Download Answers - Physics and Astronomy

Astronomy 101.003
Hour Exam 3
April 12, 2011
QUESTION 1: Some recent measurements of the expansion rate of the universe suggest a problem with
our old ideas about how the universe should be expanding. What is the problem?
a. The measurements suggest that the universe may be shrinking rather than expanding.
b. The measurements indicate that the universe is at least 30 billion years old, meaning that more than
10 billion years passed between the Big Bang and the formation of the first stars and galaxies.
c. The measurements suggest that the universe may not be expanding at all.
d. The data suggest that the expansion rate varies widely in different parts of the universe.
e. The measurements suggest that the expansion may actually be accelerating, rather than
slowing under the influence of gravity.
QUESTION 2: The cosmic microwave radiation has a spectrum most like that of:
a) Thermal radiation from a hot body.
b) Thermal radiation from a cold body.
c) An emission nebula.
d) The Sun.
e) A quasar.
QUESTION 3: What is the origin of the Cosmic Microwave Background:
a) Light remaining from the Big Bang.
b) Dirt in microwave antennae.
c) Quasars.
d) Pulsars.
e) Blackbody radiation from distant galaxy superclusters.
QUESTION 4: If the average matter density of the universe were less than the critical density, the
universe:
a) Would expand forever.
b) Would eventually collapse.
c) Would expand faster and faster.
d) Would be of constant size.
e) Would violate the law of energy conservation.
QUESTION 5: What two conditions must be present for an object to emit a black body spectrum?
a) It must be hot and dense.
b) It must be hot and opaque.
c) It must be a gas and be hot.
d) It must be a solid and have a uniform temperature.
e) It must be opaque and have a single temperature at any location inside it.
QUESTION 6: Two samples of Helium are heated, one to 3000K and the other to 3200K. What do you
know you will observe when you compare the two spectra?
a) The hotter sample will be brighter.
b) The hotter sample will be dimmer.
c) The hotter sample will be redder.
d) The hotter sample will be bluer.
e) Both samples will show mostly the same absorption lines
QUESTION 7: What assumption did Max Planck make that allowed him to fit the black body spectral
shape?
a) The radiating body must be black.
b) The energy of light is quantized.
c) The emitting object must be hotter than its surroundings.
d) The emitting object must be in an inertial frame of reference.
e) The emitting object must not contain free electrons.
QUESTION 8: How is the energy of a photon related to the frequency of the electromagnetic wave?
a) The energy is proportional to the frequency.
b) The energy is proportional to the square of the frequency.
c) The energy is inversely proportional to the frequency.
d) The energy is inversely proportional to the square of the frequency.
e) The energy is not related to the frequency.
QUESTION 9: How did Einstein’s explanation of the photoelectric effect confirm the quantum nature of
light?
a) It correctly described the scattering of photons and electrons.
b) It explained the observations with the assumption that light energy is quantized.
c) It correctly explained the presence of spectral lines in emission spectra.
d) It correctly incorporated relativity with Maxwell’s electromagnetic theory.
e) Einstein’s explanation of the photoelectric effect had nothing to do with the quantum nature of
light.
QUESTION 10: What is the central assumption of Bohr’s model of the hydrogen atom?
a) Photon energies are quantized.
b) Only certain electron states, with definite quantized energies, are allowed.
c) The electrons orbit the nucleus with elliptical orbits.
d) A hydrogen atom emits photons whenever it absorbs energy.
e) Bohr did not formulate a model of the hydrogen atom.
QUESTION 11: How does Bohr’s model explain the spectral shape of the cosmic microwave radiation?
a) The electron energy levels in atoms are quantized.
b) The emitted photon energy equals the energy difference between the initial and final states.
c) Atoms can only absorb photons with energies equal to the energy difference between the initial
and final states.
d) All of the above.
e) The Bohr model says nothing about the spectral shape of the cosmic microwave radiation.
QUESTION 12: What is the difference between the scattering of photons from free electrons and from
bound (in atoms) electrons?
a) Bound electrons are transparent to most photon energies while free electrons are opaque to
almost all photons.
b) Photons only scatter from bound electrons.
c) Photons do not scatter from free electrons.
d) Photons never scatter from bound electrons.
e) There is no difference between photon scattering from free and bound electrons.
QUESTION 13: Which of the following are properties of the cosmic microwave radiation?
a) It is a blackbody spectrum.
b) It has the same shape in all directions.
c) It has the same intensity in all directions.
d) All of the above.
e) None of the above.
