Download chapter-19-3 - High Point University

CH 19-3
Review -- Gravitational Potential Energy
and Gravitational Potential
If a ball is released from rest,
it will move in the direction of
the gravitational force on it.
Earth
If a charged particle is
released from rest, it will
move in the direction of
the electric force on it.
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Electric Potential
Electric potential is a scalar. It is analagous to
“altitude” for gravitation. A proton released from rest
will move from high potential to low potential. An
electron released from rest will move from low
potential to high potential.
Electric field always points from high potential to
low potential.
Example -- Capacitor
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+
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+
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+
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+
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








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Poll
Which point is at a higher electric potential?
+
A
B
1. A
2. B
3. Both A and B have the same electric potential
Poll
Which point is at a higher electric potential?

A
B
1. A
2. B
3. Both A and B have the same electric potential
Uniform Electric Field
The potential difference between two points along a
path that is parallel to a uniform electric field is
i
f
This equation can also be used if points are close
together.
Example -- Capacitor
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


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

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

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Poll
V=9 V
What is the potential halfway between the plates?
1. 9 V
2. 0 V
3. 4.5 V
4. 2.25 V
5. None of the above.
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V=0
A










Electric Potential Energy
When a charged particle moves from point i to point f,
the system’s change in electric potential energy is
i
f
Example -- Accelerating charged particle
in a uniform electric field
Suppose that there are
slits in the capacitor
plates. An electron is
released from rest at the
negatively charged plate.
If the potential difference
across the plates is
10,000 V, how fast is the
electron moving when it
reaches the other plate?
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



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

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Poll

q
v
++++++++++++++++++
What is the path of the electron?
1.
3.
2.
4.

2

1

3
Charged surface
The electric field near a charged
surface is shown in red. A negative
charge carrier released from rest at
point 1 will _______________.
A. travel past point 2, gaining PE as it goes
B. travel past point 2, losing PE as it goes
C. travel past point 3, gaining PE as it goes
D. travel past point 3, losing PE as it goes
Charged surface
3

1


2
The electric field near a charged
surface is shown in red. A positive
charge carrier is given an initial
velocity v0 to the right at
point 1.
Which of the following states all the
possibilities?
A. The charge carrier can reach point 2 and gain PE.
B. The charge carrier can reach point 2 and lose PE.
C. The charge carrier can reach point 3 and gain PE.
D. The charge carrier can reach point 3 and lose PE.
E. Two of the above are possible.
+ + + + + + + + +
2

1

-
An electron going from point 1 to
point 2 ______________________
A. moves toward increasing potential and gains PE.
B. moves toward increasing potential but loses PE.
C. moves toward decreasing potential and gains PE.
D. moves toward decreasing potential but loses PE.
Application - Gel Electrophoresis