Physics 171, Physics and Society Review Homework 1 Due: 1pm Tues Sept 4th, 2018

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1 Physics 171, Physics and Society Review Homework 1 Due: 1pm Tues Sept 4th, 2018 Regular question Remember, give justification for all your answers, and whenever the answer is a number always show the units, e.g. 200 J, 0.5 kg. 1. In this question we investigate the energy requirements of making tea. (a) How much energy (in Calories) does it take to boil one cup (250 ml) of water if the water starts at room temperature (about 20 C)? (b) What is that amount of energy in kilowatt-hours? (c) Roughly how much would it cost to boil the water using an electric kettle? (d) A laptop battery can hold 50 Wh of energy. How many cups of water could be boiled using a heater powered by a fully charged laptop battery? (e) A typical new alkaline battery (AA size) holds 2.6 Wh of energy. How many such batteries would it take to provide enough energy to boil one cup of water? (f) Suppose you are supplying kerosene as a cooking fuel to a group of 200 polio vaccination volunteers in India. They want enough fuel to make tea twice a day for 200 volunteers for a week. Burning 1 gram of kerosene yields about 10 Calories of energy. How much kerosene do they need? Give the answer in kilograms. {14 points} Multiple choice questions {2 points each} For most questions there is one correct choice, but in some cases there may be more than one, and to get full credit you must identify all the correct choices. In some cases none of the choices may be correct: in that case write none. The grading scheme for multiple choice questions is given on the course webpage. 1. The fact that energy is conserved means that (a) energy is never created or destroyed (b) thermal energy cannot be converted into any other form of energy (c) the total chemical energy of a system always stays the same (d) gravitational potential energy can be turned in to kinetic energy but not vice versa

2 2. One kilowatt-hour is the same as (a) 1000 Joules (b) 3600 Joules (c) 3.6 million Joules (d) 1 million Joules 3. The 1974 Chevrolet Camaro had an engine capacity of 350 cubic inches. What is that (to within 10% accuracy) in liters? (a) 2.3 liters (b) 0.89 liters (c) 5700 liters (d) 5.7 liters 4. A typical human being weighs about 150 lb and consists mostly of water. A cube of water with the same volume as a human being would be of what size (along each side, to 10% accuracy)? (a) 20 cm (b) 40 cm (c) 60 cm (d) 80 cm 5. The energy stored in fat, which will be released when it is burned or digested, is an example of (a) thermal energy (b) chemical energy (c) nuclear energy (d) kinetic energy 6. Brenda throws a package from the front door of her house up through the second floor window where it is caught by her brother. Which of the following energy transformations have occurred in this process? (a) thermal energy of the package to electrical energy in the wiring of the house (b) chemical energy in Brenda s body to kinetic energy of the package (c) gravitational potential energy of the package to chemical energy of the package (d) electrical energy in the wiring of the house to chemical energy in Brenda s body 7. When you burn natural gas to heat your house, the main process that is happening is (a) thermal energy is being converted to electrical energy (b) electrical energy is being converted to thermal energy (c) electrical energy is being converted to chemical energy (d) chemical energy is being converted to thermal energy 8. The energy density of gasoline is about 10 Calories (or Watt-hours) per gram. A full tank of gas costs $50, and the full gas tank contains about 50 kg of gasoline. To within 20% accuracy, what is the cost of the energy in gasoline, in cents per kwh? (a) 2 / kwh (b) 10 /kwh (c) 60 /kwh (d) 150 /kwh 9. Gasoline s energy density is about 10 Calories per gram. If it costs a full gas tank contains about 50 kg of gasoline, which costs how much energy does it store? (a) 1.2 kwh (b) 12 kwh (c) 0.6 kwh (d) 600 kwh

