Physics 10 Spring 2017 Final Exam Name:

Transcription

1 Physics 10 Spring 2017 Final Exam Name: (c) Randall Munroe,

2 While waiting around for an interesting-looking cat or turtle or bird or something to help illustrate physics principles, you stop to help a stranger with a broken-down DeLorian. The flux capacitor and the Mr. Fusion are both fine, he explains. It's just the regular car battery that's dead. Question #1: To recharge the batteries of the stranger's Delorean, you set up an array of solar panels. The solar panels convert sunlight to electrical energy...but where does the energy in sunlight originate? a) The sun converts chemical energy to electrical energy, then radiates it through space as electromagnetic waves. b) The sun converts chemical energy to thermal energy, then radiates it through space as sound waves. c) The sun converts nuclear energy to electrical energy, which reflects off the moon in order to reach the Earth. d) The sun converts thermal energy to gravitational energy, which affects the tides on Earth and is thus. e) The sun converts nuclear energy to thermal energy, then radiates it through space as electromagnetic waves. Question #2: You are aware that the Sun's energy source will not last forever. When the Sun reaches the end of its lifetime, what will happen? a) Its energy flow will reverse, and thermal energy will flow back into the Sun. b) The Sun will gradually radiate away leftover energy and cool until it no longer glows with visible frequencies. c) The Sun will become an ordinary ball of rock. d) The Sun will explode in a Supernova explosion. e) The Sun will expand until it becomes a galaxy much like the Milky Way. If the sun were a much more massive star than it is, would your answer to this question have been different? If so, which answer would you choose?

3 Question #3: Most of the energy used by humans on Earth comes from the Sun. Name at least one type of energy that does NOT originate from the Sun at least, not from OUR sun and explain briefly where it does come from. Grateful for your help in repairing his time machine, the stranger offers take you to visit the great scientists of other eras. Your first stop is the early 1600s, to pay a visit to Galileo. (Disclaimer: In real life, Delorians probably cannot travel through time, even if the owner wears goggles and talks really fast about flux capacitors. Just sayin', since this is a physics exam.) Question #4: You find Galileo trying an early version of his famous ball-drop experiment: he drops a feather and a lead weight at the same moment. Naturally, the feather takes much longer to fall. How would this result be different if he did the experiment on the moon? a) Neither object would fall; they would float in place. b) The lead weight would fall slower than it did on Earth. The feather would fall at the same speed as the weight. c) The lead weight would fall slower than it did on Earth. The feather would fall slower than the weight. d) The lead weight would fall just as fast as it did on Earth. The feather would fall at the same speed as the weight. e) The lead weight would fall just as fast as it did on Earth. The feather would fall faster than the weight.

4 Question #5: A few decades later, Newton would continue Galileo's work...and he would have more to say about that fallingball experiment. Which of these was one of Newton's most important discoveries? a) The force that causes the lead weight to accelerate toward Earth is the same force that keeps the moon in orbit around the earth. b) The lead weight falls to earth because the weight is positively charged, and earth is negatively charged. c) The lead weight falls to earth because the natural state of any solid object is to be as close to the center of the universe as possible. d) The lead weight falls to earth because of the force of air resistance. e) In the southern hemisphere, the weight would actually accelerate away from the Earth. Question #6: You visit Michael Faraday, who is in his lab, happily messing with compasses and magnetic fields. Which of the following statements about magnetic fields are true? (Circle all that apply; there can be more than one true answer.) a) Magnetic fields cannot change the direction a particle is moving, but they can speed it up or slow it down. b) Magnetic fields can be created by any moving charged particle. c) Magnetic fields exert force on any object with mass. d) A changing magnetic field will create electric fields. e) A changing electric field will create magnetic fields. f) None of the above are true. Question #7 Which of the statements from Question #6 is most central to the way turbines (which convert kinetic energy to electrical energy, and are used in almost every kind of electrical generator) work? Briefly explain why.

5 Next, you decide to visit J.J. Thomson as he works to discover the electron. Thomson sends a beam of electrons between two metal plates, one positively charged, one negatively charged. Question #8: Which of the paths shown below is the most accurate picture of how the electron beam will behave? What does your answer reveal about the properties of electrons? Question #9: What if, instead of a beam of electrons, Thomson were to send a beam of alpha particles between those same two charged plates? Which of the paths shown in Question #6 would be the most accurate picture in that case? Briefly explain why.

6 Question #10: Thomson used electric fields for two purposes in his experiment: 1) To speed up the electrons to a high velocity, and 2) To steer the direction of the electron beam, as discussed in Question #6. Could he have used magnetic fields instead, for purpose #1? How about for purpose #2? Briefly explain your logic. Questions #11-#13: You head forward in time a century or two, and visit Marie and Pierre Curie in their lab. They are working with samples of the following five substances: a) Hydrogen-1 (whose nucleus is a single proton) b) Helium-4 (whose nucleus contains 2 protons and 2 neutrons.) c) Carbon-12 (whose nucleus contains 6 protons and 6 neutrons.) d) Carbon-14 (whose nucleus contains 6 protons and 8 neutrons.) e) Radium-226 (whose nucleus contains 88 protons and 138 neutrons.) Question #11: Of the substances listed, which one is most likely to undergo alpha decay? Briefly explain your reasoning.

