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Chapter 21
Electric Potential
Topics:
• Electric potential energy
• Electric potential
• Conservation of energy
Sample question:
Shown is the electric potential measured on the surface of a patient.
This potential is caused by electrical signals originating in the beating
heart. Why does the potential have this pattern, and what do these
measurements tell us about the heart’s condition?
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Slide 21-1
Class Format Question
More traditional lecture method with some clicker questions
• Advantages
• More comfortable
• More examples to adapt to homework problems
• More content coverage (one chapter every 2-4 days)
• Disadvantages
• More content coverage
• Less learning of Physics concepts
• No improvement in exam scores, possible decline unless
exam problems are exactly like homework problems
• Encourages memorizing vs. learning physics
• Not as much development of useful thinking and problem
solving skills
• Will not raise grades
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Class Format Question
More conceptual approach with in-class activities
(modified to include more examples per unit)
• Advantages
• Better learning of Physics concepts
• Encourages learning physics vs. memorizing
• Less content coverage
(more time on harder chapters until last month,
then normal pace - one chapter every 2-4 days )
• Development of useful thinking and problem solving skills
• Disadvantages
• Less comfortable
• Less content coverage
• Improvement in exam scores requires practicing doing
problems using concepts and less reliance on memorizing
examples
• Possibly more frustration on homework
• Learning to think and problem solve new ways is not easy
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Class Format Question
A. More traditional lecture method with some clicker questions
Cover more topics - learn less
B. More conceptual approach with in-class activities
(modified to include more examples per unit)
Cover fewer topics - learn more
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
What to do to do well in this class
A. Focus on key physics concepts
• May seem like basics but will help you solve even complex
problems
• Focus on principle rather than recipes
• Need to have a functional understanding of key concepts
• Express key equations as sentences
• Know where they come from and what they mean
• Know how and when to apply them
• Know which equations are general and which are special cases
• Must know when not to apply special cases
• Look at a problem after a good physics diagram and maybe a
good physical diagram and know what key physics concepts apply
in that problem
• Memorize key concepts so you can look at a problem, say that’s
Newton 2, and know the associated equation in a snap
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
What to do to do well in this class
A. Focus on key physics concepts
• How to do this
• When you look at problems, mentally group problems by
the physics rather than the physical situation
• After each class or at least each week, create a notesheet
to organize a structure of the new key concepts for each
chapter and note how they fit in with previous key
concepts
• Use the note sheet to do homework problems (a) do as
many homework problems as you can just using this
sheet. (b) then go to your notes and the textbook for your
missing pieces
• Use flash cards to memorize key concepts - include the
concept description, relevant equations, diagrams, and
what types of problems benefit from using that concept
• Pay close attention to examples done in class and note the
physics and assume/observes in each example and how
these are used
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Chapter 21 Key Equations (1)
Key Energy Equations from Physics 151
Definition of Work
r
r r
r
Work W  F g r  F r cos 
Work- Energy Theorem (only valid when particle model applies)
Wnet  KE
Work done by a conservative force (Fg, Fs, & Fe)
Wg  PEg Also work done by conservative force
is path independent
Conservation of Energy Equation
KEi 

PEi  Wext  KE f 
different types
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
PE f  Eth
different types
Slide 21-16
Chapter 21 Key Equations (2)
Key Energy Equations from Physics 152
q1q2
PEe  k
r12
Electric Potential Energy for 2 point charges
(zero potential energy when charges an infinite distance apart)
Potential Energy for a uniform infinite plate
r
r
PEe  We    Fe  r cos      q E r cos 
For one plate, zero potential energy is at infinity
For two plates, zero potential energy is at one plate or
inbetween the two plates
Electric Potential V and Change in Electric Potential => V
PEe
V
qtest
PEe
We
V 

qtest
qtest
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Slide 21-16
Example: Electric Potential Energy
A cart on a track has a large, positive charge and is located between
two sheets of charge. Initially at rest at point A, the cart moves
from A to C.
a. Draw qualitative force diagrams for
the cart at positions A, B and C.
b. Draw qualitative energy bar charts
for the cart when it is at each position
A, B and C. List the objects that
make up your system:
c. How would your force and energy diagrams change (if at all) if the sheet to
the right were also positively charged?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Changes in Electric Potential Energy PEe
For each situation below, identify which arrangement (final or initial) has more
electrical potential energy within the system of charges and their field.
Initial
Final
Greatest PEe
(a)
(b)
(c)
(d)
Hydrogen Atom
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Changes in Electric Potential Energy PEe
For each situation below, identify which arrangement (final or initial) has more
electrical potential energy within the system of charges and their field.
Initial
Final
Greatest PEe
(e)
(f)
(g)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Changes in Electric Potential Energy PEe
Is the change ∆PEe of a charged particle positive, negative,
or zero as it moves from i to f?
(a) Positive (b) Negative (c) Zero (d) Can’t tell
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-11
Chapter 21 Key Equations (3)
Key Points about Electric Potential
Electric Potential increases as you approach positive source
charges and decreases as you approach negative source
charges (source charges are the charges generating the electric
field)
A line where V= 0 V is an equipotential line
(The electric force does zero work on a test charge that moves
on an equipotential line and PEe= 0 J)
For a point charge
q
1 q
VK 
r 4 0 r
For very large charged plates, must use
r
r
r
r
r
r r
PEe
We
Fe g r
qtest E g r
r
V 



  E g r   E r cos 
qtest
qtest
qtest
qtest
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Electric Potential and E-Field for Three Important Cases
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Slide 21-25
Checking Understanding
Rank in order, from largest to smallest, the electric
potentials at the numbered points.
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Slide 21-14
Example
A proton has a speed of 3.5 x 105 m/s at a point where the
electrical potential is 600 V. It moves through a point where the
electric potential is 1000 V. What is its speed at this second point?
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Slide 21-16
A Topographic Map
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Slide 21-12
Graphical Representations of Electric Potential
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Slide 21-13
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Slide 21-15
Example
A proton has a speed of 3.5 x 105 m/s at a point where the
electrical potential is 600 V. It moves through a point where the
electric potential is 1000 V. What is its speed at this second point?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Electric Potential Energy
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Slide 21-9
Electric Potential
U elec  qV; V  U elec / q
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-10