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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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-1
Conceptual Example Problem
All charges in the diagrams below are equal magnitude. In each
case, a small positive charge is placed at the blank dot. In which
cases is the force on this charge:
A. to the right?
B. to the left?
C. zero?
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Slide 20-28
Checking Understanding
All charges in the diagrams below are of equal magnitude. In each
case, a small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the largest?
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Slide 20-29
Answer
All charges in the diagrams below are of equal magnitude. In each
case, a small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the largest?
C
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Slide 20-30
Thought for the day
If you don’t have time to do things right, when will you have time to
do it over.
Benjamin Franklin
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Slide 20-3
Strategic Problem Solving Strategy (PSS)
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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Solving Problems - Prepare (also identify key physics)
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Solving Problems (continued)
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Using Mastering Physics
If
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Coulomb’s Law
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Slide 20-15
Example Problem
Two 0.10 g honeybees each acquire a charge of +23 pC as they fly
back to their hive. As they approach the hive entrance, they are 1.0
cm apart. What is the magnitude of the repulsive force between the
two bees? How does this force compare with their weight?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-33
Charge & Forces
1. Draw individual and net forces acting on object B for the four situations below.
2. Calculate the magnitude and direction of the net force on object B.
Be sure to state your assumptions
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Slide 20-3
Three charges in a line
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Slide 20-3
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.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-3
The Electric Field
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Slide 20-34
The Electric Field of a Point Charge
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Slide 20-35
Checking Understanding
Positive charges create an electric field in the space around them.
In which case is the field at the black dot the smallest?
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Slide 20-36
Answer
Positive charges create an electric field in the space around them.
In which case is the field at the black dot the smallest?
D
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Slide 20-37