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Electric Potential
Chapter 26
physics chapter 26
1
Review from ch 7
W12  U1  U 2

When positive work is done, the potential
energy decreases.
K1  U1  K2  U 2

Total energy is conserved.
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Work done by E field
Wab  Fd  qEd

Using calculus and the coulomb force
equation, we can come up with
Wab
q1q2  1 1 
  

40  ra rb 
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Work done by E field

We can write using potential energy
Wab  U a  U b

Thus
1
q1q2
U
40 r
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Example
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A small plastic ball with a mass of 2.0 g and
an electric charge of 0.10 mC moves in the
vicinity of a stationary metal ball with a
charge of 2.0 mC. When the plastic ball is
0.10 m from the metal one, the plastic ball is
moving directly away from the metal one with
a speed of 5.0 m/s.
What is the speed of the plastic ball when the
two balls are 0.20 m apart?
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You try

What if the plastic ball had a charge of
-0.10 mC?
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Potential

Potential energy per unit charge
U
V
q

U  qV
Measured in volts
J
1V  1
C
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Potential from a group of
charges
qi
V

40
ri
1
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Example

Pages 583-584
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Equipotential surfaces
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A surface such that every point on the
surface has the same potential.
Since the potential is the same, E does no
work as a charge moves along an
equipotential surface.
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Millikan oil-drop experiment
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1909-1913 in Chicago by Robert Millikan
Very small electrically charged oil drops
sprayed into electric field
Field adjusted until the gravitational force
down balanced the electric force up.
Figured out charge on the oil drops.
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Millikan oil-drop experiment

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Figured out that the charge was always a
whole number multiple of 1.6 x 10-19
“discovered” the charge on the electron
Validation of atomic theory
Combined with earlier research of J.J.
Thomson to determine mass of electron
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Mass of oil drops
mg
q
E
mgd
q
V
4 r gd
q
3 V
3 3
d  vt
q  18
V 2 g
3
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Electronvolt

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Unit of energy (like J)
Useful in atomic and nuclear calculations
1 eV = 1.6 x 10-19 J
The change in energy as 1 electron moves
through a potential difference of 1 V
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Rest energy
E0 = mc2
 For an electron, 81.87 x 10-15 J or 0.511 MeV
 Mass of subatomic particles is often
expressed in MeV/c2 to make the numbers
easier to work with
 Mass of electron = 0.511 MeV/c2
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Cathode ray tubes
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Found in computer monitors and
oscilloscopes
Similar to TV picture tubes
Use an electron beam – historically called a
cathode ray
A near vacuum
Electrons sent through two sets of electrically
charged plates

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One to aim it horizontally
One to aim it vertically
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Equations
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We did this kind of problem last chapter
There are more equations on pages 594 595
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