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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?
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Slide 20-1
Checking Understanding
When a ball on the end of a string is swung in a vertical circle,
the ball is accelerating because
A.
B.
C.
D.
the speed is changing.
the direction is changing.
the speed and the direction are changing.
the ball is not accelerating.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 6-13
Answer
When a ball on the end of a string is swung in a vertical circle,
the ball is accelerating because
A.
B.
C.
D.
the speed is changing.
the direction is changing.
the speed and the direction are changing.
the ball is not accelerating.
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Slide 6-14
Checking Understanding:
Circular Motion Dynamics
When the ball reaches the break in the circle, which path will it
follow?
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Slide 6-21
Answer
When the ball reaches the break in the circle, which path will it
follow?
C.
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Slide 6-22
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:
• to the right?
• to the left?
• zero?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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
Three charges in a line
Two charged particles, with charges q1= +q and q2 = 4q,
are located at a distance d = 2 cm apart on the x axis.
A third charged particle, with charge q3= +q , is placed on
the x axis such that the magnitude of the force that
charge 1 exerts on charge 3 is equal to the force that
charge 2 exerts on charge 3.
Find the position of charge 3 when = 1.0
For equilibrium (not asked) there are 3 conditions
• Charges must be on x-axis
• Fe,1=>3 = Fe,2=>3
• The two forces must point in opposite directions
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The Force of Gravity
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Slide 6-35
Example Problem
A typical bowling ball is spherical, weighs 16 pounds, and has a
diameter of 8.5 in. Suppose two bowling balls are right next to
each other in the rack.
•What is the gravitational force between the two—magnitude and
direction?
•What is the magnitude and direction of the force of gravity
exerted by the Earth on one of the bowling balls?
•What is the magnitude and direction of the force of gravity on a
60 kg person?
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Slide 6-36
Introduction to Electric Field
1. Find the Electric force from a 9.00e-6 C charge on the
following charges at 1m, 2m, and 3 m.
a) 3.00e-6 C
b) -4.00e-6 C
c) -10.0e-6 C
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Introduction to Electric Field
1. Find the Electric force from a 9.00e-6 C charge on the
following charges at 1m, 2m, and 3 m.
a) 3.00e-6 C
b) -4.00e-6 C
c) -10.0e-6 C
2. Find the force per charge from a 9.00e-6 C charge on the
charges above at 1m, 2m, and 3 m.
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Nature of Electric Field
E-field Applet 1
http://physics.weber.edu/schroeder/software/EField/
What observations can we make about E-fields?
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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.
A
B
C
E
D
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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.
A
B
C
E
D
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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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