Transcript Chapter 17: Electric Potential

Chapter 17: Electric Potential
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 As
in earlier chapters on mechanics we
learned that energy is conserved; it is
neither created nor destroyed but is
transferred from one object to another or
transformed into another type of energy
 Energy and its interactions can help us
understand nature
 Work performed on a charged particle in an
electric field can result in the particle
gaining electric potential energy (PE), kinetic
energy (KE) or both
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 The
electric
field does
work when it
moves the
charged
particle from
location a to b
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
Electric potential is defined as the electric
potential energy per unit charge and is measured
with a voltmeter

Va= Electric Potential: Units=volt (V). Named
after Alessandro Volta, inventor of the electric
battery

PEa= Electric Potential Energy: Unit= joule (J)

q = Charge on particle: Unit=Coulomb (C)
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 Also
called voltage
 Electric potential difference is the
difference in electric potential (V) between
the final and initial position
Δ
 Also
the ratio of work needed to move a
charge between two points divided by the
magnitude of the charge
ΔV=W
q
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17.2 Relation between Electric Potential
and Electric Field

A uniform electric field can made by placing two
large flat conducting plates of opposite charge
parallel to each other

The electric field can be calculated by dividing the
potential difference between the plates by the
distance between the plates (in meters)
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17.3 Equipotential Lines
 Electric
potential can be
represented by drawing
equipotential lines (green)
 An
equipotential is a line
over which the potential is
constant
 Equipotential
lines are
perpendicular to the
Electric field (red)
 Conductors
are
equipotential surfaces
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The electric field is strongest where the
equipotential lines are closest together.
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17.4 The Electron Volt, a Unit of Energy
 The
joule is a large unit to deal with energies
of electrons, atoms or molecules
 The electron volt (eV) is used
 An eV is the energy gained by an electron
moving through a potential difference of one
volt.
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17.7 Capacitance
A capacitor consists of two conductors that are
close but not touching. A capacitor has the
ability to store electric charge
 In general capacitance increases as the plates
become larger and decreases as the separation
between plates increases

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 Capacitors
are
used widely in



electronic
circuits
power failure
back ups
Blocking
surges of
charge and
energy
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 (a)
Parallel-plate
capacitor connected to
battery. When connected
to a battery the plates
become charged; one +
and one –
 (b)
In a circuit diagram
the capacitor is
represented as seen here
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 When
a capacitor is connected to a battery,
the amount of charge (Q) on its plates is
proportional to the potential difference
(voltage) between them
• C is the capacitance and depends on the size,
shape, position and separating material of the
capacitor
•Unit of capacitance: farad (F) 1 F = 1 C/V
(coulomb/volt)
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17.8 Dielectrics

Most capacitors have an
insulating sheet of
between the plates

This insulator is a
dielectric

Do not break down and
allow charge to flow as
easily as air, allowing
higher voltages

Allow plates to be closer
together

Increase the capacitance
by a factor of K; a
dielectric constant
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If the electric field in a dielectric becomes too
large, it can tear the electrons off the atoms,
thereby enabling the material to conduct. This is
called dielectric
breakdown; the
field at which this
happens is called
the dielectric
strength
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17.9 Storage of Electric Energy
A
charged capacitor stores electric
energy by separating + and –
charges
 The energy stored is equal to the
work done to charge it
 Stored energy in a capacitor can
cause burns or shocks, even when
the external power is off!
 There are many uses for
capacitors; a camera flash, a
cardiac defibrillator, etc.
 An essential part of most
electrical devices used today
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A
defibrillator is a
capacitor charged
to a high voltage.
Once charged it
sends a brief charge
through the heart.
This can stop the
heart and
(hopefully!) allow it
to resume normal
beating
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

Zitewitz. Physics: Principles and
Problems. 2004
Giancoli, Douglas. Physics: Principles with
Applications 6th Edition. 2009.
 Walker,
James. AP Physics, 4th Edition.
2010
 http://commons.wikimedia.org/wiki/File:Ca
pacitor_schematic_with_dielectric.svg
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