Transcript Today`s 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
Electric Field Vectors and Electric Field Lines
Applet 1: Exploring E-field from point
chargeshttp://physics.weber.edu/schroeder/software/EField/
E-field Applet 2: Exploring E-field vectors, E-field lines and
trajectories for point charges
http://vnatsci.ltu.edu/s_schneider/physlets/main/efield.shtml
E-field Applet 3: E-field vector fields for point charges
& charged plates (tricky – make notes on how to use)
http://www.falstad.com/vector2de/
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
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Are these Field Vectors 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)
A)
B)
C)
D)
E)
A is possible but B is not
A is not possible but B is possible
Both A and B are possible
Neither A nor B are possible
It is not possible to tell
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
Consider an infinite sheet of charge
h
Q
E=
where h =
2e 0
A
• Epsilon nought, e 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
materials
Discuss charge density
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 ?
Case 1:
A
B
C
Qleft = +Q
Qright = -Q
Case 2:
Qleft = 2Q
Qright = Q
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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.
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)
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.
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.
Begin Energy Review (time permitting)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.