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Week 3 Day 1 => 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
How does an electroscope work?
Three cases
A. When you put charge on a
neutral electroscope
B. When you bring a charged
object near a neutral
electroscope
C. When you bring a charged
object near a charged
electroscope
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Slide 20-3
Soda Can Electroscope
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Slide 20-3
Checking Understanding
Two spheres are touching each other. A charged rod is brought
near. The spheres are then separated, and the rod is taken away. In
the first case, the spheres are aligned with the rod.
After the charged rod is removed, which of the spheres (A & B) is:
i) Positive
ii) Negative
iii) Neutral
A => Sphere A is + and sphere B is –
B => Sphere A is – and Sphere B is +
C => Spheres A and B are both +
D => Spheres A and B are both –
E => Spheres A and B are both neutral
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-13
Checking Understanding
Two spheres are touching each other. A charged rod is brought
near. The spheres are then separated, and the rod is taken away. In
the second case, they are perpendicular. After the charged rod is
removed,
which of the spheres (C & D) is:
i) Positive
ii) Negative
iii) Neutral
A => Sphere C is + and sphere D is –
B => Sphere C is – and Sphere D is +
C => Spheres C and D are both +
D => Spheres C and D are both –
E => Spheres C and D are both neutral
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Slide 20-13
Van de Graff Generator
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Slide 20-3
Charged Spheres & Forces
Two identical metal spheres are firmly fastened to and electrically insulated from
frictionless plastic air pucks that ride on an air table as shown below. The pucks
are held in place as a charge of 2.0 x 10-8 C is placed on sphere A on the left and
a charge of 6.0 x 10-6 C is placed on sphere B on the right. The pucks are then
released so that the pucks with the spheres attached are now free to move without
across the table.
A. Draw Free-Body Diagrams for the pucks and spheres
B. How do the Coulomb forces acting on spheres A & B
compare? (Use a ratio)
C. Which sphere has the greater acceleration?
How would your answer change if the mass of the puck under
sphere A was reduced by 50%?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-3
Charged Spheres & Forces
Two identical metal spheres are firmly fastened to and electrically insulated from
frictionless plastic air pucks that ride on an air table as shown below. The pucks
are held in place as a charge of 2.0 x 10-8 C is placed on sphere A on the left and a
charge of 6.0 x 10-6 C is placed on sphere B on the right. The pucks are then
released so that the pucks with the spheres attached are now free to move without
across the table.
D. As the two spheres get farther away from one another, how would (if at all)
the following quantities change?
1) Force
2) Speed
3) Acceleration
Choices:
a) Increase
b) Decrease
c) Stay the same
d) Can’t tell
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Slide 20-3
Coulomb’s Law
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Slide 20-15
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 20-29
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:
A. to the right?
B. to the left?
C. 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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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
The Electric Field
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Slide 20-30
The Electric Field of a Point Charge
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Slide 20-31
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