Download Powerpoint

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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-1
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-25
Checking Understanding
Rank in order, from largest to smallest, the electric
potentials at the numbered points.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
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
A Topographic Map
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-12
Graphical Representations of Electric Potential
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-13
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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