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Electric Forces and Fields
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Vocab 16 Electric forces & fields
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Section 1
Electric Forces and Fields
Today’s special
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HW check
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Section 1
Electric Forces and Fields
Section 1
Electric forces and fields
Pa.10:
Pd.1:
Pd.2:
Use appropriate safety procedures when conducting
investigations.
Recognize the characteristics of static charge and explain
how a static charge is generated.
Use diagrams to illustrate an electric field (including point
charges and electric field lines).
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Electric Forces and Fields
Section 1
All great achievements require
time.
Maya Angelou
Fact: Quasars are believed to be stars, but may actually be distant galaxies.
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Electric Forces and Fields
Preview
Section 1 Electric Charge
Section 2 Electric Force
Section 3 The Electric Field
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Section 1
Electric Forces and Fields
What do you think?
• In the top picture, the girl has rubbed
the balloon on her hair, and now there
is a force of attraction between them.
Normally, a balloon and hair would
not attract each other.
• What happened to each to produce this
force?
• In the lower picture, the two balloons
are repelling each other.
• How was this force of repulsion produced?
© Houghton Mifflin Harcourt Publishing Company
Section 1
Electric Forces and Fields
What do you think?
• Suppose that after this balloon
is rubbed against the girl’s
hair, it is held against the wall.
It will be attracted to the wall
and stick to it.
• Explain why the balloon is
attracted to the wall.
• Why does it eventually fall?
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Section 1
Electric Forces and Fields
Electric Charge
• There are two types of
charge, positive and
negative.
• Like charges repel.
– Positive and positive
– Negative and negative
– The two balloons
• Opposite charges attract.
– Positive and negative
– The balloon and the hair.
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Section 1
Electric Forces and Fields
Section 1
Transferring Electric Charge
• Atoms have smaller particles called protons
(+ charge), neutrons, and electrons (- charge).
– Number of protons = number of electrons
• Atoms are neutral (no net charge).
– Electrons are easily transferred from one atom to
another.
• Protons and neutrons remain in nearly fixed positions.
• When rubbing a balloon on your hair, electrons
are attracted to the balloon and transfer.
– The balloon is left with excess electrons (- charge).
– The hair is left with an equal excess of protons (+
charge).
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Electric Forces and Fields
Section 2
The Nobel Prize in Physics 1923
Robert A. Millikan
Robert Andrews Millikan was born on the 22nd
of March, 1868, in Morrison, Ill. (U.S.A.)
Millikan was an enthusiastic tennis player, and
golf was also one of his recreations.
Professor Millikan married Greta Erwin
Blanchard in 1902; they had three sons: Clark
Blanchard, Glenn Allen, and Max Franklin.
He died on the 19th of December, 1953, in San
Marino, California.
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Electric Forces and Fields
Section 1
Millikan Oil Drop Experiment
• Millikan sprayed oil drops between charged metal plates.
• The oil drops were negatively charged by friction.
• By adjusting the voltage on the plates, he could make
the drops rise and fall.
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Electric Forces and Fields
Section 1
Millikan’s Results
• Millikan found that the amount of charge on objects was
always a multiple of some fundamental charge (e).
• In other words, charge is quantized.
– e turned out to be the amount of charge on an electron.
• e = 1.602176  10-19 coulombs
• Coulomb is the SI unit of charge.
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Electric Forces and Fields
Millikan's Oil Drop Experiment
Click below to watch the Visual Concept.
Visual Concept
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Section 1
Electric Forces and Fields
Section 1
Coulombs and Electrons
• What amount of charge
does a single electron
carry?
• How many electrons are
needed to produce an
amount of charge equal
to -1.00 C?
• What is the mass of this
number of electrons?
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• -1.60  10-19 C/electron
• 6.25  1018 electrons/C
• 5.69  10-12 kg
– about 5 billionths of a gram
Electric Forces and Fields
Section 1
Conductors and Insulators
• What is meant by the term
electrical conductor?
– Provide a few examples.
• What is meant by the term
electrical insulator?
– Provide a few examples.
• Why do conductors and
insulators behave differently?
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• Conductors allow electrons to
flow freely through them.
– Silver, copper, aluminum, and
other metals
• Electrons do not flow freely
though insulators.
– Plastic, rubber, glass
• Outer electrons in metals are
loosely bound to the nucleus
and relatively free to move.
Electric Forces and Fields
Section 1
Charging by Contact
• Both insulators and conductors can be charged
by contact.
– Rubbing two materials together results in a transfer of
electrons.
– When charging metal, the charge may move through
your body into the ground.
• The metal and your body are conductors, so the charge
moves through them.
• You must hold the conductor with an insulating material, such
as rubber gloves, to keep the charge on the metal.
