Physics 102: Lecture 3 Electric Potential

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Physics 102 Lecture 3Electric Potential

• Todays lecture will cover Textbook Sections 17.1
  - 3

Honors Credit info on web under course
description.
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Overview for Todays Lecture

• Electric Potential Energy/ Work
• Uniform fields
• Point charges
• Electric Potential (like height)
• Uniform fields
• Point charges

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Recall Work from P101

• Work done by the force given by
• W F d cos(q)
• Positive Force is in direction moved
• Negative Force is opposite direction moved
• Zero Force is perpendicular to direction moved
• Careful ask WHAT is doing work!
• Opposite sign for work done by you!
• Conservative Forces
• D Potential Energy -Wconservative

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Preflight 3.1
C
A
B
Uniform E?

• In what direction does the force on a negative
  charge at point A point?
• left
• right
• up

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Preflight 3.2
C
A
B
Uniform E?

• When a negative charge is moved from A to C the
  ELECTRIC force does
• positive work.
• zero work.
• negative work.

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Preflight 3.3
C
A
B
Uniform E?

• When a negative charge is moved from A to B the
  ELECTRIC force does
• positive work.
• zero work.
• negative work.

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Preflight 3.5
C
A
B

• When a negative charge is moved from A to B, the
  electric potential energy of the charge
• Increases
• is constant
• decreases

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ACT Electric Potential Energy

C
B
A
When a negative charge is moved from A to B, the
electric potential energy of the
charge (1) increases (2) is constant (3) decreases
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Work and D Potential Energy
W F d cos(q) Gravity Electric

• Brick raised yi? yf

• Charge moved 8 ? rf
• FE kq1q2/r2 (left)
• WE -kq1q2/rf
• DUE kq1q2/rf

• FG mg (down)
• WG -mgh
• DUG mgh

yf?
h
yi?
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Work done by YOU to assemble 3 charges
Example

• W1

• W2

• W3
• Wtotal
• WE
• D UE

3
5 m
5 m
2
1
5 m
Watch signs!
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ACT Work done by YOU to assemble 3 negative
charges
How much work would it take YOU to assemble 3
negative charges?
3

• W 19.8 mJ
• W 0 mJ
• W -19.8 mJ

5 m
5 m
2
1
5 m
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Preflight 3.11
1

5 m
5 m
-

3
2
5 m
The total work required by you to assemble this
set of charges is (1) positive (2) zero (3) ne
gative
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Electric Potential (like height)

• Units Joules/Coulomb ?Volts
• Batteries
• Outlets
• EKG
• Really Potential differences
• Equipotential lines at same height
• Field lines point down hill
• V k q/r (distance r from charge q)
• V(8) 0

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Preflight 3.7

• The electric potential at point A is _______ at
  point B
• greater than
• equal to
• less than

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Preflight 3.9
conductor

• The electric potential at point A is _______ at
  point B
• greater than
• equal to
• less than

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ACT Electric Potential

C
B
A

• The electric potential at A is ___________ the
  electric potential at B.
• greater than
• equal to
• less than

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Electric Potential due to Proton

• What is the electric potential a distance r
  0.53?10-10 m from a proton? (Let V(?) 0)

V U/q k q/ r
What is the electric potential energy of an
electron a distance r 0.53?10-10 m from a
proton?
U Vq

rf 0.5?10-10 m

-
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ComparisonElectric Potential Energy vs.
Electric Potential

• Electric Potential Energy (U) - the energy of a
  charge at some location.
• Electric Potential (V) - found for a location
  only tells what the EPE would be if a charge
  were located there (usually talk about potential
  differences between two locations)
• U Vq
• Neither has direction, just location. Sign
  matters!

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Two Charges
Example

• Calculate electric potential at point A due to
  charges
• Calculate V from 7mC charge
• Calculate V from 3.5mC charge
• Add (EASY!)
• V kq/r
• V7
• V3
• Vtotal

A
4 m
6 m
Q7.0mC
Q-3.5 mC
WDUDVq
How much work do you have to do to bring a 2 mC
charge from far away to point A?
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ACT Two Charges

• In the region II (between the two charges) the
  electric potential is

1) always positive 2) positive at some points,
negative at others. 3) always negative
I
II
III
Q-3.5 mC
Q7.0mC
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To Do

• Read 17.5-6, 18.1-2, 6
• Bring Problem Solver to discussion section
• Complete preflight before 800AM on day of next
  lecture.