Download Powerpoint

Chapter 20
Electric Forces and Fields
Topics:
•
•
Coulomb Force
The field model and the
electric field
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
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Checking Understanding
A set of electric field lines is directed as below. At which of the noted
points is the magnitude of the field the greatest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-46
Answer
A set of electric field lines is directed as below. At which of the noted
points is the magnitude of the field the greatest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-47
Are the Fields Real???
Are either or both of these a possible electric field?
Explain the reasoning behind your answer
(Focus on the vectors, not the source charges)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-32
Answer
Positive charges create an electric field in the space around them.
In which case is the field at the black dot the smallest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-33
Checking Understanding
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which of the cases below is the net
field to the right?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-36
Answer
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which of the cases below is the field
to the right?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-37
Checking Understanding
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which case is the magnitude of the
field at the black dot the largest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-41
Answer
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which case is the magnitude of the
field at the black dot the largest?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-42
The Electric Field
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-30
The Electric Field of a Point Charge
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-31
E-field Superposition Examples
1. Determine the magnitude and the direction of the electric field
at point A.
2. Determine the individual forces and the net force on charge B
for each of the following cases.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-66
Electric Field Vectors and Electric Field Lines
E-field Applet 2
http://qbx6.ltu.edu/s_schneider/physlets/main/efield.shtml
What observations can we make about E-field lines?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Nature of Electric Field
E-field Applet 1
http://physics.weber.edu/schroeder/software/EField/
What observations can we make about E-fields?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.