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Chapter 21
Electric Potential
Topics:
• Electric potential energy
• Electric potential
• Conservation of energy
• Capacitors and
Capacitance
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.
Discussion of other units for Energy and E-field
eV – electron Volts => Unit of energy for particle accelerators
The energy gained by an electron that goes through a potential
difference of one volt
1 eV = 1.60 x 10-19 J
V/m – Volts per meter => Unit of Electric Field
|Delta V| = |E||Delta r| => |E| = |Delta V| / |Delta r|
[E] = V / m
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
Slide 22-12
Parallel Plate Capacitor
A capacitor consists of two conductors that are
close but not touching. A capacitor has the
ability to store electric charge.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Parallel Plate Capacitor
(a) Parallel-plate capacitor connected to battery.
(b) Battery and Capacitor in a circuit diagram.
Relationship of E-field & Delta V?
Delta V
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Define Capacitance
Capacitance is a measure of how much charge can be stored in a
capacitor for a given amount of voltage
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
The Capacitance of a Parallel-Plate Capacitor
C=
e0 A
d
Slide 21-31
Capacitance and Capacitors
The charge ±Q on each
electrode is proportional to the
potential difference ΔVC between
the electrodes:
Q = CDVC
Slide 21-29
Charging a Capacitor
Slide 21-30
Capacitors
Note: Battery is a source of constant potential
What happens when you pull the plates of a capacitor
apart?
• With a Battery connected
• With no Battery connected
Do the following quantities (a) increase, (b) decrease, or
(c) remain the same:
• Charge
• E-Field
• Delta V
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Dielectrics and Capacitors
Dielectrics and Capacitors
The molecules in a dielectric tend to become oriented in a way that
reduces the external field.
This means that the electric field within the dielectric is
less than it would be in air, allowing more charge to be
stored for the same potential.
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=
ke 0 A
d
Dielectric strength is the maximum
field a dielectric can experience
without breaking down.
E' 
E0

Energy stored in Capacitor – Storing Energy in E-field
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Energy stored in Capacitor – Storing Energy in E-field
A charged capacitor stores electric energy; the energy stored is
equal to the work done to charge the capacitor.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Storage of Electric Energy
The energy density, defined as the energy per unit
volume, is the same no matter the origin of the
electric field:
(17-11)
The sudden discharge of electric energy can be
harmful or fatal. Capacitors can retain their charge
indefinitely even when disconnected from a
voltage source – be careful!
Capacitors and Capacitance (Key Equations)
Capacitance
• C = |Q| / |Delta V|
• Property of the conductors and the dielectric
Special Case - Parallel Plate Capacitor
• C = Kappa * Epsilon0*A / d
Energy
• Pee = 1/2 |Q| |Delta V|
• |Delta V| = Ed
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 21-16
Properties of a Current
Slide 22-8
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
Definition of a Current
Slide 22-9
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
W
chem
∆Vbat = ____
=
q
Slide 22-12
Storage of Electric Energy
Heart defibrillators use electric
discharge to “jump-start” the heart,
and can save lives.
The Electrocardiogram (ECG or EKG)
The electrocardiogram
detects heart defects by
measuring changes in
potential on the surface of
the heart.
Capacitors
Note: Battery is a source of constant potential
What happens when you insert a dielectric?
• With a Battery connected
• With no Battery connected
Do the following quantities (a) increase, (b) decrease, or
(c) remain the same:
•
Charge
•
E-Field
•
Delta V
•
Energy stored
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.