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2/2/13 Chapter 21: Electric Potential
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Key Terms:
¤  Electric
Potential
Potential Energy
¤  Capacitance
¤  Electric
Electric Potential Energy
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Doing work on a charge can increase the charge’s
potential energy.
A charged particle’s potential energy is
proportional to its charge.
The ratio between a charged particle’s potential
energy and its charge is called electric potential.
1 2/2/13 Electric Potential Difference (Voltage)
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A potential difference is
created by separating
positive charge from
negative charge.
A common source of a
fixed potential difference is
a battery.
The potential difference
between two points is often
called voltage.
Sample Problem #1 (ex. 21.1, page
678)
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A 15 nC charged particle moves from point A,
where the electric potential is 300 V, to point B,
where the electric potential is – 200 V.
¤  By
how much does the electric potential change?
how much does the particle’s electric potential
energy change?
¤  How would your answers differ if the particle’s charge
were -15 nC?
¤  By
Electric Potential and Conservation of
Energy
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Energy is conserved.
Therefore, charges will speed up or slow down
while moving through regions of changing potential.
2 2/2/13 Sample Problem #2 (ex. 21.2, page
682)
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A proton moves through an electric potential
created by a number of charges. Its speed is 2.5 x
105 m/s at a point where the potential is 1500 V.
What will be the proton’s speed a short time later
when it reaches a point where the potential is
-500V?
The Electron Volt
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The joule is a unit of appropriate size in mechanics
and thermodynamics.
With atomic and nuclear events, it is often more
appropriate to use the “electron volt” as our unit for
energy.
1 eV is the kinetic energy gained by an electron (or
proton) if it accelerates through a potential
difference of 1 volt.
Sample Problem #3 (ex. 21.3, page
683)
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Atomic particles are often characterized by their
kinetic energy in MeV. What is the speed of an 8.7
MeV proton?
3 2/2/13 Electric Potential Inside A Capacitor
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A uniform electric field exists between the
oppositely charged plates of a parallel-plate
capacitor.
The potential difference across the capacitor
depends upon this field and the distance between
the plates.
The negative plate will have a lower potential.
Sample Problem #4 (page 707 #14)
Which capacitor plate
is positive?
¨  What is the electric
field strength inside
the capacitor?
¨  What is the potential
energy of a proton at
the midpoint of the
capacitor?
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4 2/2/13 Sample Problem #5 (ex. 21.4, page
686)
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A parallel-plate capacitor is constructed of two
disks spaced 2.00 mm apart. It is charged to a
potential difference of 500 V. A proton is shot
through a small hole in the negative plate with a
speed of 2.0 x 105 m/s. Does it reach the other
side? If not, what is the farthest distance from the
negative plate that the proton reaches?
The Potential of a Point Charge
Sample Problem #6 (ex. 21.4, page
687)
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An interaction between two elementary particles
causes an electron and a positron (a positively
charged electron) to be shot out back-to-back with
equal speeds. What minimum speed must each
particle have when they are 100 fm apart in order
to end up far from each other?
5 2/2/13 Sample Problem #7 (ex. 21.6, page
688)
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What is the electric potential 1.0 cm from a 1.0 nC
charge? What is the potential difference between a
point 1.0 cm away and a second point 3.0 cm
away?
The Electric Potential of a Charged
Sphere
Sample Problem #8 (ex. 21.7, page
689)
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A proton is released from rest at the surface of a
1.0 cm diameter sphere that has been charged to
+1000 V.
¤  What
is the charge of the sphere?
is the proton’s speed when it is 1.0 cm from the
sphere?
¤  When the proton is 1.0 cm from the sphere, what is its
kinetic energy in eV?
¤  What
6 2/2/13 The Electric Potential of Many Charges
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Electric potential is a scalar…
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No directions, no triangles, etc!
Sample Problem #9 (ex. 21.8, page
690)
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What is the electric potential at the point indicated
in the figure below?
Capacitance
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The charge of a capacitor is directly proportional
to the potential difference between its electrodes.
The ratio of the the capacitor’s charge to its
potential difference is called capacitance.
A charged capacitor stores energy as electric
potential energy.
7 2/2/13 Sample Problem #10 (ex. 21.11, page
697)
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The spacing between the plates of a 1.0 μF
parallel-plate capacitor is a 0.070 mm.
¤  What
is the surface area of the plates?
much charge is on the plates if this capacitor is
attached to a 1.5 V battery?
¤  How
Sample Problem #12 (ex. 21.13, page
700)
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How much energy is stored in a 220 μF cameraflash capacitor that has been charged to 330 V?
What is the average power delivered to the flash
lamp if this capacitor is discharged in 1.0 ms?
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