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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
Why is Electric Field important?
• Helps us understand action at a distance
• Often we can measure electric field when we can’t measure or
know source charges directly
• Tells us how much force per charge we can expect when we
place a charged object at a point in space
• Lets us analyze what would happen for different charges
• Later, E-field will help us better understand circuits
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Additional Clicker Questions
A small sphere is suspended from a string in a uniform electric field.
Several different cases of sphere mass and sphere charge are
presented in the following table. In which case is the angle at which
the sphere hangs the largest?
Sphere mass (g)
A.
2.0
B.
3.0
C.
2.0
D.
3.0
E.
4.0
Sphere charge (nC)
4.0
4.0
6.0
8.0
9.0
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-61
Answer
A small sphere is suspended from a string in a uniform electric field.
Several different cases of sphere mass and sphere charge are
presented in the following table. In which case is the angle at which
the sphere hangs the largest?
Sphere mass (g)
C.
2.0
Sphere charge (nC)
6.0
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-62
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?
What symmetries can you see?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Consider an infinite sheet of charge
• What kind of symmetry would we expect?
• What will the field look like?
• Is the field (A) converging, (B) diverging, or
(C) neither -- (D) can’t tell
• What can we say about E-field strength?
• A charged sheet can be considered to be like
an infinite sheet when we look at points a
distance d away where d << L, where L is the
length of a side of the sheet
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Consider an infinite sheet of charge

Q
E
where  
2 0
A
• Epsilon nought,  0  8.85  10
12
C2
N  m2
is electric permitivity of free space
• Electric permitivity is a measure of how well
electric field can pass through space or
material
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Consider two infinite sheets of charge
What is the E-field at
points A, B, and C ?
A
B
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C
Electric Field Vectors and Electric Field Lines
E-field Applet 3
http://www.falstad.com/vector2de/
What observations can we make about the E-field
for parallel Plates?
Define capacitor as any two conductors with
equal and opposite charges
Discuss electric permittivity, spacing and charge density
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
Checking Understanding
Two parallel plates have charges of equal magnitude but opposite
sign. What change could be made to increase the field strength
between the plates?
A.
B.
C.
D.
E.
increase the magnitude of the charge on both plates
decrease the magnitude of the charge on both plates
increase the distance between the plates
decrease the distance between the plates
increase the area of the plates (while keeping the magnitude of
the charges the same)
F. decrease the area of the plates (while keeping the
magnitude of the charges the same)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-50
Answer
Two parallel plates have charges of equal magnitude but opposite
sign. What change could be made to increase the field strength
between the plates?
A. increase the magnitude of the charge on both plates
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-51
Checking Understanding
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-57
Answer
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
D. The dipole rotates counterclockwise.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-58
E-fields and Conductors
Inside => E=field is zero
Outside => E-field is perpendicular to the surface
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Checking Understanding
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-55
Answer
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 20-56