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Chapter 21
Electric
Potential
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PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
21 Electric Potential
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Slide 21-2
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Slide 21-3
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Slide 21-4
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Slide 21-5
Example Problem
Is the change ∆U of the particle positive, negative,
or zero as it moves from i to f?
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Slide 21-14
Electric Potential Energy
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Slide 21-15
Electric Potential
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Slide 21-16
Conceptual Example Problem
Rank in order, from largest to smallest, the electric
potentials at the numbered points.
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Slide 21-17
Charged Particle Moving Through a Potential
Difference
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Slide 21-18
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Slide 21-19
Example Problem
A proton has a speed of 3.5 x 105 m/s at a point where the
electrical potential is 600 V. It moves through a point where the
electric potential is 1000 V. What is its speed at this second point?
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Slide 21-20
A Topographic Map
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Slide 21-21
Graphical Representations of Electric Potential
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Slide 21-22
Example Problem
A proton is released from rest at point a. It then travels past
point b. What is its speed at point b?
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Slide 21-23
Potential of a Parallel-Plate Capacitor
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Slide 21-24
The Potential Inside a Parallel-Plate Capacitor
Uelec
Q
V=
= Ex =
x
q
Î0 A
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Slide 21-25
Example Problem
A parallel-plate capacitor is held at a potential difference of 250 V.
A proton is fired toward a small hole in the negative plate with a
speed of 3.0 x 105 m/s. What is its speed when it emerges
through the hole in the positive plate? (Hint: The electric potential
outside of a parallel-plate capacitor is zero).
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Slide 21-26
Electric Potential of a Point Charge
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Slide 21-27
Electric Potential: Charged Sphere
Outside of a sphere of charge Q the potential has the same form as
for a point charge Q:
If the sphere has radius R and the potential at its surface is V0, then
the potential a distance r from its center can also be written
R
V = V0
r
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Slide 21-28
Example Problem
For the situation shown in the figure, find
A. The potential at points a and b.The potential difference
between a and b.
B. The potential energy of a proton at a and b.
C. The speed at point b of a proton that was moving to the right at
point a with a speed of 4.0 x 105 m/s.
D. The speed at point a of a proton that was moving to the left at
point b with a speed of 4.0 x 105 m/s.
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Slide 21-29
Example Problem
A 2.0-mm-diameter plastic bead is charged to –1.0 nC.
A. A proton is fired at the bead from far away with a speed of
1.0 x 106 m/s, and it collides head-on. What is the impact
speed?
B. An electron is fired at the bead from far away. It “reflects,”
with a turning point 0.10 mm from the surface of the bead.
What was the electron’s initial speed?
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Slide 21-30
Connecting Potential and Field
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Slide 21-31
Potential and Field for Three Important Cases
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Slide 21-32
Example Problem
Source charges create the
electric potential shown.
A. What is the potential at point
A? At which point, A, B, or C,
does the electric field have its
largest magnitude?
B. Is the magnitude of the
electric field at A greater than,
equal to, or less than at point
D?
C. What is the approximate magnitude of the electric field
at point C?
D. What is the approximate direction of the electric field at
point C?
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Slide 21-33
A Conductor in Electrostatic Equilibrium
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Slide 21-34
Example Problem
What is Q2?
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Slide 21-35
Capacitance and Capacitors
The charge ±Q on each
electrode is proportional to the
potential difference ∆VC between
the electrodes:
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Slide 21-36
Charging a Capacitor
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Slide 21-37
The Capacitance of a Parallel-Plate Capacitor
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Slide 21-38
Dielectrics and Capacitors
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Slide 21-39
Dielectric Constant
With a dielectric between its
plates, the capacitance of a
parallel-plate capacitor is
increased by a factor of the
dielectric constant κ:
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Slide 21-40
Summary
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Slide 21-41
Summary
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Slide 21-42