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Quiz 5
Physics 55: Introduction to Astronomy
Professor Henry Greenside
Friday, November 4, 2005
This quiz is closed book. You can use a scientific calculator but no other electronic device. If you have any
questions during the quiz, please do not hesitate to ask me.
The following formulas and data may be useful:
E = mc2 ,
C = 2πr,
c ≈ 3.0 × 108
m
,
s
the sum of mvr is a conserved quantity for isolated systems .
1 u ≈ 1.7 × 10−27 kg,
1 MeV = 1.6 × 10−13 J.
True or False Questions (1 point each)
Please circle “T” for true or “F” for false to indicate the truth of the following statements.
1. T / F The physical unit for the Sun’s luminosity is watts per square meter (W/m2 ).
2. T / F The Sun is a young star, near the beginning of its life.
3. T / F Although the Sun does not generate energy today by gravitational contraction, this energygeneration mechanism was important when the Sun was forming.
4. T / F The annihilation of matter with antimatter (electrons with positrons) to produce gamma rays
is the sole source of energy production in the core of the Sun.
5. T / F Although the Sun’s luminosity has increased by about 30% since the Sun was born out of
a nebula, the Earth’s temperature has been relatively constant during this time because biological
organisms like plants have been able to alter the composition of the atmosphere to compensate for the
increased amount of energy received by the Sun.
Multiple Choice Questions (4 points each)
Circle the letter that best answers each of the following questions.
1. With your right hand, you swing a baseball of mass 0.15 kg around your head in a circle on a string of
length 1 m with a speed of 20 m/s. If with your left hand, you now pull the string through your right
hand until the length of the string is decreased to 0.5 m, what is the new speed of the baseball as you
swing it in a circle around your head?
(a) Not enough information to get an answer.
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(b) 10 m/s.
(c) 20 m/s.
(d) 30 m/s.
(e) 40 m/s.
2. What two forces are balanced in what is called “gravitational equilibrium”?
(a) the electromagnetic force and gravity.
(b) the electromagnetic force and the strong force.
(c) gravity and the strong force.
(d) outward pressure and gravity
(e) outward pressure and kinetic energy.
3. The cloud chamber that I showed the class at the beginning of a recent lecture is a valuable way to
(a) to visualize the formation of a proton-antiproton pair.
(b) to visualize the radioactive decay of an atomic nucleus.
(c) to visualize the de-excitation of an atom from an excited state to its ground state, with the
emission of a photon.
(d) to visualize the path of fast neutral particles like neutrons and neutrinos.
(e) demonstrate conservation of energy.
4. In the nuclear reaction
2
1H
+ 31 H → 42 He + 10 n,
the scientific principle that requires that the sum of the subscripts on the left side equal the sum of
the subscripts on the right side is called
(a) conservation of energy.
(b) conservation of nucleon number (also called baryon number).
(c) conservation of charge.
(d) conservation of angular momentum.
(e) conservation of momentum.
5. In the nuclear reaction
2
1H
+ 31 H → 42 He + 10 n,
the scientific principle that requires that the sum of the superscripts on the left side equal the sum of
the superscripts on the right side is called
(a) conservation of energy.
(b) conservation of nucleon number (also called baryon number).
(c) conservation of charge.
(d) conservation of angular momentum.
(e) conservation of momentum.
6. Circle the two nuclei in the list
4
2 He,
7
3 Li,
7
4 Be,
10
5B
that could most easily undego a nuclear reaction in the core of the Sun,
2
7. What do sunspots, solar prominences, and solar flares all have in common?
(a) The are all strongly influenced by magnetic fields on the Sun.
(b) They all have the same temperature.
(c) They are all shaped by the solar wind.
(d) They all occur only in the Sun’s photosphere.
8. Suppose that somehow the core of the Sun suddenly became hotter so that the rate of nuclear fusion
increased. What would happen next?
(a) The rate of fusion would skyrocket and the Sun would explode, turing eventually into a white
dwarf.
(b) The temperature would continue to increase, causing higher fusion rates.
(c) The core would expand, causing the pressure and temperature to drop, and the rate of fusion to
decrase.
(d) The Sun would quickly run out of hydrogen to fuse, causing the temperature to return to its
original value.
9. Why does the Sun emit neutrinos?
(a) Solar flares create neutrinos as the magnetic field lines snap.
(b) The proton-proton chain in the Sun’s core creates neutrinos as a byproduct.
(c) Convection of the plasma releases neutrinos, which then random walk through the radiation zone.
(d) The Sun does not emit neutrinos.
Open Questions
For the following, you need to show how you got your answer. You do not have to write in complete sentences
but you do have to provide enough information and write clearly enough so that your logic and answer are
easily understood.
1. (14 points) Draw a schematic picture of the the six layers of our Sun, indicating with arrows the
names of the parts and their approximate absolute temperatures (in kelvin). Also indicate with one or
more asterisks * those parts of the Sun where fusion reactions occur.
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2. (8 points) A highly promising way to solve the world’s energy crisis in an environmentally benign
way is to create a hot star-like plasma on Earth in which the following fusion reaction takes place
2
1H
+ 31 H → 42 He + n,
in which a deuterium nucleus 21 H and tritium nucleus 31 H collide and react to produce a helium nucleus 42 He, a neutron n, and a lot of energy.
Given that the masses of the particles 21 H, 31 H, 42 He, and n are respectively 2.014102 u, 3.016049 u,
4.002603 u, and 1.008665 u, what is the amount of energy produced by this reaction to two significant
digits and in units of millions of electron volts (MeV)?
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