Download 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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-1
What to do to do well in this class
A. Focus on key physics concepts
• May seem like basics but will help you solve even complex
problems
• Focus on principle rather than recipes
• Need to have a functional understanding of key concepts
• Express key equations as sentences
• Know where they come from and what they mean
• Know how and when to apply them
• Know which equations are general and which are special cases
• Must know when not to apply special cases
• Look at a problem after a good physics diagram and maybe a
good physical diagram and know what key physics concepts apply
in that problem
• Memorize key concepts so you can look at a problem, say that’s
Newton 2, and know the associated equation in a snap
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
What to do to do well in this class
A. Focus on key physics concepts
• How to do this
• When you look at problems, mentally group problems by
the physics rather than the physical situation
• After each class or at least each week, create a notesheet
to organize a structure of the new key concepts for each
chapter and note how they fit in with previous key
concepts
• Use the note sheet to do homework problems (a) do as
many homework problems as you can just using this
sheet. (b) then go to your notes and the textbook for your
missing pieces
• Use flash cards to memorize key concepts - include the
concept description, relevant equations, diagrams, and
what types of problems benefit from using that concept
• Pay close attention to examples done in class and note the
physics and assume/observes in each example and how
these are used
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Coulomb Force Problems
1. Sum of Electric Forces from Point Charges
2. Levitation
3. Static Equilibrium
4. Comparing Electric Forces and Gravitational Forces
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
The Force of Gravity
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 6-35
Example Problem 1
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 20-33
Example Problem 2
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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 6-36
Coulomb’s Law and Three Charges
Two charged particles, with charges q1= +q and q2 = +4q,
are located at a distance d = 2 cm apart on the x axis.
Where can a 3rd charge be placed so that this 3rd charge
has Fenet = 0 N, i.e the 3rd charge is in equilibrium.
Hint: Draw a diagram of the electric forces on the 3rd
charge in various positions.
What are the conditions of equilibrium?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Coulomb’s Law and Three Charges
Two charged particles, with charges q1= +q and q2 = +4q,
are located at a distance d = 2 cm apart on the x axis.
Where should a third charged particle, with charge
q3= +q , is placed so that Fenet = 0 (equilibrium).
For equilibrium, there are 3 conditions
• Charges must be on x-axis
• Fe,1=>3 = Fe,2=>3
• The two forces must point in opposite directions
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Three charges in a line (change the problem)
Two charged particles, with charges q1= -q and q2 = +4q,
are located at a distance d = 2 cm apart on the x axis.
Where should a third charged particle, with charge
q3= +q , is placed so that Fenet = 0 (equilibrium).
For equilibrium, there are 3 conditions
• Charges must be on x-axis
• Fe,1=>3 = Fe,2=>3
• The two forces must point in opposite directions
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
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.
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/
Back-up
applet:https://phet.colorado.edu/en/simulation/charges-andfields
What observations can we make about E-fields?
Source Charges and Test Charges?
Superposition of E-fields?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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.
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.
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.
Coulomb Force Problems
1. Sum of Electric Forces from Point Charges
2. Levitation
3. Static Equilibrium
4. Comparing Electric Forces and Gravitational Forces
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
The Electric Field
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-34
The Electric Field of a Point Charge
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-36
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
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
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.
E-field Superposition Example
1. Determine the magnitude and the direction of the electric field
at point A.
In your physical diagram, make sure you label
your r’s
as well as your angles
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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://vnatsci.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.
Dipole and Uniform Electric Fields
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-45
Electric Field Lines
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-50
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
Nature of Electric Field Vectors
• 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 E-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
• E-fields add as vectors, at a point in space Enet,x = E1x + E2x + …
• For a point charge E = Fe / |q| = [k |Q| |qt| / r2] / |qt| = k |Q| / r2
• Electric Force
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Nature of Electric Field Lines
• E-Field lines start on + charges and end on -- charges
• Larger charges will have more field lines going out/coming in
• Density of Field lines is a measure of field strength – the higher
the density the stronger the field
• The E-field vector at a point in space is tangent to the field line
at that point. If there is no field line, extrapolate
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Conductors and Electric Fields
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-55