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
Coulomb’s Law
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-15
Example Problem
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
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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
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