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Chapter 21
Electric Potential
Topics:
•
•
•
•
Conservation of energy
Work and Delta PE
Electric potential energy
Electric potential
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
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
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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
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Slide 21-16
Chapter 21 Key Equations (Physics 151)
Key Energy Equations from Physics 151
Types of Energy
Conservation of Energy Equation (key concept)
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Slide 21-16
Chapter 21 Key Ideas (Physics 151)
Dot Product
Method for multiplying two vectors to get a scalar
Definition of Work
Work is how forces add energy to or take away energy from a
system. It is the effect of a force applied over a displacement.
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Chapter 21 Key Equations (Physics 151)
Key Energy Equations from Physics 151
Definition of Work
Work W = F i Dr = F Dr cos a
Where a = angle between the vectors
Work- Energy Theorem (only valid when particle model applies)
Wnet = DKE
Work done by a conservative force (Fg, Fs, & Fe)
Also work done by conservative force
Wg = -DPEg
is path independent
Conservation of Energy Equation
KEi +
å
PEi + D Esys = KE f +
different types
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å
PE f + DEth
different types
Slide 21-16
Energy Bar Graph Sample
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Energy Bar Chart Example 1
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Energy Bar Chart Example II
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Energy Bar Chart Example III
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Review of Work
Definition of Work: Work W = F i Dr = F Dr cos a
where a = angle between the vectors
• Calculate the work done in moving each ball from y = 0 meters to y = 5 meters
• Calculate the work per kg for moving each ball from y = 0 m to 5 m
• Calculate the change in gravitational potential energy per kg for moving each
ball from = 0 m to 5 m
• Calculate the speed each ball would have as it reached the ground if released
from 5 meters above the ground
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Slide 21-16
Electric Potential Energy
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Slide 21-9
Electric Potential Energy
Case A - Book starts & stops at rest
WNet = DEK = 0J
Whand + Wg = 0 Þ Whand = -Wg = DEg
Case C - Charge at rest at A and B
WNet = DEK = 0J
Whand + We = 0 Þ Whand = -We = DEe
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Slide 21-9
Electric potential energy: A qualitative analysis
•
•
A positively charged
cannonball is held near
another fixed positively
charged object in the
barrel of the cannon.
Some type of energy
must decrease if
gravitational and kinetic
energies increase in this
process.
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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
DPEe = -We = - éë Fe × Dr cos a ùû = - ( q E ) Dr cos a
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 => elta V
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Slide 21-16
Graphing the electric potential energy versus distance
•
Because of the 1/r dependence, the electric
potential energy approaches positive infinity
when the separation approaches zero, and it
becomes less positive and approaches zero as
like charges are moved far apart.
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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?
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Slide 21-16
Changes in Electric Potential Energy - elta 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 (A)
Final (B)
Greatest
elta PEe
(a)
(b)
(c)
(d)
Hydrogen Atom
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Slide 21-16
Changes in Electric Potential Energy – Delta 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 (A)
Final (B)
Greatest
elta PEe
(e)
(f)
(g)
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Slide 21-16
Changes in Electric Potential Energy – Delta 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
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Slide 21-11
Electric Potential Model Worksheet 2:
Energy and Potential in Uniform Fields
Rank the change in gravitational potential energy for the following lettered objects in the
Earth’s gravitational field.
a. . most  _______ _________ ________ ________ _______ _______ ________
b. Explain your ranking, stating why each is greater than, less than, or equal to its
neighbors.
c. Where is the energy stored? What gains or loses energy as the masses move from one
place to another?
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Slide 21-16
Electric Potential Energy Example Problem
The electric field between two
charged plates is uniform with a
strength of 4 N/C.
a. Draw several electric field lines in the
region between the plates.
b. Determine the change in electrical
potential energy in moving a positive
4 microCoulomb charge from A to B.
c. Determine the change in electrical potential energy in moving a
negative 12 microCoulomb charge from A to B.