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Electric Forces and Fields
Section 1
Charging by Induction
• A charged rod is held near
a metal sphere. Why do
the charges in the metal
arrange themselves as
shown?
• The metal sphere is
connected to the ground
with a conductor. Why did
some of the electrons
move off the sphere?
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Electric Forces and Fields
Section 1
Charging by Induction
• The conductor connecting
the sphere to ground is
removed. What type of net
charge does the sphere
now possess?
• The negatively charged
rod is removed. Why do
the charges move into the
positions shown?
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Electric Forces and Fields
Surface Charges
• Why does a charged balloon stick
to the wall?
• A positive surface charge is
induced on the wall by the
negatively-charged balloon.
– Electrons shift within atoms due to
attraction or repulsion.
– The insulator does not have a net
charge.
• The diagram shows the opposite
case.
• Why can a charged comb pick up
little pieces of paper?
© Houghton Mifflin Harcourt Publishing Company
Section 1
Electric Forces and Fields
Now what do you think?
• In the top picture, the girl has rubbed
the balloon on her hair, and now there
is a force of attraction between them.
Normally, a balloon and hair would
not attract each other.
• What happened to each to produce this
force?
• In the lower picture, the two balloons
are repelling each other.
• How was this force of repulsion produced?
© Houghton Mifflin Harcourt Publishing Company
Section 1
Electric Forces and Fields
Now what do you think?
• Suppose that after this balloon
is rubbed against the girl’s
hair, it is held against the wall.
It will be attracted to the wall
and stick to it.
• Explain why the balloon is
attracted to the wall.
• Why does it eventually fall?
© Houghton Mifflin Harcourt Publishing Company
Section 1
Electric Forces and Fields
Section 2
What do you think?
• Electric forces and gravitational forces are both
field forces. Two charged particles would feel the
effects of both fields. Imagine two electrons
attracting each other due to the gravitational
force and repelling each other due to the
electrostatic force.
• Which force is greater?
• Is one slightly greater or much greater than the other, or are
they about the same?
• What evidence exists to support your answer?
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 2
Coulomb’s Law
• The force between two charged particles depends on the
amount of charge and on the distance between them.
– Force has a direct relationship with both charges.
– Force has an inverse square relationship with distance.
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Electric Forces and Fields
Section 2
Coulomb’s Law
• Use the known units for q, r, and F to determine the units
of kc.
– kc = 8.99  109 N•m2/C2
• The distance (r) is measured from center to center for
spherical charge distributions.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 2
Classroom Practice Problem
• The electron and proton in a hydrogen atom are
separated, on the average, a distance of about
5.3  10-11 m. Find the magnitude of both the
gravitational force and the electric force acting
between them.
– Answer: Fe = 8.2  10-8 N, Fg = 3.6  10-47 N
• The electric force is more than 1039 times
greater than the gravitational force.
– Atoms and molecules are held together by electric
forces. Gravity has little effect.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 2
Classroom Practice Problem
• A balloon is rubbed against a small piece of
wool and receives a charge of -0.60 C while the
wool receives an equal positive charge. Assume
the charges are located at a single point on each
object and they are 3.0 cm apart. What is the
force between the balloon and wool?
• Answer: 3.6 N attractive
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Electric Forces and Fields
Superposition Principle
• The net force on a charged
object is the sum of all of the
forces due to other charged
objects.
• Charge q3 shown has two
forces acting on it.
– q2 pulls to the left.
– q1 pushes up and to the right.
• The vector sum is shown in
the lower diagram.
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Section 2
Electric Forces and Fields
Section 2
Classroom Practice Problem
• Two charges, q1 and q2, lie on the x-axis. The
first charge is at the origin and the second
charge is at x = 1.0 m. Determine the force on a
third charge, q3, placed at x = 0.75 m. The
charges are as follows:
q1 = +10.0C , q2 = +7.5C, q3 = -5.0C
• Answer: Fleft = 0.80 N and Fright= 5.4 N, so
Fnet = 4.6 N to the right
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Electric Forces and Fields
Section 2
Electric Force
• Like gravity, the electric force is a field force.
• Similarities
– Both forces are related to distance in the same way.
• Differences
– Two types of charge and only one type of mass
– Electric forces can attract or repel while gravity only
attracts.
– Electric forces are far stronger than gravitational
forces.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 2
Born on June 14, 1736, in Angoulême, France,
Charles-Augustin de Coulomb studied
engineering and plied his trade with the military
before winning accolades for his work in torsion
balances. He offered pioneering theories in the
force found between electrical charges, as well
as magnetic attraction and repulsion. The unit
of measurement known as the coulomb is
named in his honor. He died in Paris on August
23, 1806.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 2
Coulomb’s Apparatus
• Coulomb developed his
law using a torsion
balance like that shown.
• He measured the force
between the two charged
spheres by the amount of
twisting in the wire.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 2
Now what do you think?