3 Physics 171, Physics and Society Review Homework 2 Due: 1pm Tues Sept 11th, 2018 Short report {20 points} Write a short critical summary (about 200 to 300 words) of a recent published article which is related to the topics covered so far in Chapter 1 of the textbook, namely energy storage, energy cost, etc. You can choose the article, but first read the detailed instructions for writing the report which are on the course web page, Your report should be printed on letter-size (8.5 by 11 inch) paper and stapled at the end of the form for the multiple choice questions. Multiple choice questions {2 points each} For most questions there is one correct choice, but in some cases there may be more than one, and to get full credit you must identify all the correct choices. In some cases none of the choices may be correct: in that case write none. The grading scheme for multiple choice questions is given on the course webpage. 1. Which of the following are true statements about ways to store energy: (a) batteries have a higher energy density than chocolate chip cookies (b) gasoline has a higher energy density than batteries (c) Uranium has a lower energy density than gasoline (d) coal has about the same energy density (to within 20%) as an explosive like TNT 2. A typical alkaline AA battery weighs about 20 g. Using table 1.1 in the textbook, calculate how much energy it contains. Roughly (to within a factor of 2) how much would this amount of electrical energy cost if it was obtained from the electrical outlets in your home? (a) 0.3 cents (b) 10 cents (c) 0.03 cents (d) 1 cent 3. To withon 20% accuracy, how many kilowatt-hours of energy are stored in a gallon of gasoline? (a) 0.25 kwh (b) 2.5 kwh (c) 7 kwh (d) 30 kwh 4. Suppose you have a car that is powered only by lead-acid batteries. We want the car to be able to go 75 miles on a full charge. To within 20% accuracy, how much weight of batteries will the car need to contain? (a) 8000 lb (b) 4000 lb (c) 2000 lb (d) 6 lb 5. A Tesla electric car has a battery that holds 80 kwh of energy, whereas the gas tank of a conventional car holds about 500 kwh of energy. If the conventional car has a range of 300 miles on one full tank, what (to within 20%) do you expect is the range of the Tesla car on a full charge? (a) 50 mi (b) 100 mi (c) 200 mi (d) 400 mi

4 6. Which of the following are major advantages of electric cars compared to gasoline cars? (a) batteries explode more readily than gasoline (b) batteries have a lower energy density than gasoline (c) electric motors are more efficient than gasoline engines (d) compared to buying a gallon of gasoline, it is much cheaper to buy the equivalent amount of energy in the form of electricity 7. A non-plugin hybrid car (like a Toyota Prius) (a) has a gasoline engine (b) can get over 100 miles per gallon (c) contains a fuel cell (d) turns all its kinetic energy into heat when it brakes 8. Which of the following features of hybrid cars help them have better gas mileage than regular cars: (a) the battery pack is lighter than a filled fuel tank (b) they use regenerative braking (c) their gasoline engine runs at maximum efficiency (d) they use a larger capacity engine 9. In a Hydrogen-powered car, the role of the fuel cell is (a) converting chemical energy to electric energy (b) capturing the heat produced when hydrogen is burned (c) capturing carbon dioxide emitted by the engine (d) converting electric energy to kinetic energy of the car 10. If we used hydrogen-powered cars, where would the hydrogen most likely come from? (This question has one correct answer.) (a) mining pockets of hydrogen found underground in the United States (b) hydrogen made from water using energy from burning fossil fuels (c) hydrogen produced when Uranium decays in nuclear reactors (d) hydrogen gas extracted from the atmosphere

