Download Ch. 42 Molecules, Condensed Matter

Chapter 42
Molecules and
Condensed Matter
PowerPoint® Lectures for
University Physics, Thirteenth Edition
– Hugh D. Young and Roger A. Freedman
Lectures by Wayne Anderson
Copyright © 2012 Pearson Education Inc.
Goals for Chapter 42
• To understand the bonds holding atoms together
• To see how rotation and vibration of molecules affect
spectra
• To learn how and why atoms form crystalline structures
• To apply the energy-band concept to solids
• To develop a model for the physical properties of metals
• To learn how impurities affect semiconductors and to
see applications for semiconductors
• To investigate superconductivity
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Introduction
• Why do CO2 molecules raise
the temperature of Venus so
high? Why are they raising the
earth’s temperature?
• After looking at individual
atoms in earlier chapters, we
shall now look at molecules and
condensed matter, formed when
atoms combine.
• We’ll see how energy bands
help us understand solids and
we’ll look at semiconductors,
which are used in calculators,
computers, and much more.
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Ionic bonds
• An ionic bond is an
interaction between
oppositely charged ionized
atoms.
• Figure 37.1 (right) shows a
graph of the potential
energy of two oppositely
charged ions.
• Follow Example 42.1.
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Covalent bonds
• In a covalent bond, the wave
functions are distorted and become
more concentrated in certain places.
• Figure 42.2 (right) shows the
hydrogen covalent bond, and Figure
42.3 (below) shows the methane
molecule.
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Q42.1
The difference between an ionic bond and a covalent bond is
A. ionic bonds are only found in crystals such as sodium
chloride (NaCl) where there are many atoms in close
proximity.
B. covalent bonds are only found in molecules with three
or more atoms.
C. ionic bonds are highly directional, while covalent
bonds are not.
D. ionic bonds involve the transfer of an electron from
one atom to another, while covalent bonds involve
electrons that spend much of their time between atoms.
Copyright © 2012 Pearson Education Inc.
A42.1
The difference between an ionic bond and a covalent bond is
A. ionic bonds are only found in crystals such as sodium
chloride (NaCl) where there are many atoms in close
proximity.
B. covalent bonds are only found in molecules with three
or more atoms.
C. ionic bonds are highly directional, while covalent
bonds are not.
D. ionic bonds involve the transfer of an electron from
one atom to another, while covalent bonds involve
electrons that spend much of their time between atoms.
Copyright © 2012 Pearson Education Inc.
Rotational energy levels
• Follow the text discussion of rotational energy levels for
diatomic molecules.
• Figure 42.4 (left) shows a model of a diatomic molecule.
• Figure 42.5 (right) shows some rotational energy levels for
a diatomic molecule.
• Follow Example 42.2.
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Vibrational energy levels
• Follow the text discussion of vibrational energy levels of
a diatomic molecule.
• Figure 42.6 (left) shows a model of a diatomic molecule.
• Figure 42.7 (right) shows some vibrational energy levels
of a diatomic molecule.
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Rotation and vibration combined
• Follow the text discussion. Figure
42.8 (right) shows an energy-level
diagram for rotational and
vibrational energy levels of a
diatomic molecule. Figure 42.9
(below) shows a typical molecular
band spectrum.
• Follow Example 42.3.
Copyright © 2012 Pearson Education Inc.
Q42.3
This diagram shows the vibrational
and rotational energy levels of a
diatomic molecule. Consider two
possible transitions for this molecule:
A. n = 2, l = 5 to n = 1, l = 4
B. n = 2, l = 1 to n = 1, l = 0
The energy change is
A. greater for transition A.
B. greater for transition B.
C. the same for both transitions.
D. any of the above, depending on circumstances.
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A42.3
This diagram shows the vibrational
and rotational energy levels of a
diatomic molecule. Consider two
possible transitions for this molecule:
A. n = 2, l = 5 to n = 1, l = 4
B. n = 2, l = 1 to n = 1, l = 0
The energy change is
A. greater for transition A.
B. greater for transition B.
C. the same for both transitions.
D. any of the above, depending on circumstances.
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Crystal lattices
• A crystal lattice is a repeating pattern of
mathematical points. Figure 42.11 (below)
shows some common types of lattices.
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Crystal lattices and structures
• Follow the text discussion of
crystal lattices and structures
using Figures 42.12 (top) and
42.13 (bottom). Figure 42.14
(below) shows diamond.
• Follow Example 42.4.
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Types of crystals
• Follow the text discussion of the types of crystals.
• Figure 42.15 (left) shows a metallic solid, and Figure
42.16 (right) shows an edge dislocation in two
dimensions.
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Energy bands
• Follow the text analysis,
using Figures 42.18
(right) and 42.19 (below).
• Follow Example 42.5.
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Q42.4
At absolute zero (T = 0 K), what is the difference between
a semiconductor and an insulator?
A. The conduction band is empty in a semiconductor but
partially filled in an insulator.
B. The conduction band is partially filled in a
semiconductor but empty in an insulator.
C. The energy gap between the valence and conduction
bands is large in a semiconductor but small in an insulator.
