Physics 212 Lecture 4, Slide 1

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Physics 212 Lecture 5
Today's Concept Electric Potential
Energy Defined as Minus Work Done by Electric
Field
Extra 3
Preflight 1
Preflight 5
Preflight 3
Extra 1
Extra 2
Music
Preflight 9
Preflight 7
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Things we saw you struggle with
1) Sign of work potential energy
2) Picking a place where U is zero
3) DU for a collection of charges
Other comments
Preflight deadlines
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Preflight 1
The work done by gravity is negative because the
direction of the gravitational force is opposite
the direction of motion. Its potential energy
increases because the change in potential energy
is equal to the opposite of the work done.
Since the ball is moving against the force of
gravity, the work is negative. The other part of
the answer doesn't even deserve justification
since we all had to pass 211 to get here. )
D
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Recall from physics 211
5
FE
FH
dr
E
WH is the work done by the hand on the ball WE is
the work done by the electric field on the
ball Which of the following statements is
true A) WH 0 and WE 0 B) WH 0 and WE
0
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FE
FH
dr
E
B) WH 0 and WE

Is DU positive or negative?

Positive

Negative

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Preflight 5
D
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F
W 0
dr
The sign will usually be simple.
F
W dr
But to get the magnitude right you may have to do
an integral
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Example
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We often say U 0 at infinity
FE
W 0 so DU dr
FE
W 0
dr
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DU from a collection of many charges simply adds
up
Q1
Q2
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Two positive charges of equal magnitude are
placed at equal distances from point A.
If a third positive is added to the
system at point A, how does the electric
potential energy of the system change? A)
IncreasesB) DecreasesC) Doesnt change
A
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Two negative charges of equal magnitude are
placed at equal distances from point A.
If a third positive is added to the
system at point A, how does the electric
potential energy of the system change? A)
IncreasesB) DecreasesC) Doesnt change
A
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Preflight 3
A
B
C
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Preflight 9
Since the sign of the work done by each of the
positive charges will always be the same, they
can never be added together to sum to zero.
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Preflight 9
Since the sign of the work done by each of the
positive charges will always be the same, they
can never be added together to sum to zero.
W W1 W2 0
DU -W 17
Preflight 7

• Since there are two more terms added to the
  potential energy equation, the terms would have
  to cancel each other, but this is possible
  because the existing charges are opposite sign.
• No matter the magnitude or sign of the charge,
  it will always have a non-zero net force on it,
  therefore the total P.E. of the system will have
  to change.

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Preflight 7
Since there are two more terms added to the
potential energy equation, the terms would have
to cancel each other, but this is possible
because the existing charges are opposite sign.
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ACT
Where is the change in the potential energy of
the system equal to zero when a positive charge
q is added to the (q, -5q) system?
(C)
(B)
(A)
(D)
(E)
The force doesnt have to be zero at the point
where the change in potential energy is zero !
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Music

• Who is the Artist?
• Grateful Dead
• Jefferson Airplane
• CCR
• The Band
• ZZ Top

Have you seen the movie? Best music movie
ever?? 1978 Martin Scorcese Clapton, Dylan. Dr.
John, Joni Mitchell, Van the Man, Neil Young,
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Extra 1
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Extra 2
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Extra 3