Download Sample Final #1

Sample Final Exam #1:
Section #1:
Please answer all the questions from #1 to #18. In most cases, your reasoning is much more
important than which answer you choose!
Following in the footsteps of Tycho Brahe and Johannes Kepler, you and a few friends manage
to schmooze a king or a prime minister or someone into granting you an island where you can
build an astronomical observatory...
Your first job is to set up a power source on the island.
Question #1:
Which of the following sources of energy CANNOT be traced back to the sun? (At least, not to
OUR sun?)
a) Human muscle power.
b) Fossil fuels.
c) Geothermal power.
d) Wind power.
e) Hydroelectric power.
f) None of the above. ALL energy available on the Earth comes
ultimately from our sun.
Question #2:
Explain your answer to Question #1. If you chose an energy source, explain where that energy
DOES come from. If you chose “None of the above”, explain how you know that.
You decide to build a coal-fired plant to get your electrical grid up and running. After a brief
building- the-power-grid montage scene, you are zipping across the island in an
electrically-powered tram.
Question #3:
What kind of energy does your tram car definitely have, as it races across the island?
Question #4:
Where did the tram car GET that energy? Trace the energy backwards, explaining where it came
from with each step, until you connect it to the fusion reactions in the sun.
(Remember that,
despite its dismal carbon footprint, you've chosen a coal-burning power plant.)
Question #5:
“Our coal-fired plan emits way to much greenhouse gas,” remarks one of your colleagues. “We
should build a direct fusion-powered plant, instead.”
“That would be incredibly difficult,” warns another. She’s right, of course; humans have not yet
managed to use nuclear fusion as a viable energy source on earth. Why not? What is it about the
process of nuclear fusion that makes it so difficult?
Question #6:
Nonetheless, you decide to go ahead and try to build a fusion plant anyway. Which of the
following would be the best fuel for a fusion reaction? (Hint: the Sun is powered by fusion...)
a) Hydrogen (whose nucleus is a single proton.)
b) Helium-4 (whose nucleus is 2 protons and 2
neutrons.)
c) Carbon-14 (whose nucleus is 6 protons and 8 neutrons.)
d) Iron-56 (whose
nucleus is 26 protons and 28 neutrons.)
e) Uranium-238 (whose nucleus is 92 protons and 146
neutrons.) f) All of the above nuclei would make equally good fusion fuels.
Question #7:
Of the five nuclei listed in Question #6, which one would be the most likely to emit an alpha
particle? Briefly explain why. (It might help to explain what an alpha particle is, and what forces
cause it to be “spit out” of a nucleus...)
Question #8:
Of the five nuclei listed in Question #6, which one would be the most likely to emit a beta
particle? Briefly explain why. (It might help to explain what a beta particle is, and what forces
cause it to be “spit out” of a nucleus...)
Having wired the island for electrical power, you set up your telescopes and begin to observe.
NOTE: Questions #9 - #13 involve the lecture on cosmology, which was given only in the
Mon/Wed 9:30 AM class. Students in any other class time will not be responsible for
cosmology on the final exam. (But I wanted them to think about the questions anyway, for
their own edification and enlightenment, because cosmology is cool!)
Question #9:
When you observe the “milky way” — that diffuse, cloudlike structure that sweeps across the
night sky — you realize that it consists of a vast number of very faint-appearing dots.
Why do
these dots appear so faint?
a) They are very young stars.
b) They are very old stars.
c) They are stars much like the other
stars we see in the night sky, but much further away.
d) They are not stars at all, but are planets.
e) They are not stars at all, but are balls of ether.
f) We still do not know the answer.
Question #10:
You use your telescope to photograph one small region of the night sky, containing several
observable stars.
If you want to use stellar parallax to attempt distance measurements on those stars, when would
be the best time to observe that region of the sky again?
a) As soon as possible.
b) At low tide. (Assuming your first measurement was at high tide.) c)
Several days from now.
d) One month from now.
e) Six months from now.
f) Many years
later.
Question #11:
Name at least one other method you could use to measure the distance to a star. (You do not need
to describe the method in great detail right now, though you can do so later for a long-answer
question if you like!)
Question #12:
Which would be the best way for you to measure how fast a star or other celestial object is
moving toward or away from you?
a) Measure the doppler shift of the sound waves it emits.
b) Measure the doppler shift of the light waves it emits.
c) Bounce a radar signal off the star.
d) Observe how its position on the sky changes over time.
e) Observe the tidal forces on any planets orbiting the star.
f) There is no known way to measure this.
