Download Check to see if your answer is reasonable

Chapter 20
Electric Forces and Fields
Topics:
• Electric charge
• Forces between charged
objects
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
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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Checking Understanding
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 20-16
Answer
A small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the largest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-17
Electric Field
How do non-contact forces work?
How do we get action at a distance without contact?
Need concept of a field
Field is something that has a value at every point in space
Field is the influence a mass or charge has at every point on the
space around it
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Slide 20-3
The Electric Field
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Slide 20-30
Visualizing Charge
•
•
Charges on an insulator do not move.
Charges on a conductor adjust until
there is no net force on any charge.
We call this electrostatic
equilibrium.
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Slide 20-12
Examples
If a charged plastic rod is brought near an uncharged metal rod on an
insulating stand, an uncharged metal ball near the other end of the
metal rod is attracted to this end of the rod. Explain the motions of
charges that give rise to this force.
Follow-up: Describe a procedure by which you could give two
identical metal spheres exactly equal charges.
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Slide 20-14
How does an electroscope work?
Three cases
A. When you put charge on a
neutral electroscope
B. When you bring a charged
object near a neutral
electroscope
C. When you bring a charged
object near a charged
electroscope
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Slide 20-3
Van de Graff Generator
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Van de Graff Generator
From Worksheet 2, Problem 9:
What happens to Mike Piepan?
1.
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Coulomb’s Law
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Slide 20-15
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 20-29
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
smallest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-27
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
smallest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-28
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 charges
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Slide 20-3