5 Physics 171, Physics and Society Review Homework 3 Due: 1pm Tues Sept 18th, 2018 Regular question Remember, give justification for all your answers, and whenever the answer is a number always show the units, e.g. 200 J, 0.5 kg. 1. In this question we will estimate the energy released in the 9/11 terrorist attack on the world trade center. (a) A fully-laden Boeing 767 airplane weighs about 100 tons. How much is that in kilograms? (b) Explain which one of the following is the best estimate of the speed (in meters/second) at which the airplane flew in to the world trade center: 10 m/s; 50 m/s; 200 m/s. (c) Using your answer from the previous part, roughly how much kinetic energy (in Joules) was released when the plane hit the tower? (d) How many tons of TNT would one have to detonate to release the same amount of energy? (e) If the airplane had been going twice as fast, how would this affect the amount of kinetic energy released? (f) A Boeing 767 carries about 25 tons of jet fuel, whose chemical energy content is similar to that of gasoline. If that fuel burned completely after the collision, how much chemical energy in Joules was released? (g) How many tons of TNT is that equivalent to? (h) Power is defined as amount of energy released time duration of energy release. Make a reasoned estimate of which had higher power, the release of kinetic energy or the release of chemical energy. {14 points} Multiple choice questions {2 points each} In answering these questions, please use the data presented in lectures. For most questions there is one correct choice, but in some cases there may be more than one, and to get full credit you must identify all the correct choices. In some cases none of the choices may be correct: in that case write none. The grading scheme for multiple choice questions is given on the course webpage.

6 1. The efficiency of a typical coal or natural gas power plant is (to within ±5%) (a) 10% (b) 30% (c) 50% (d) 70% 2. In terms of the total cost ( levelized cost ) per kilowatt-hour, which of the following is the cheapest way to provide electrical energy? (a) wind power (b) geothermal power (c) solar panels on the roofs of houses (d) nuclear power 3. Which of the following statements about the levelized cost of generating electricity are correct? (a) solar rooftop panels are cheaper than a natural gas combined cycle power plant (b) biomass is usually cheaper than nuclear power (c) large solar power plants ( utility scale ) are more expensive than coal (d) the most efficient way to use natural gas is in a combined cycle power plant 4. Kinetic energy can be measured in (a) Calories (b) Watts per second (c) Watts (d) meters per second 5. A pitcher throws a ball at 80 mi/hr. The ball s kinetic energy is measured as 90 J. What is the mass of the ball (to within 20% accuracy)? (a) 5 kg (b) 150 g (c) 70 g (d) 2500 g 6. Suppose your electric bill for a whole year is $1000. If your electricity comes from burning coal, how much coal (to 20% accuracy) was burned to supply you with electricity for a year? (a) 400 kg (b) 2 tons (c) 4 tons (d) 8 tons

7 Physics 171, Physics and Society Review Homework 4 Due: 1pm Tues Sept 25th, 2018 Short report {20 points} Write a short critical summary (about 200 to 300 words) of a recent published article which is related to the topics covered so far in Chapter 1 and 2 of the textbook, namely power, temperature, heat, etc. You can choose the article, but first read the detailed instructions for writing the report which are on the course web page, Your report should be printed on letter-size (8.5 by 11 inch) paper and stapled at the end of the form for the multiple choice questions. Multiple choice questions {2 points each} In answering these questions, please use the data presented in lectures. For most questions there is one correct choice, but in some cases there may be more than one, and to get full credit you must identify all the correct choices. In some cases none of the choices may be correct: in that case write none. The grading scheme for multiple choice questions is given on the course webpage. 1. Which of the following sentences use the correct units? (a) One horsepower is about 750 Watts/sec. (b) The power used by a refrigerator is about 200 Watts (c) A running person is developing about 1000 J of power. (d) The power in sunlight is about 1 kj per square meter 2. In a year, roughly (to within 15%) how much petroleum energy does the U.S. consume? (a) 35 quads (b) 25 quads (c) less than 1 quad (d) 100 quads 3. Which forms of energy supply more than 5% of the U.S. s total power needs? (a) solar (b) wind (c) biofuels (d) hydroelectric 4. Which of the following are true statements about U.S. energy use? (a) Of the natural gas that we use, most of it goes to generate electricity (b) Transportation uses more energy than houses (i.e. residential consumption) (c) We generate more electricity from wind power than from biomass (d) We get more energy from renewable sources than from coal 5. A large animal s body produces about 1 horsepower in the form of heat. Roughly (to within 20%) how many Calories does this require the animal to eat in a day? (a) 60,000 Cal (b) 15,000 Cal (c) 8 million Cal (d) 20 Cal 6. An atom with A = 197 and Z = 79 has (a) 197 protons and 79 neutrons (b) 118 protons and 197 neutrons

