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Exam Grading
Lecture 10
• I try to “clean up” scan-sheets as best I can
• Follow directions: mark what test you have!
• If the class average is lower than about a B-,
I adjust the grades upward.
• Exam 1 median & average: 191 (B-)
Matter and its
interactions with light.
Hmwk 3 opens this
week, due 3/15 (Th).
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5th Most Difficult Question
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4th Most Difficult Question
What direction does the Sun set in Amherst during
summertime?
1. Southwest (tied with 3)
2. Due west
3. Northwest (tied with 3)
4. Due East
A lunar eclipse would coincide with a
1. Low tide.
2. Spring tide.
3. Neap tide. (favorite ~ 47%)
4. Ultra Tide®
Part 2: 39 57
Part 2: 36 64
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3rd
Most Difficult Question
You have been drugged and kidnapped….You notice a
star our of the cell window and through the night it
traces the following pattern on the sky: You must be looking…
1. north, from somewhere in the Northern hemisphere
(favorite, ~ 40%)
2. north, from somewhere in the Southern hemisphere
3. south, from somewhere in the Southern hemisphere
4. south from somewhere in the Northern hemisphere
Part 2: 32 48
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2nd
Most Difficult Question
From a city at latitude 40 degrees, the highest
above the horizon that the Sun can ever get in
the sky is
1. 23.5 degrees
2. 50 degrees (favorite choice 35%)
3. 66.5 degrees
4. 73.5 degrees
5. 90 degrees
Part 2: 28 47
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Matter and Light
Most Difficult Question
Saturn’s mass is about 100 times larger than the
mass of the Earth. What is the ratio of the
gravitational force of the Earth on Saturn to
the gravitational force of Saturn on the Earth?
1. 10000 to 1
2. 100 to 1
3. 1 to 1
4. 1 to 100 (favorite choice ~ 35%)
5. 1 to 10000
• What is matter made of?
– Nucleus: massive, p and n, p determines what the atom is
– Electrons: stuck in discrete energy orbitals, equal # to protons
• Temperature:
– How fast things are moving
– What phase matter is in
– What kind of light comes out
• Electrons’ orbitals:
–
–
–
–
Changes in electrons’ states makes or absorbs light (EM energy)
Quantized: only specific energies!
Each atom has it’s own specific lines
Ionization: electrons escape for ANY energies above the minimum
• Molecules:
– Overlapped orbitals hold them together
– Stretching, vibrating and motion of bonds can produce lines too.
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PRS: Suppose an atomic nucleus contains
64 protons+neutrons. About how many
times bigger in diameter would it be than a
hydrogen nucleus containing just 1 proton?
(1)
(2)
(3)
(4)
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64
V ∝ r3
V ↑ 64 x
r ↑ 3 64 x = 4 x
Answer: 1
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Isotopes
Same
number
of
protons
but
differing
numbers
of
neutrons.
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Molecules
• Atoms can join together by sharing electrons
to make molecules. These chemical bonds
hold the molecules together.
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Phases of Matter
• Matter can take many forms depending mostly
on its temperature or internal energy.
• As temperature climbs, the strength of bonding
of the particles to each other grows weaker,
while the kinetic energy grows larger.
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Temperature Scales
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• Fahrenheit scale
– zero at freezing
saltwater
– 100 at body
(whose?!)
temperature
• Kelvin scale
– zero at zero energy
– step size same as
Celsius
− only scale with temperature proportional to kinetic
energy: T goes as KE = ½ mv2, v=avg. velocity of particles
Electron orbitals are quantized
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PRS: The LN2 in the beaker is boiling at
77K, that’s about 1/3 of room temperature
(303 K). The molecules of nitrogen in the
air are therefore moving with velocities
____ on average compared to the beaker.
(1)
(2)
(3)
(4)
27 x lower
9 x lower
3 x lower
√3 x lower
(5) 27 x higher
(6) 9 x higher
(7) 3 x higher
(8) √3 x higher
Answer: 8
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Atomic Excitation and Ionization
• As the energy increases, electrons go into
more energetic orbits.
• Electron orbitals depend on
the energy of the electron
• Only certain energies work—
complete “wavelengths”
• The energies depend on what
kind of atom it is (nucleus)
and how many electrons are
present. This is important!
• At high enough energies the electrons are
stripped off -- Plasma.
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Analyzing Spectra: Gratings and prisms
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Spectra
• Get out your diffraction grating.
• The diffraction grating is like a prism--it
splits up light into its component
frequencies when it goes through the
grating at an angle.
• You can see a source of light directly but
off to its left and right you will see all of its
component colors.
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Atoms in a Gas or Plasma
Atomic Spectra
• When we look at light generated by isolated
atoms, the appearance of the spectrum is
different.
• Use your diffraction grating to compare
several sources of light.
• Note the individual colors that are present—
these are called “spectral lines
• Isolated atoms or molecules absorb and
emit at specific wavelengths characterized
by the particular type of atom or molecule.
• The only possible wavelengths are those
that correspond to the energy changes for
electrons shifting between orbitals.
• The wavelengths of spectral lines can tell us
what the composition of a gas is.
Photon Energies and
Atomic Structure
• Particles of light
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(photons) have
energies inversely
proportional to their
wavelength.
• The colors emitted by
an atom are related to
the energies of atomic
orbits.
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Interactions of Light and Matter
• Atoms can absorb a photon, raising an
electron to a higher energy orbit.
• Atoms emit a photon when an electron
drops to a lower energy orbit.
• If an atom absorbs enough energy, the
electron flies off (ionization).
What you are seeing with the Hydrogen Tube:
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Hydrogen Spectrum
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