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Slide 21-16
Gravitational Potential Energy: Example Problem 2
A spacecraft is launched away from earth
a. Draw several gravitational field lines
in the region around Earth.
b. Determine the change in
gravitational potential energy when
the spacecraft moves from A to B,
where A is 10 million miles from
Earth and B is 30 million miles from
Earth.
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Slide 21-16
Electric Potential Energy: Example Problem 3
A small charge moves farther from a
positive source charge.
a. Draw several electric field lines in the region
around the source charge.
b. Determine the change in electrical potential
energy in moving a positive 4 nC charge
from A to B, where A is 3 cm from the source
charge and B is 10 cm away.
c. Determine the change in electrical potential
energy in moving a negative 4 nC charge
from A to B.
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Slide 21-16
Electric Potential
Uelec = qV; V = U elec / q
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<= Replace
before using
Slide 21-10
Chapter 21 Key Equations (3)
Key Points about Electric Potential
Electric Potential is the Electric Potential Energy per Charge
PEe
V=
qtest
DPEe
We
DV =
=qtest
qtest
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 elta V= 0 V is an equipotential line
(The electric force does zero work on a test charge that moves
on an equipotential line and elta PEe= 0 J)
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Slide 21-16
Electric Potential and E-Field for Three Important Cases
For a point charge
q
1 q
V=K =
r 4pe 0 r
For very large charged plates, must use
DPEe
We
Fe i Dr
qtest E i Dr
DV =
==== -E i Dr = - E Dr cos a
qtest
qtest
qtest
qtest
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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
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
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Slide 21-15
Chapter 22
Current and Resistance
Topics:
• Current
• Conservation of current
• Batteries
• Resistance and resistivity
• Simple circuits
Sample question:
How can the measurement of an electric current passed through a
person’s body allow a determination of the percentage body fat?
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Slide 22-1
Properties of a Current
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Slide 22-8
Brainstorm: what is used up when current flows?
(Penguin Racetrack Demo)
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Slide 21-16
Definition of a Current
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Slide 22-9
Simple Circuits
The current is determined by
the potential difference and
the resistance of the wire:
∆V
_____
chem
I = R
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Slide 22-13
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Slide 22-3
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Slide 22-4
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
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Slide 22-20
Series and Parallel
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Slide 21-16
Resistivity
The resistance of a wire
depends on its dimensions
and the resistivity of its
material:
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Slide 22-22
Kirchhoff’s Laws
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Slide 23-11
The V field
• Can we describe electric fields using the
concepts of work and energy?
• To do so, we need to describe the
electric field not as a force-related E field,
but as an energy-related field.
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Electric potential due to a single charged
object
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Finding the electric potential energy when
the V field is known
•
If we know the electric potential at a specific
location, we can rearrange the definition of the
V field to determine the electric potential
energy:
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The superposition principle and the V field
due to multiple charges
• where Q , Q , Q , … are the source charges
1
2
3
(including their signs) creating the field and r1,
r2, r3, … are the distances between the source
charges and the location where we are
determining the V field.
• So Electric Potentials (V) add just like
Electric Potential Energies
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•
Quantitative Example
Suppose that the heart's dipole charges −Q and +Q are
separated by distance d. Write an expression for the V field
due to both charges at point A, a distance d to the right of the
+Q charge.
1.Simplify and diagram.
2.Represent mathematically.
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Finding the electric potential energy when
the V field is known
•
If we know the electric potential at a specific
location, we can rearrange the definition of the
V field to determine the electric potential
energy:
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Potential difference – Delta V
• The value of the electric potential
depends on the choice of zero level, so
we often use the difference in electric
potential between two points.
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Particles in a potential difference
• A positively charged object accelerates
from regions of higher electric potential
toward regions of lower potential (like an
object falling to lower elevation in Earth's
gravitational field).
• A negatively charged particle tends to do
the opposite, accelerating from regions of
lower potential toward regions of higher
potential.
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