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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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-1
A Conductor in Electrostatic Equilibrium
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Slide 21-27
Potential and a Conducting Sphere
Outside the Sphere (Just like a point charge)
• E = k|q| / r2
• V = kq / r
Inside the sphere (not like a point charge)
• E=0
• Delta V = 0 => V = constant
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Exercise
What is Q2?
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Slide 21-28
Equipotential Maps (Contour Maps)
1.Describe the charges that
could create equipotential lines
such as those shown above.
2.
2.Describe the forces a proton
would feel at locations A and B.
3. Describe the forces an
electron would feel at locations
A and B
4.
4.Where could an electron be
placed
that is
it would
not
5. At
whichsopoint
the magnitude
of the electric field the greatest?
move?
6. Is it possible to have a zero electric field, but a non-zero electric potential?
7. Is it possible to have a zero electric potential, but a non-zero electric field?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
3D view
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Slide 21-16
E-field lines and Equipotential lines
E-field Lines
• Go from + charges to - charges
• Perpendicular at surface of conductor or charged surface
• E-field in stronger where E-field lines are closer together
• More charge means more lines
Equipotential Lines
• Parallel to conducting surface
• Perpendicular to E-field lines
• Near a charged object, that charges influence is greater, then blends as
you to from one to the other
• E-field is stronger where Equipotential lines are closer together
• Spacing represents intervals of constant V
• Higher potential as you approach a positive charge; lower potential as you
approach a negative charge
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Two last Points
Electron Volt - unit of energy
• The energy an electron gains as it goes through Delta V = 1 V
• PEe = qV = (1.6e-19 C)(1 V) = 1.6e-19 J
• 1.6e-19 J = 1 eV
Path Independence
• Delta V does not depend on path
• Delta V = 0 around any closed path
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Capacitance and Capacitors
The charge ±Q on each
electrode is proportional to the
potential difference ΔVC between
the electrodes:
C = Q/Vc
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Slide 21-29
Charging a Capacitor
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Slide 21-30
The Capacitance of a Parallel-Plate Capacitor
C
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0 A
d
Slide 21-31
Dielectrics and Capacitors
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Slide 21-32
Dielectric Constant
With a dielectric between its
plates, the capacitance of a
parallel-plate capacitor is
increased by a factor of the
dielectric constant κ:
C
 0 A
d
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-33
Light the Bulb
Can you light a bulb when you have
• 1 battery
• 1 Bulb
• 1 wire
• A - yes
• B - no
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Light the Bulb
Can you light a bulb when you have
• 1 battery
• 1 Bulb
• 1 wire
• A - yes
• B - no
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
W
chem
∆Vbat = ____
=
q
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 22-12
Properties of a Current
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Slide 22-8
Definition of a Current
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Slide 22-9
Kirchhoff’s Laws
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Slide 23-11
Using Kirchhoff’s Laws
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Slide 23-12