QUESTION 14: The Cosmic Microwave Radiation is observed to come from:
a) The center of our galaxy.
b) The center of the universe.
c) A radio antenna in New Jersey.
d) Equally in all directions.
e) Primarily in the direction of the Earth’s motion.
QUESTION 15: How does the Bohr model explain the stability of the hydrogen atom?
a) The ground state cannot emit a photon, as there is no lower energy state.
b) The quantization of light energy requires stability of the atom.
c) The hydrogen atom is not a black body
d) The electrons in the atom do not scatter photons.
e) The statement is false: the Bohr theory does not explain the stability of the hydrogen atom.
QUESTION 16: The Earth is bombarded by neutrinos produced in the Sun. Why don’t we notice them?
a) There aren’t enough to notice.
b) Because they are so light.
c) Because they pass through virtually everything on Earth without interacting.
d) Because we just got used to them.
e) The light from the Sun overwhelms the neutrinos.
QUESTION 17: The universe became transparent when electrons and protons formed hydrogen atoms
because:
a) Atoms only absorb certain wavelengths of light
b) The density of matter was so low
c) Nucleosynthesis finally ceased
d) There is less chance that a photon will hit an atom than an electron
e) The statement is not correct: the universe was always transparent.
QUESTION 18: What is the average temperature of the universe now?
a) It is not well defined: it depends on which direction you are looking.
b) 300 K.
c) Absolute zero.
d) 2.7 K
e) 0 ° C
QUESTION 19: The photons in the cosmic microwave radiation that are detected now have:
a) Low energies because they were produced with low energies.
b) Low energies because they have experienced a cosmological red shift.
c) Low energies because they lost energy in collisions with electrons.
d) High energies because they come from the early moments of the universe.
e) High energies because they were produced in high-energy collisions.
QUESTION 20: The Sun produces energy by what kind of reaction?
a) Burning (a chemical reaction).
b) Fusion.
c) Fission.
d) Dark energy.
e) Chemical reduction.
QUESTION 21: A fusion reaction results in:
a) The building up of heavier nuclei from light nuclei.
b) The breaking apart of heavy nuclei into lighter ones.
c) The annihilation of nuclei.
d) The annihilation of antimatter.
e) Production of carbon dioxide.
QUESTION 22: The most tightly bound nucleus is:
a) Hydrogen.
b) Helium.
c) Carbon.
d) Iron.
e) Uranium.
QUESTION 23: Fusion reactions release energy by:
a) Converting mass into energy.
b) Reducing nuclear binding energies.
c) Creating simpler forms of matter.
d) Producing neutrinos.
e) The weak nuclear force.
QUESTION 24: The production of energy on the Sun results in:
a) The Sun getting less massive.
b) The Sun getting more massive.
c) The Sun getting hotter.
d) The Sun getting cooler.
e) The Sun rotating faster.
QUESTION 25: Fusion occurs only near the Sun’s center because:
a) Only near the center is there enough hydrogen that is not mixed with other elements.
b) Only near the center is it opaque enough for fusion.
c) Heat is transferred down to the center to ignite fusion.
d) Only near the center are the temperature and pressure high enough to sustain fusion.
e) The statement is false: fusion occurs throughout the Sun.
QUESTION 26: The mercury found on Earth came from:
a) A supernova.
b) A nova.
c) Nuclear fusion in a star.
d) Nuclear fission in a star.
e) Chemical reactions after the Earth was formed.
QUESTION 27: Which of the following was not produced in significant quantities in the Big Bang?
a) Hydrogen.
b) Deuterium.
c) Helium.
d) Carbon.
e) Photons.
QUESTION 28: What force is responsible for overcoming the electric repulsion between the protons in a
nucleus?
a) Gravity.
b) Electric force.
c) Magnetic force.
d) Strong nuclear force.
e) Weak nuclear force.
QUESTION 29: When an iron nucleus is split into two lighter nuclei:
a) Mass is lost.
b) Energy is absorbed.
c) Electric charge is lost.
d) Fusion energy is produced.
e) Fission energy is produced.
QUESTION 30: Why does nuclear fusion of helium require higher temperatures than fusion of
hydrogen?
a) Helium has lower density.
b) To overcome the stronger electrostatic repulsion of the helium nuclei.
c) To strip the electrons from the atoms.
d) Because of the neutrons in helium.
e) The statement is false – helium fuses at the same temperature as hydrogen.
QUESTION 31: Neutrinos have which of the following properties?
a) They are charged.
b) Their mass equals that of an electron.
c) They can pass through a light-year of matter without interacting.
d) They are constituents of nuclei.
e) Their mass approximately equals that of a proton.
