force has three
properties:
– It is attractive or repulsive
depending upon the sign of the
charges.
Coulomb’s Law
• An electric force has three
properties:
– It is attractive or repulsive
depending upon the sign of the
charges.
– It is directly proportional to the
product of the magnitudes of the
charges (q1.q2).
Coulomb’s Law
• An electric force has three
properties:
– It is attractive or repulsive
depending upon the sign of the
charges.
– It is directly proportional to the
product of the magnitudes of the
charges (q1.q2).
– It is inversely proportional to the
square of the separation (r2).
Coulomb’s Law
• Coulomb’s formula:
 q1q2 
F  k 2 
 r 
Coulomb’s Law
• Definitions
– Coulomb
• The amount of charge that has
passed a given point in one
second when one ampere of
current is flowing
q  It
q  n e
Coulomb’s Law
• Definitions
– Ampere
•One coulomb of charge
passing a given point in
one second
–Analogy: Like water
flowing through a pipe
Coulomb’s Law
• Coulomb’s constant:
ke = 8.9875 x 10
9
N.m2/C2
Coulomb’s Law
• Charges and masses of
particles
– Table 15.1 (pg. 501)
Coulomb’s Law
• Reminders:
– Force is a vector quantity
– r is the distance between
centers
– Newton’s Third Law applies
– The Coulomb force is a field
force just like …?
Coulomb’s Law
• Reminders:
– Force is a vector quantity.
– r is the distance between
centers.
– Newton’s Third Law applies.
– The Coulomb force is a field
force just like gravity.
Coulomb’s Law
• The two field force formulas
are mathematically
identical.
Coulomb’s Law
• The two field force formulas
are mathematically
identical.
F = k q1q2/r2
Coulomb’s Law
• The two field force formulas
are mathematically
identical.
F = k q1q2/r2
F = G m1m2/r2
Coulomb’s Law
• Differences between
electrical and gravitational
forces
– Gravity only attracts
Coulomb’s Law
• Differences between
electrical and gravitational
force
– Gravity only attracts
– Gravity is weaker
Coulomb’s Law
• The Superposition Principle
may be used to find the
resultant force between
two charges:
•In a straight line
•Not in a straight line
–Trigonometry is required
QUESTIONS
1–7
Pg. 525
The Electric Field
• The concept of an electric field
was first introduced by Michael
Faraday.
15.10, 15.11
The Electric Field
• Description of an electric field
157, 158, 159, 161
The Electric Field
• Effects upon particles
entering an electric field
– Charged particles
156
The Electric Field
• Effects upon particles
entering an electric field
– Charged particles
– Uncharged particles
The Electric Field
• Electric field strength
E = F/qo
• Gravitational field strength
?
The Electric Field
• Electric field strength
E = F/qo
• Gravitational field strength
g = F/m
The Electric Field
• The magnitude of the
electric field due to charge q
can also be found by using:
E  ke
q
r
2
The Electric Field
• The electric field is a vector
quantity
– Magnitude (N/C)
– Direction
The Electric Field
• Direction (defined)
•The direction of the force
on a small + test charge
Electric Field Lines
• Point in the direction of the
electric field.
160
Electric Field Lines
• Point in the direction of the
electric field
• Are tangent to the electric field
vector (E)
Electric Field Lines
• Point in the direction of the electric
field
• Are tangent to the electric field
vector (E)
• The number of lines per unit area are
proportional to the electric field
strength
• 15.16
Electric Field Lines
• Point in the direction of the
electric field
• Are tangent to the electric field
vector (E)
• The number of lines per unit
area are proportional to the
electric field strength
• Are always directed away from
a + charge
Electric Field Lines
• Point in the direction of the electric
field
• Are tangent to the electric field
vector (E)
• The number of lines per unit area are
proportional to the electric field
strength
• Are always directed away from a +
charge
• Are always closer together near the
charge
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Electric Field Lines
• For an electric dipole
– The number of lines leaving
a + charge must equal the
number of lines entering a charge
Electric Field Lines
• For an electric dipole
– The number of lines leaving
one charge must equal the
number of lines entering a
second charge
– The lines can never cross
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What is the ratio of q1 to q2?
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Conductors In
Electrostatic Equilibrium
• In electrostatic equilibrium:
– There is no net motion of
charges within the
conductor
15.20
Conductors in
Electrostatic Equilibrium
• Properties of an isolated
Conductor:
Conductors in
Electrostatic Equilibrium
• Properties of an isolated
Conductor:
– The electric field is zero
everywhere inside the conductor
Conductors in
Electrostatic Equilibrium
• Properties of an isolated
Conductor:
– The electric field is zero
everywhere inside the conductor
– Any excess charge resides on the
surface
Conductors in
Electrostatic Equilibrium
• Properties of an isolated
Conductor:
– The electric field is zero
everywhere inside the conductor
– Any excess charge resides on the
surface
– The outside electric field is always
perpendicular to the surface
Conductors in
Electrostatic Equilibrium
• Properties of an isolated
Conductor:
– The electric field is zero
everywhere inside the conductor
– Any excess charge resides on the
surface
– The outside electric field is always
perpendicular to the surface
– The charge tends to accumulate at
the sharpest points
Conductors In
Electrostatic Equilibrium
• More applications involving
static electricity:
– Lightning rods
– Tesla coils
– Van De Graaff generators
Parallel Plate Capacitor
• The device consists of
plates of positive and
negative charge
• The total electric field
between the plates is
given by
•

E 
The field outside
 o the
plates is zero
The Van De Graaff
Generator
• How does it work?
15.23
Van de Graaff
Generator
• An electrostatic generator
designed and built by Robert
J. Van de Graaff in 1929
• Charge is transferred to the
dome by means of a rotating
belt
• Eventually an electrostatic
discharge takes place
The Oscilloscope
• Creates a visual display of
waveforms from external
applications
The Oscilloscope
• Uses a CRT (cathode ray
tube) similar to those found
in radar systems, computers,
televisions, and hospital
monitors
15.24
The Oscilloscope
• How does it work?
– Electron gun
• Filament
• Cathode
• Anode
– Horizontal and vertical deflection
plates
– Screen
QUESTIONS
9 - 13
Pg. 525