Transcript Today`s Powerpoint

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
For uniform circular motion, the acceleration
A.
B.
C.
D.
points toward the center of the circle.
points away from the circle.
is tangent to the circle.
is zero.
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For uniform circular motion, the acceleration
A.points toward the center of the circle.
B. points away from the circle.
C. is tangent to the circle.
D.is zero.
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Circular Motion
There is an acceleration
because the velocity is
changing direction.
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Slide 3-35
Uniform Circular Motion
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Slide 6-13
Uniform Circular Motion
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Circular motion vs. Angular (Rotational) motion
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Slide 6-14
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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Dryer Sheet Problem - SPS Problem
You and a friend are doing the laundry when you unload the
dryer and the discussion comes around to static electricity. 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 20-15
Dryer Sheet Problem - SPS Problem
You and a friend are doing the laundry when you unload the
dryer and the discussion comes around to static electricity. 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 20-15
Ski Ramp - SPS Problem
You are working over winter break to design a new skiing ride at Cliff’s
Amusement Park. The ride consists of a ramp that acts like a beginners ski
hill. The ramp has 2 parts. Both parts are inclined at 15 degrees to the
horizontal. The skiers will start at the top of the upper part of the ramp and
ski down both parts of the ramp.
The upper part of the ramp is made from a low-friction (friction is negligible)
surface so that skiers pick up speed going down the upper part of the ramp.
The skiers should be at 40.0 MPH at the end of the upper part of the ramp.
Assume the skier has a mass m = 75 kg.
The lower part of the ramp, which is 1000. m long, has just enough friction
force so that the riders maintain constant speed over this part of the ramp.
At the end of the 2nd part of the ramp is a horizontal braking area.
As part of the design of the ramp, determine the following:
•How long the upper part of the ramp has to be so that the skiers reach 40.0
MPH starting from rest.
•What is the magnitude and direction of the friction force of the lower ramp
on the skier so that skiers on the lower ramp stay at constant velocity.
•How long would it take skiers to reach the bottom of the ramp
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Slide 20-15
Van de Graff Generator
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Slide 20-3
Peter Piepan Demo
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Slide 20-3
Coulomb’s Law
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Slide 20-15
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 20-25
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?
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Slide 20-26
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