7 Question #12: Of the substances listed, which one is most likely to undergo beta decay? Briefly explain your reasoning. Question #13: Of the substances listed, which one would be the best fuel for a fusion reactor (such as the Mr. Fusion reactor that powers the stranger's Delorian?) Briefly explain your reasoning. (Hint: remember that the Sun is also a fusion reactor.)

8 Question #14: Nuclear fusion is an exceptionally difficult energy source to harness; both the natural fusion reactions that power stars, and the fusion reactions in any human-made fusion reactor, require extremely high temperatures. Briefly explain why. (Your answer should involve at least two (or possibly more) of the fundamental forces of nature.) Extra Credit: A different isotope of the same element you chose in Question #12would be an even better fuel for a fusion reactor. Which isotope am I talking about, and why would it be easier to fuse? Answer on one of the attached sheets if (as is likely) you need more space. Question #15: You stop and pay a visit to Nikola Tesla, who is putting the finishing touches on a radio transmitter. Your fellow philosophers begin to debate among themselves exactly what a radio wave is. Which of the following arguments is the most convincing? a) Radio waves are sound waves; we know this because they can broadcast voices and music. b) Radio waves are light waves; we know this because in a dark enough room, radio waves are faintly visible to our eyes. c) Radio waves are sound waves; we know this because they grow weaker as they move away from their source. d) Radio waves are light waves; we know this because they can travel through the vacuum of empty space. e) Radio waves are sound waves; we know this because they have an amplitude, a frequency, and a wavelength. Extra credit: Can you describe at least one other experiment or observation that could help prove that your answer to this question is correct?

9 Question #16: As you continue to slide down the hill (it is a very long hill), you reflect on interesting people you have met. Turtles live very long lives at least, you seem to so there are quite a few such people, and they've done many interesting thing. Thinking of the following experiments: a) Ben Franklin's work with pith balls and static charge b) Lise Meitner and Otto's Hahn's work bombarding Uranium with neutrons. c) Edwin Hubble's observations of galaxies outside the Milky Way d) Ernest Rutherford's gold foil experiment e) JJ Thomson's experiments with cathode ray tubes f) Michael Faraday's work with electric currents and compass needles Which of these experiments gave the first strong evidence that atoms exist? Which of these experiments gave the first strong evidence that electrons exist? Which of these experiments gave the first strong evidence that atoms have a nucleus? Which of these experiments gave the first strong evidence that opposite charges attract; like charges repel? Which of these experiments was an early step toward understanding magnetic fields? Which of these experiments gave the first strong evidence that the universe is expanding, and has existed for only a finite amount of time?

10 Question #17: Choose any two of the experiments listed in Question #16. For each of your two choices, briefly describe the key observations that were made, and how those observations led to the breakthrough in understanding that you already identified. You don't have to go into great detail though you could do so later, as a long-answer topic but for now, a couple sentences summarizing the most important aspect of each experiment is fine. Choice #1: Choice #2:

11 Long-Answer Problems: Choose and answer, on the attached pages, at least two of the following seven topics. If you want to answer more than two, you can, but please decide which two are 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 to galaxies outside the Milky Way. The first two steps on the 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. Topic #2: Lord Kelvin's Estimates. One of Lord Kelvin's most famous claims was that he could accurately estimate the age of the Sun. By his calculations, the Sun could not be more than 10 or 20 milltion years old. Today, we believe the Sun is much older than that. What discoveries or phenomena, that Lord Kelvin did not know about, have changed our views on this question? Topic #3: Structure of an Atom. Over 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 inner workings of an atom. Describe at least two of the experiments or discoveries that led to this shift in understanding. (Names you might consider include Brown, Thomas, Curie, Rutherford...and many others. For inspiration, think about how some of the particles that make up atoms, or the forces that hold them together, were discovered!) Topic #4: Nuclear Physics in Detail. Describe in reasonable detail how at least one of the following phenomena works: alpha decay, beta decay, or nuclear fusion. Explain the roles, if any that two or more of the fundamental forces the Strong Nuclear Force, the Electromagnetic Force, the Weak Nuclear Force, and Gravity play in the process you have described Topic #5: The lifetime of a star. During a star's lifetime, all four of the fundamental forces of nature the Strong Nuclear Force, the Electromagnetic Force, the Weak Nuclear Force, and Gravity play a crucial role. Describe at least one important role that each of these four forces plays in the evolution and behavior of a star.. Topic #6: The future is another country. Describe at least one interesting unanswered question in science, and explain how we might go about investigating the topic in the future. What observations would we need to make to learn the answer? What difficulties prevent us from answering the question now? What possible answers might we find, and what new questions would they lead to?

12 Topic #7: Wildcard. As always, you are more than welcome to choose a topic of your own that you would rather write about. Please talk to me briefly before beginning to write, to make sure that we agree that the topic will make for a good essay!