Chapter 20
Electric Forces and Fields
Topics:
• 
• 
• 
• 
Electric charge
Forces between charged
objects
The field model and the
electric field
Forces and torques on
charged objects in electric
fields
Sample question:
In electrophoresis, what force causes DNA fragments to migrate
through the gel? How can an investigator adjust the migration rate?
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Slide 20-1
Strategic Problem Solving Strategy (SPS)
When solving problems, it is important to show how you reasoned from the information
given in the problem and key physics ideas to your final answer. The correct final
answer with units is only worth 1-3 points. The remainder of the points (70-90% of
credit) are awarded for the quality of your solution. You are expected to include the
following to receive full credit:
Prepare
•  Identify the Physics: State explicitly which physics principle(s) apply to the
problem situation and that you will use to solve the problem
•  Drawing a Picture: Draw at least one picture to visualize the physics of the
problem and define your variables and constants. For motion problems this could
be a motion diagram, motion graph, or pictorial diagram
•  Collecting Necessary Information: State all the information given in the problem
with correct units. Include preliminary calculations such as unit conversions
•  Assume/Observe: State assumptions or observations that would be useful
Solve
•  Start with key equation(s) in symbol form
•  Solve for the unknown quantity in symbols before numeric calculations
•  Then substitute numbers with units and calculate the numeric answer
Assess
•  Check to see if your answer is reasonable
•  Does it answer the question that was asked
•  Does it have the right units?
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SPS Steps – Prepare Step
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SPS Steps (continued – Solve and Assess steps)
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Two hanging cans
You and a friend are doing the laundry when you unload the dryer and
your friend wants to get some idea of the amount of charge that
causes static cling. You immediately take two empty soda cans, which
each have a mass of 120 grams, from the recycling bin. You tie the
cans to the two ends of a string (one to each end) and hang the center
of the string over a nail sticking out of the wall. Each can now hangs
straight down 30 cm from the nail. You take your flannel shirt from the
dryer and touch it to the cans, which are touching each other. The
cans move apart until they hang stationary at an angle of 10º from the
vertical. Assuming that there are equal amounts of charge on each
can, you now calculate the amount of charge transferred from your
shirt.
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Slide 20-3
A model of the mechanism for electrostatic interactions
• A model for electric
interactions, suggested by
Michael Faraday, involves
some sort of electric
disturbance in the region
surrounding a charged object.
• Physicists call this electric
disturbance an electric field.
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Gravitational field due to a single object with mass
•  We find a mathematical description of the "strength"
of Earth's gravitational field at a particular location
that is independent of the test mass:
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Electric field due to a single point-like charged object
•  We use a similar approach of test charges to construct a
physical quantity for the "strength" of the electric field:
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The Electric Field
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Slide 20-34
Electric field due to a single point-like charged object
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Electric field due to a single point-like charged object
• We can interpret this field as follows:
• The E field vector at any location points
away from the object creating the field if Q is
positive, and toward the object creating the
field if Q is negative.
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Observational experiment
Go through Force and Force / qtest
Movie Link
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Observational experiment (movie link)
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Superposition principle
• When multiple charged objects are present,
each object makes its own contribution to
the E field.
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Using the superposition principle
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Nature of Electric Field
E-field Applet 1
http://physics.weber.edu/schroeder/software/EField/
Back-up applet:
https://phet.colorado.edu/en/simulation/charges-and-fields
What observations can we make about E-fields?
Source Charges and Test Charges?
Superposition of E-fields?
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Electric Field Warm-up
1.  Check your predicted vectors with the applet at the locations
marked
with an “x”.
2.  Compare your predictions and resolve discrepancies.
3.  Draw the positive charge off the bottom of screen – check
predictions.
4.  Note the magnitude and direction of the top, middle vector
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Nature of Electric Field
•  Test charge is a small positive charge to sample the E-Field
•  Charge of test charge is small compared to source charges
(source charges are the charges that generate the field)
•  E-field vectors
•  E -field is the force per charge
•  E-field vectors points away from + charges
•  E-field vectors point towards - charges
•  E -field for point charges gets weaker as distance from
source point charges increases
•  For a point charge E = Fe / q = [k Q q / r2] / q = k Q / r2
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Find the Electric Field
Given the following forces that a positive test charge feels if placed
at these three points, find the E-field vectors at these points.
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Find the Electric Field
Given the following forces that a positive test charge feels if placed
at these three points, find the E-field vectors at these points.
How would the Force vectors and E-field vectors change at point 3
for the following changes:
•  Replace the positive test charge (+q) with a negative test
charge (-q)
•  Replace the positive test charge (+q) with a test charge twice
as large (+2q)
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Lab Prep: Work, Potential Energy,
Conservative Forces, Conservation of Energy, and
Electric Potential
1.  Dot Product
2.  Work
3.  Potential Energy
4.  Conservation of Energy, Bar Charts, Potential Energy and
Work
5.  Electric Energy
6.  Conservation of Energy
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Electric potential energy: A qualitative analysis
•  A positively charged
cannonball is held near
another fixed positively
charged object in the
barrel of the cannon. •  Some type of energy must
decrease if gravitational
and kinetic energies
increase in this process.
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Electric potential energy: A qualitative analysis
•  Consider two oppositely charged blocks, one of
which can slide without friction. When the negatively
charged block is released and moves nearer the nut,
the kinetic energy of the system increases.
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Electric potential energy: A quantitative analysis
•  Use the generalized work-
energy principle
to analyze a situation where
only the electric potential
energy changes when work is
done.
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Electric potential energy
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Example
•  Two oppositely charged objects (with positive charge
+q and negative charge –q) are separated by distance
ri. Will the electric potential energy of the system
decrease or increase if you pull the objects farther
apart? Explain.
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Electric potential energy of multiple charge systems
• Each pair of charged objects has an
associated electric potential energy, and the
total electric potential energy of the system
is the sum of the energies of all pairs. • For three charged objects, the total electric
potential energy is:
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Skills for analyzing processes involving electric force
and electric potential energy
In conjunction with your problem-solving strategy:
• Decide whether you can consider the charged
objects to be point-like or plate-like.
• If you are using the Conservation of energy
principle, construct an energy bar chart. Decide
where the zeros are.
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Lab Prep: Work, Potential Energy,
Conservative Forces, Conservation of Energy, and
Electric Potential
• 
Using Work by the electric force to find equipotential
surfaces => surfaces where the work done by the electric
field on a test charge is zero are equipotential surfaces.
• 
In equilibrium, no work is done by the electric force to move
a charge around on a conductor, the charge has the same
the same electric potential everywhere on a conductor =>
therefore, the surface of a conductor is an equipotential
surface.
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Equipotential surfaces and E field
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Summary
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Summary
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Summary
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Summary
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Summary
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