Transcript 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
Newton’s Laws of Motion
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Questions on P151 Force & Motion Review
Note sheet for Mastering Physics?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Coulomb’s Law
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-15
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-3
Two hanging cans
You and a friend are doing the laundry when you unload the dryer and
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 20-3
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/
What observations can we make about 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.
A
B
C
E
D
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.
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
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-37