QUESTION 32: Which force is responsible for the decay of the neutron (n → p e- ν )?
a) The electromagnetic force
b) The strong nuclear force
c) The weak nuclear force
€
d) Gravity
e) Dark energy
QUESTION 33: Which of the following is a property of the strong nuclear force?
a) It falls off as 1/r2, where r is the distance between two protons.
b) It is either attractive or repulsive, depending on whether it is acting on protons or neutrons.
c) It is equal in strength to the electromagnetic force.
d) All of the above.
e) None of the above.
QUESTION 34: When studying nuclear beta decay, scientists found that energy is not conserved. As a
consequence of this:
a) Pauli postulate an unseen light, neutral particle that carried off the missing energy.
b) Physicists abandoned the Law of Conservation of Energy.
c) Physicists realized that the measurements must be wrong.
d) Physicists used E = m c2 to account for the missing energy.
e) Physicists accepted this as a small error in the powerful Law of Conservation of Energy.
QUESTION 35: When 2 protons are fused with 2 neutrons to form He4:
a) The mass of He4 must equal the combined mass of the protons and neutrons.
b) The mass of He4 is less than the combined mass of the protons and neutrons.
c) The mass of He4 is greater than the combined mass of the protons and neutrons.
d) Combining 2 protons and 2 neutrons would not produce He4.
e) Combining 2 protons and 2 neutrons to form He4 would violate Conservation of Energy.
QUESTION 36: What keeps the Sun from collapsing under its own weight?
a) Dark energy.
b) The strong nuclear force.
c) Neutrinos produced at the center of the Sun.
d) The radiation pressure produced at the center of the Sun by nuclear fusion.
e) The Sun is actually collapsing – it’s just doing so very slowly.
QUESTION 37: How do we know that the Sun was formed from exploded remnants of previous stars?
a) Because it is a main-sequence star.
b) Because it contains some oxygen, carbon, and nitrogen.
c) It’s too young.
d) Because it is producing Helium.
e) The statement is false – the Sun is not made from exploded remnants of previous stars.
QUESTION 38: Nearly all of the space in an atom is taken up by:
a) The nucleus.
b) The electrons.
c) The protons.
d) Empty space.
e) Electron bonds.
QUESTION 39: Hubble’s constant measures:
a) The density of galaxies in the universe.
b) The luminosity of distant galaxies.
c) The reddening of light from gravitational fields.
d) The radial velocity of a galaxy.
e) The rate of expansion of the universe.
QUESTION 40: What key event happened during the decoupling epoch?
a) Pairs of neutrons and protons were created.
b) Electrons and positrons were created.
c) Expansion cooled the universe enough that protons could capture electrons in orbit.
d) Dark energy accelerated the cosmos on to infinity.
e) The universe underwent a brief period of very rapid expansion.
QUESTION 41: Why does the Big Bang theory predict that the cosmic background radiation
should have a perfect thermal radiation spectrum?
a. The spectrum of pure hydrogen is a perfect thermal radiation spectrum.
b.
c.
d.
e.
7
It doesn't predict that the cosmic background radiation should have a perfect thermal
radiation spectrum.
The light from all the stars and gas in the sky averaged over the entire universe is a
perfect thermal radiation spectrum.
The spectrum of 75% hydrogen and 25% helium is a perfect thermal radiation
spectrum.
The background radiation came from the heat of the early universe, a black
body.
8
QUESTION 42: Which of the following is not evidence for the Big Bang Theory?
a) The presence of dark energy.
b) The cosmic microwave background radiation.
c) The abundance of helium in the universe
d) Hubble’s Law
e) The abundance of lithium in the universe.
QUESTION 43: What will be the fate of the Sun when it uses up all of its hydrogen?
a) It will blow off its outer layer and become a white dwarf.
b) It will undergo a nova explosion.
c) It will undergo a supernova explosion and become a neutron star.
d) It will undergo a supernova explosion and become a black hole.
e) It will just stop producing energy and gradually cool off.
QUESTION 44: We know that the Big Bang theory is true because:
a) It explains the expansion of the universe.
b) It explains the existence and properties of the cosmic microwave radiation.
c) It explains the abundance of deuterium, He3, He4 and Li7.
d) All of the above.
e) We do not know that it is true – it is a scientific theory, which can’t be proven
true.
QUESTION 45: How did the Bohr model explain the existence of spectral lines?
a) Since light is quantized, only certain frequencies are allowed.
b) Light is emitted only when an electron falls from one quantized energy level to a
lower quantized energy level.
c) Light is emitted only when an electron jumps from one quantized energy level to a
higher quantized energy level.
d) Spectral lines are emitted whenever the source is hot.
e) The Bohr model does not explain the existence of spectral lines.
