Download Chapter 21 Electric Potential Reading Quiz Reading Quiz Answer

Chapter 21
Electric Potential
Reading Quiz
1. What are the units of potential difference?
Topics:
•
•
•
•
•
Electric potential energy
Electric potential
Conservation of energy
Potential and field
Capacitors and capacitance
Sample question:
Shown is the electric potential me
easured on the surface of a patient.
This potential is caused by electric
trical signals originating in the beating
heart. Why does the potential have this pattern, and what do these
measurements tell us about the heart’s condition?
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Reading Quiz
Answer
2. New units of the electric field were introduced in this
chapter. They are:
A. V/C
B. N/C
C. V/m
D. J/m2
E. Ω/m
F. J/C
2. New units of the electric field were introduced in this
chapter. They are:
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C. V/m
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Reading Quiz
Answer
3. The electric potential inside a parallel-plate
parallel
capacitor
A. is constant.
B. increases linearly from the negative to the positive plate.
C. decreases linearly from the negative to the positive plate.
D. decreases inversely with distance from the negative plate.
E. decreases inversely with the square of the distance from
the negative plate.
3. The electric potential inside a parallel-plate
parallel
capacitor
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B. increases linearly from the negative to the positive plate.
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Reading Quiz
Answer
4. The electric field
A. is always perpendicular to an equipotential surface.
B. is always tangent to an equipotential surface.
C. always bisects an equipotential surface.
D. makes an angle to an equipotential surface that depends
on the amount of charge.
4. The electric field
A. is always perpendicular to an equipotential surface.
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Electric Potential Energy
Potential Energy
WAB = mghA − mghB = GPE A − GPE B
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Potential Energy
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10
Potential Energy
WAB = EPE A − EPE B
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The Electric Potential Difference
The Electric Potential Difference
DEFINITION OF ELECTRIC POTENTIAL
The electric potential at a given point is the electric potential energy
of a small test charge divided by the charge itself:
WAB EPE A EPE B
=
−
qo
qo
qo
V=
SI Unit of Electric Potential:
The potential energy per unit charge
is called the electric potential.
joule/coulomb = volt (V)
VB − VA =
EPE B EPE A − WAB
=
−
qo
qo
qo
∆V =
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13
∆(EPE ) − WAB
=
qo
qo
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14
Exercise
Electric Potential
Is the change ∆U of the particle positive, negative, or
zero as it moves from i to f?
U elec = qV; V = U elec / q
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EPE
qo
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A Topographic Map
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Graphical Representations of Electric Potential
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Checking Understanding
Rank in order, from largest to smallest, the electric
potentials at the numbered points.
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Example
Example
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?
A proton is released from rest att p
point a. It then travels past point b.
What is its speed at point b?
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The Potential Inside a Parallel-Plate
Parallel
Capacitor
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Example
A parallel-plate
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
plate capacitor is zero).
V = Ex =
Q
∆V
x= C x
ε0 A
d
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Electric Potential: Charged Sphere
Electric Potential of a Point Charge
Outside of a sphere of charge Q the
th potential has the same form as
for a point charge Q:
V=
1 Q
4πε
π 0 r
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
V=K
1 q
q
=
r 4πε 0 r
V=
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R
V0
r
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Example
Example
For the situation shown in the figure, find
A 2.0-mm-diameter
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.
head
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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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 21-22
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Connecting Potential and Field
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Potential and Field for Three Important Cases
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A Conductor in Electrostatic Equilibrium
Example
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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Exercise
Capacitance and Capacitors
The charge ±Q on each electrode
is proportional to the potential
difference ∆VC between the
electrodes:
Q = C∆VC
What is Q2?
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The Capacitance of a Parallel-Plate
Parallel
Capacitor
Charging a Capacitor
C=
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ε0 A
d
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Dielectrics and Capacitors
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:
C=
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κε 0 A
d
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