Question #13:
When Edwin Hubble carried out the measurements you just described, he found that:
a) Most galaxies outside the Milky Way are moving toward us.
b) Most galaxies outside the
Milky Way are moving away from us.
c) Galaxies outside the Milky Way are not moving at
all.
d) Most stars outside the Milky Way galaxy have a much shorter lifetime than our sun.
e) Most stars outside the Milky Way galaxy have a much longer lifetime than our sun.
f) AIIIIIEEEEE! Run for your lives! We’re all DOOMED!
Even on your wonderful island observatory, some nights are too cloudy to make any
observations. So on those nights, you and your friends start talking about past discoveries of
science.
Question #14:
Rank these discoveries in order, from the earliest (first discovered) to the most recent.
a) The Sun is powered by nuclear fusion. b) The Earth orbits the sun.
c) Matter is made up of
atoms.
d) The Earth is a sphere.
e) Atoms consist of a nucleus surrounded by electrons.
f) Two negative electrical charges will
repel each other, but a negative and a positive charge will attract.
Question #15:
In one of J.J. Thomson’s cathode-ray experiments, he sent a beam of electrons between two
metal plates, one positively charges, one negatively charged,
Which of the paths shown below
is the best picture of how the electron beam would behave?
Question #16:
What property did this prove that electrons must have?
Question #17:
Instead of an electric field, could Thomson have used a magnetic field to deflect the path of the
electron beam?
a) Yes, as long as the electrons were moving.
b) Yes, as long as electrons have more mass than
protons.
c) Yes, as long as electrons have no charge.
d) No; a magnetic field will only deflect
magnets.
e) No; a magnetic field cannot exert forces on anything.
f) The experiment was
done, once, but the results are still classified.
Question #18:
Name at least one interesting scientific question whose answer is not yet known, and briefly
explain how people might go about investigating it in the future.
Section II: LongAnswerProblems
Choose and answer, on the attached pages, at least two of
the following six problems.
If you wish to answer more than two, you can, but please decide which two will be your “main”
answers, and clearly label the others as “Extra Credit”.
Topic #1: Astronomical Distance Measurements.
One of the most difficult problems in astronomy is the “Cosmic Distance Ladder,” the set of
methods we use to measure the distance to stars outside our solar system, or galaxies outside the
Milky Way.
The two first steps on the distance ladder are Parallax measurements, and Cepheid variable stars.
Explain how each of these methods can be used to find the distance to a star outside our solar
system. (Describe both the measurements an astronomer must make to use the technique, and the
reasoning that leads from there to a measurement of distance.)
Topic #2: Lord Kelvin's Age Measurements.
One of Lord Kelvin's famous claims was that he could accurately measure the age of both the
Earth and the Sun – and for both objects, he calculated ages of roughtly 10 to 20 million years.
Today, we estimate that both the Earth and Sun are much older than this. Explain why Lord
Kelvin's calculations gave such a different answer from our calculations today. (What
discoveries or phenomena did he not yet know about, which would change the calculated ages of
these objects dramatically?)
Topic #3: Structure of an Atom.
In the space of a hundred years or so – say, from the mid-1800s to the mid-1900s – humans went
from being uncertain whether atoms exist at all, to a fairly impressive understanding of the
internal structure of an atom.
Describe two of the experiments or discoveries that led to this
shift in our understanding. (Names you might think of are Brown, Thomson, Curie,
Rutherford...and many others. You can probably get away with focusing on only one experiment,
if you do an impressive enough job of explaining what they discovered and why it was
important!)
Topic #5: Fission and Fusion.
Nuclear fission and nuclear fusion are both processes that can release a tremendous amount of
energy. Choose at least one of these process, and explain in reasonable detail how it works. What
forces are involved in the process? Which of these forces try to “prevent” the reaction from
occuring, and which forces are actually responsible for the release of energy? What type of
nuclei are most likely to undergo the process, and why?
Topic #6: The four fundamental forces.
The four fundamental forces of nature are gravity, the weak nuclear force, the strong nuclear
force, and electromagnetism. Choose at least two of these forces, and describe them in some
detail. What type of particles do they act on? What are their limitations, and what, if anything,
can each force do that none of the others can do? What type of behavior does this lead to for
subatomic particles and the larger objects that are made up of them?
Topic #7: Roll your own.
As always, I invite you to invent your own question. Pick any topic we have discussed this
semester (and which you have not already written about in the Long-Answer sections of Midterm
#1 or #2) and come up with a question you would like to answer.
If you choose this option,
please talk to me before beginning to write, to make sure that we both agree on a good question.