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Exam 3 Answers 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.
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