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
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?
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Slide 20-29
Charged Spheres & Forces
Two identical metal spheres are firmly fastened to and electrically insulated from
frictionless plastic air pucks that ride on an air table as shown below. The pucks
are held in place as a charge of 2.0 x 10-8 C is placed on sphere A on the left and
a charge of 6.0 x 10-6 C is placed on sphere B on the right. The pucks are then
released so that the pucks with the spheres attached are now free to move without
across the table.
A. Draw Free-Body Diagrams for the pucks and spheres
B. How do the Coulomb forces acting on spheres A & B
compare? (Use a ratio)
C. Which sphere has the greater acceleration?
How would your answer change if the mass of the puck under
sphere A was reduced by 50%?
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Slide 20-3
Charged Spheres & Forces
Two identical metal spheres are firmly fastened to and electrically insulated from
frictionless plastic air pucks that ride on an air table as shown below. The pucks
are held in place as a charge of 2.0 x 10-8 C is placed on sphere A on the left and a
charge of 6.0 x 10-6 C is placed on sphere B on the right. The pucks are then
released so that the pucks with the spheres attached are now free to move without
across the table.
D. As the two spheres get farther away from one another, how would (if at all)
the following quantities change?
1) Force
2) Speed
3) Acceleration
Choices:
a) Increase
b) Decrease
c) Stay the same
d) Can’t tell
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Slide 20-3
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
Van de Graff Generator
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Slide 20-3
How to make an object move in a circle
Consider coordinate system with radial,
tangential, and z components
Consider Force Diagrams for
• Ball on String on table
• Ball with plastic circle
• Ball on string hanging
• Ball on string in vertical circle
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Slide 6-14
Uniform Circular Motion
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Slide 6-13
Forces in Circular Motion
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Slide 6-21
Circular Motion
There is an acceleration
because the velocity is
changing direction.
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Slide 3-35
Circular Motion
Old vinyl records are 12" in diameter, and spin at 33⅓ rpm when
played. What’s the acceleration of a point on the edge of the
record?
Two friends are comparing the acceleration of their vehicles. Josh
owns a Ford Mustang, which he clocks as doing 0 to 60 mph in a
time of 5.6 seconds. Josie has a Mini Cooper that she claims is
capable of a higher acceleration. When Josh laughs at her, she
proceeds to drive her car in a tight circle at 13 mph. Which car
experiences a higher acceleration?
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Slide 3-36
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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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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Acceleration
The average acceleration of a moving object is
defined as the vector
As an object moves, its velocity vector can change in two
possible ways.
1.The magnitude of the velocity can change, indicating
a change in speed, or
2. The direction of the velocity can change, indicating
that the object has changed direction.
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Tactics: Finding the
acceleration vector
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Tactics: Finding the
acceleration vector
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Uniform Circular Motion
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