8 (c) 197 protons and 118 neutrons (d) 79 protons and 118 neutrons 7. Absolute zero (zero degrees Kelvin) is the temperature at which (a) molecules move at roughly half the usual speed of sound (760 mi/hr) (b) molecules have fallen apart into atoms (c) molecules have no random movement (d) water freezes 8. When you cool an object from room temperature (20 C) to freezing (0 C), its thermal energy (a) decreases by about 25% (b) decreases by about 10% (c) decreases to zero (d) stays about the same 9. The density of air is 1.2 grams per liter. The amount of heat energy stored in one liter of air at room temperature is roughly (to within 20%): (a) 60 J (b) 0.6 Cal (c) 60 kj (d) 60 Cal 10. A jug of water is at the same temperature as the air around it. This means that the molecules in the water and in the air have the same (a) average acceleration relative to earth (b) atomic mass (c) average velocity (d) average kinetic energy

9 Physics 171, Physics and Society Review Homework 5 Due: 1pm Tues Oct 2nd, 2018 Regular question For each part of the question please circle or underline your final answer to make it clearly visible to the grader. 1. A Canadian teenager has invented a flashlight that is powered by the heat of the human body. Please see this article: The flashlight uses a Peltier tile which is a simple heat engine that turns heat energy into electrical energy. (a) What is human body temperature in Kelvin? (b) What is room temperature in Kelvin? (c) What is the maximum possible efficiency of the Peltier tile used in the humanpowered flashlight? (d) What does the article say is the efficiency of the Peltier tiles used in the flashlight? Comment on the number given in the article. (e) Now we are going to estimate how much heat power flows into the flashlight, so we can estimate the electrical power it could produce. (Note that this is instead of using the value 5.7 mw/cm 2 that is given in the article.) According to the course textbook, roughly what is the heat power output of the whole human body? (f) We want to estimate how much heat power is emitted by the part of the hand and fingers that is in contact with the flashlight when holding it. Roughly what is the area, in cm 2, of that part of the hand and fingers? Justify your estimate by making measurements on your own hand, and comment on how your results compares to the estimate given in the article. (g) Estimate the total surface area of the human body, assuming it can be approximated as a cylinder of height 5 feet 6 inches and waistline (circumference) 36 inches. Ignore the two flat ends of the cylinder. Give your result in square centimeters. (h) Using the results you have calculated above, estimate how much heat power could be flowing into the flashlight. (i) Estimate how much electrical power could be produced by Peltier tiles in the flashlight. (j) The article says that an LED needs about 0.5 mw of electrical power to get it to light with reasonable brightness. Does your calculation indicate that the LED flashlight has a good chance of working? {20 points}