QUESTION 46: Most of the photons in the universe are:
a) The cosmic microwave radiation.
b) Emitted by stars.
c) Emitted by quasars.
d) High-energy gamma rays in the cosmic rays that bombard the Earth.
e) Concentrated at the center of the universe.
QUESTION 47: Most of the matter in the universe is:
a) Hydrogen and Helium.
b) Contained in galactic disks.
c) Contained in dark-matter halos surrounding galaxies.
d) Small dark stars like brown dwarfs and MACHOS.
e) Exotic non-baryonic dark matter such as WIMPS.
QUESTION 48: As the universe expanded after the Big Bang, it:
a) Got cooler and denser.
b) Got hotter and denser.
c) Got cooler and less dense.
d) Got hotter and less dense.
e) Got cooler but its density did not change.
QUESTION 49: What is deuterium?
a) An isotope of hydrogen.
b) An isotope of helium.
9
c) An isotope of Lithium.
d) A weakly interacting particle believed to be responsible for most of dark matter.
e) A particle responsible for the strong nuclear force.
10
QUESTION 50: The universal abundance of deuterium, He , He , and Li fixes what quantity?
a) The baryon-to-photon ratio.
b) The fraction of matter that is non-baryonic.
c) The dark matter-to-dark energy ratio.
d) The value of the Hubble constant.
e) The neutron-to-proton mass ratio.
QUESTION 51: The cosmic microwave radiation (CMB) was produced at approximately what
temperature?
a) 2.7 K.
b) 300 K.
c) 0° C.
d) 40° C.
e) 3000 K.
QUESTION 52 The large-scale structure of the universe (walls and voids) results from what
interaction?
a) Gravity.
b) Radiation pressure.
c) Strong nuclear force.
d) Electromagnetic force.
e) Weak nuclear force.
QUESTION 53: Olber’s Paradox is related to:
a) Why is (most of) the sky dark?
b) How can the Universe expand when there is nothing to expand into?
c) How could the Universe have once fit into a single point?
d) How can the speed of light not depend on the velocity of the source?
e) Why does light bend in a gravitational field?
QUESTION 54: The most common element in the Universe is:
a) Hydrogen.
b) Helium.
c) Carbon.
d) Iron.
e) Silicon.
QUESTION 55: Most of the helium in the universe was produced by:
a) Stars.
b) The Big Bang.
c) Galaxies.
d) Black Holes.
e) Quasars.
QUESTION 56: Why do we find no plutonium naturally occurring on Earth?
a) Because plutonium is not produced in stars.
b) Because plutonium is not naturally produced anywhere in the universe.
c) Because it has all sunk to the Earth’s core.
d) Because all of the plutonium that existed when the Earth formed has decayed
away.
e) There is plutonium naturally occurring on Earth.
QUESTION 57: How did the sun get hot after it was first formed?
a) Nuclear fusion.
b) Nuclear fission.
c) Heating due to gravitational collapse.
3
4
7
11
d) Heat left over from the Big Bang.
e) Combustion.
12
QUESTION 58: How did physicists learn that the atom has a small (~10 m) heavy nucleus
with the rest of the atom’s size (~10-10 m) made up of empty space with electrons in orbit?
a) When Bohr’s model explained the spectral lines in hydrogen.
b) When Planck studied the black body spectrum.
c) When Rutherford studied alpha-particle scattering from a gold foil.
d) When Einstein explained the photoelectric effect.
e) When Maxwell formulated the theory of electromagnetism.
QUESTION 59: The production of certain light elements early in the life of the universe is
called:
a) Baryogenesis.
b) Decoupling.
c) Big Bang nucleosynthesis.
d) Big Bang WIMP production.
e) Recombination.
QUESTION 60: Evidence that the cosmic background radiation really is the remnant of a Big
Bang comes from predicting characteristics of remnant radiation from the Big Bang and
comparing these predictions with observations. Four of the five statements below are real.
Which one is fictitious?
a) The cosmic background radiation is expected to look essentially the same in all
directions, and it does.
b) The cosmic background radiation is expected to have tiny temperature fluctuations at
the level of about 1 part in 100,000. Such fluctuations were found in recent
measurements.
c) The cosmic background radiation is expected to have a perfect thermal spectrum, and
observations verify this prediction.
d) The cosmic background radiation is expected to have a temperature just a few degrees
above absolute zero, and its actual temperature turns out to be about 2.7 K.
e) The cosmic background radiation is expected to contain spectral lines of
hydrogen and helium, and it does.
-15