• Electric forces and gravitational forces are both
field forces. Two charged particles would feel the
effects of both fields. Imagine two electrons
attracting each other due to the gravitational
force and repelling each other due to the
electrostatic force.
– Which force is greater?
• Is one slightly greater or much greater than the other, or are
they about the same?
• What evidence exists to support your answer?
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 2
Today’s special
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HW check; Q & A
Special presentations
Lab: Capacitance…a Discovery lab
Lab due next
HW problems 19 & 21 due next to be turned in
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Electric Forces and Fields
Section 2
Today’s special
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Turn in lab on front table now!
Turn in problems 19 & 21 on front table now!
Trip to CERN interest
Notes II
HW II due next
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Electric Forces and Fields
Section 3
What do you think?
• In the chapter “Circular Motion and
Gravitation,” you learned about the
gravitational field (g). The diagram
shows the “g” field around Earth.
• In this section, we will study the
electric field (E) around charged
particles. On the next slide are
three different diagrams. Make a
sketch of the “E” field for each
charge or combination of charges.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
What do you think?
• Make a sketch of the
“E” field for each
charge or combination
of charges.
– How are your sketches
similar?
– How are they different?
– Explain.
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Section 3
Electric Field Strength
• Electric fields (E) have magnitude and direction.
– The direction is defined as the direction of the force on a small,
positive test charge (q0) placed in the field caused by Q.
– The magnitude of the field is defined as the force per unit
charge on q0.
Felectric
E
q0
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Electric Forces and Fields
Electric Fields and Test Charges
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Section 3
Test Charges
• A small test charge
will not significantly
affect the field.
• If the test charge (q0) is large,
it will affect the way the
charges are distributed on the
charged conductor.
– This would
change the
field around
the
conductor.
• Test charges will always be considered small
enough to have no effect on the field.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 3
Electric Field Strength
• Combine Coulomb’s law with
the definition of electric field
to derive an equation for E
due to a point charge.
Felectric
qq0
E
 kC 2
q0
r q0
• SI unit: N/C
• The field strength does not depend on the test charge.
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Sample Electric Field Strengths
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Section 3
Electric Forces and Fields
Section 3
Classroom Practice Problems
• An electric field around a charged object is
5.95  106 N/C at a distance of 0.100 m. Find
the charge on the object.
– Answer: 6.62  10-6 C or 6.62 C
• Suppose a small test charge of 0.200 C was
placed at the point that is 0.100 m from the
charged object. What force would be exerted on
the test charge and on the object?
– Answer: 1.19 N for both test charge and object
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 3
Classroom Practice Problems
• A charge q1 = 4.50 C experiences an attractive
force of 1.35 N at a distance of 0.150 m from a
charged object, q2. Find the strength of the
electric field due to q2 at a distance of 0.150 m
from q2.
– Answer: 3.00  105 N/C
• Find the charge, q2.
– Answer: 0.751 C
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Electric Forces and Fields
Electric Field Lines - Rules
• Apply the above
rules and sketch the
E field around the
charge shown.
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Section 3
Electric Forces and Fields
Electric Field Lines - Rules
• Apply the above
rules and sketch the
E field around the
charge shown.
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Electric Field Lines - Rules
• Apply the above rules
and sketch the E field
around the charge
combination shown.
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Electric Field Lines - Rules
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Electric Field Lines - Rules
• Apply the above rules
and sketch the E field
around the charge
combination shown.
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Electric Field Lines - Rules
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Section 3
Rules for Drawing Electric Field Lines
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Electric Forces and Fields
Section 3
Rules for Sketching Fields Created by Several
Charges
Click below to watch the Visual Concept.
Visual Concept
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Electric Forces and Fields
Section 3
Electrostatic Equilibrium
• Electrostatic equilibrium occurs in conductors when no
net motion of charges exists within the conductor.
• Charges in a conductor are free to move, but are not
moving when equilibrium exists.
– The rules below result from this fact.
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Electric Forces and Fields
Section 3
Now what do you think?
• What is an electric field?
• When sketching electric fields, what information
is conveyed by the direction of the field lines?
• When sketching electric fields, what information
is conveyed by the density of the field lines?
• Why must electric field lines just outside a
conductor be perpendicular to the conductor?
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Electric Forces and Fields
Today’s special
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HW check; Q & A
The Cosmos
Review
Test next time!
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Section 3
Electric Forces and Fields
Today’s special
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HW check; Q & A
SAT question of the day
Demo
Review
Test next time!
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Section 3
Electric Forces and Fields
Van de Graaff generator
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electric Forces and Fields
Today’s special
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Test 16; enter mc in Smart response
Pick up vocab 17 when finished
Vocab 17 due next time
Final sign-ups for CERN trip
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Section 3