D. The energy gap between the valence and conduction
bands is small in a semiconductor but large in an insulator.
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A42.4
At absolute zero (T = 0 K), what is the difference between
a semiconductor and an insulator?
A. The conduction band is empty in a semiconductor but
partially filled in an insulator.
B. The conduction band is partially filled in a
semiconductor but empty in an insulator.
C. The energy gap between the valence and conduction
bands is large in a semiconductor but small in an insulator.
D. The energy gap between the valence and conduction
bands is small in a semiconductor but large in an insulator.
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Free-electron model of metals
• The free-electron model assumes that electrons are
completely free inside the metal, but that there are
infinite potential-energy barriers at the surface.
• The density of states, dn/dE, is the number of states per
unit energy range.
• Follow the text analysis using Figures 42.20 (left) and
42.21 (right).
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Fermi-Dirac distribution
• The Fermi-Dirac
distribution f(E) is the
probably that a state with
energy E is occupied by
an electron.
• Follow the analysis of the
Fermi-Dirac distribution
in the text, using Figures
42.22 (top) and 42.23
(bottom).
• Follow Example 42.6.
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Electron concentration and free-electron energy
• Follow the text analysis of electron
concentration and Fermi energy.
• Follow Example 42.7 on the Fermi energy in
copper.
• Follow the text analysis of the average freeelectron energy.
• Follow Example 42.8 which compares a freeelectron gas with an ideal gas.
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Q42.5
If you increase the temperature of a block of copper from 300 K
to 600 K, what happens to the average kinetic energy of the
electrons in the conduction band? (Copper remains a solid at
these temperatures.)
A. The average kinetic energy increases by a factor of 4.
B. The average kinetic energy increases by a factor of 2.
C. The average kinetic energy increases by a factor of
2.
D. The average kinetic energy changes by only a small factor.
Copyright © 2012 Pearson Education Inc.
A42.5
If you increase the temperature of a block of copper from 300 K
to 600 K, what happens to the average kinetic energy of the
electrons in the conduction band? (Copper remains a solid at
these temperatures.)
A. The average kinetic energy increases by a factor of 4.
B. The average kinetic energy increases by a factor of 2.
C. The average kinetic energy increases by a factor of 2 .
D. The average kinetic energy changes by only a small factor.
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Semiconductors
• A semiconductor has an electrical resistivity that
is intermediate between those of good conductors
and good insulators.
• Follow Example 42.9 using Figure 42.24 below.
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Q42.6
How would you expect the electric conductivity of a
semiconductor to vary with increasing temperature?
A. It should increase, because more electrons are thermally
excited from the valence band into the conduction band.
B. It should increase, because more electrons are removed
from their parent atoms and added to the valence band.
C. It should decrease, because the added thermal energy
breaks apart correlated electron pairs.
D. It should decrease, because the atoms in the crystal will
vibrate more and thus block the flow of electrons.
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A42.6
How would you expect the electric conductivity of a
semiconductor to vary with increasing temperature?
A. It should increase, because more electrons are thermally
excited from the valence band into the conduction band.
B. It should increase, because more electrons are removed
from their parent atoms and added to the valence band.
C. It should decrease, because the added thermal energy
breaks apart correlated electron pairs.
D. It should decrease, because the atoms in the crystal will
vibrate more and thus block the flow of electrons.
Copyright © 2012 Pearson Education Inc.
Holes
• A hole is a vacancy in a
semiconductor.
• A hole in the valence band
behaves like a positively
charged particle.
• Figure 42.25 at the right
shows the motions of
electrons in the conduction
band and holes in the
valence band with an
applied electric field.
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Impurities
• Doping is the deliberate addition of impurity
elements.
• In an n-type semiconductor, the conductivity is
due mostly to negative charge (electron) motion.
• In a p-type semiconductor, the conductivity is due
mostly to positive charge (hole) motion.
• Follow the text analysis of impurities.
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n-type and p-type semiconductors
• Figure 42.26 (left) shows an n-type semiconductor, and
Figure 42.27 (right) shows a p-type semiconductor.
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Photocell
• A photocell is a simple semiconductor device.
• See Figure 42.28 below.
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The p-n junction
• A p-n junction is the boundary in a semiconductor
between a region containing p-type impurities and
another region containing n-type impurities.
• Figure 42.29 below shows the behavior of a
semiconductor p-n junction in a circuit.
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Currents through a p-n junction
• Follow the text analysis of currents through a p-n
junction.
• Figure 42.30 below shows a p-n junction in equilibrium.
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Forward and reverse bias at a p-n junction
• Figure 42.31 (left) shows a p-n junction under forwardbias conditions.
• Figure 42.32 (right) shows a p-n junction under reversebias conditions.
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Transistors
• Follow the analysis of transistors in the text.
• Figure 42.33 (left) shows a p-n-p transistor in a circuit.
• Figure 42.34 (right) shows a common-emitter circuit.
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Integrated circuits
• Follow the text discussion of integrated circuits.
• Figure 42.35 (left) shows a field-effect transistor.
• Figure 42.36 (right) shows an actual integrated
circuit chip.
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Superconductivity
• Follow the text summary of BCS theory
and superconductivity.
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