10 Multiple choice questions {2 points each} For most questions there is one correct choice, but in some cases there may be more than one, and to get full credit you must identify all the correct choices. In some cases none of the choices may be correct: in that case write none. The grading scheme for multiple choice questions is given on the course webpage. 1. The atomic mass of naturally-occurring Oxygen is From this you can deduce that (a) some Oxygen atoms have less than 8 protons (b) some Oxygen atoms have less than 8 neutrons (c) naturally-occurring Oxygen contains impurities of other elements (d) a small fraction of Oxygen atoms clump together to form molecules 2. A thermal expansion joint (a) connects the hot region to the cold region in a heat engine (b) allows a large structure to undergo temperature changes without suffering damage (c) must be made of material with zero expansion coefficient in order to work properly (d) is only needed for structures made of a mixture ef metal and concrete 3. The linear expansion coefficient of olive oil is per C. Suppose a bottle of olive oil is heated so that the depth of oil increases from 8 inches to 8.1 inches. To an accuracy of 10%, by how much did the temperature of the olive oil change when it was heated? (a) 500 C (b) 60 C (c) 20 C (d) 7 C 4. To make a gasoline engine more efficient at converting heat to work, (a) the surrounding environment should be as warm as possible (b) the burning fuel should be spread out over a large volume (c) the gasoline should be kept cool (d) the burning fuel should be as hot as possible 5. A heat pump (a) extracts work from heat flowing out of a hot object (b) uses heat from a hot object to warm up a cold object (c) transfers heat from a cold object to a hot object (d) outputs an amount of heat that is equal to the work it takes as input 6. A coal-burning power plant is (a) a heat engine (b) a heat pump (c) a device for converting chemical energy to electrical energy (d) a device for obtaining heat from the atmosphere and converting it to electricity 7. If two objects are at the same temperature then (a) you can extract useful work from the heat flow between them

11 (b) they each have the same total thermal energy (c) it takes a lot of work to move heat from one object to the other (d) the molecules have the same average kinetic energy in each object 8. If a heat pump s coefficient of performance is 4.0, that means that for every kilowatt of heat it produces, (a) it consumes 4 kilowatts of electric power (b) it takes 4 kilowatts from the cold reservoir (c) it takes 250 Watts from the cold reservoir (d) it consumes 250 Watts of electric power 9. Which of the following are true: (a) heat pumps can have an efficiency of more than 100% (b) a heat engine s main purpose is to use work to drive heat from a cold object to a hot one (c) an air conditioner is a type of heat engine (d) a heat pump needs a large temperature difference to work efficiently 10. Suppose you use a ground-source heat pump as the heating system to keep your house at 70 F. The heat pump will pump heat from the rocks 50 feet beneath your house, where the temperature is 45 F. To within 20%, what is the maximum possible efficiency ( COP ) of the heat pump? (a) 3 (b) 23 (c) 1.5 (d) 15

12 Physics 171, Physics and Society Review Homework 6 Due: 1pm Tues Oct 23rd, 2018 Short report {20 points} Write a short critical summary (about 200 to 300 words) of a recent published article which is related to the topics covered so far in Chapter 4 of the textbook, namely radioactivity, its health effects, applications of radioactivity, etc. You can choose the article, but first read the detailed instructions for writing the report which are on the course web page, Your report should be printed on letter-size (8.5 by 11 inch) paper and stapled at the end of the form for the multiple choice questions. Multiple choice questions {2 points each} For most questions there is one correct choice, but in some cases there may be more than one, and to get full credit you must identify all the correct choices. In some cases none of the choices may be correct: in that case write none. The grading scheme for multiple choice questions is given on the course webpage. 1. Relative to the size of a whole atom, a nucleus is roughly (to within a factor of 10) (a) 100 times bigger (b) 100,000 times bigger (c) 100 times smaller (d) 100,000 times smaller 2. Alpha rays can pass through (a) aluminum foil (b) a metal plate (c) thick concrete (d) lead shielding 3. A terrorist wants to build a dirty bomb that, when it explodes, will cover a 50 m by 100 m area with enough radioactive material so that a person in that area will get a dose of 20 msv per hour. Before detonation, the bomb contains the radioactive material packed into a 1 m by 1 m area. Roughly what radiation dose will the unexploded bomb give to someone standing next to it? (a) 100 Sv/hour (b) 10 Sv/hour (c) 200 Sv/day (d) 20 msv/hour 4. The death rate for driving is about deaths per mile driven. Given that only half of all cancers are fatal, and assuming the linear hypothesis is correct, how many miles of driving (to within 20% accuracy) has the same death risk as a mammogram (0.4 msv whole body equivalent)? (a) 130,000 miles (b) 250 miles (c) 130 miles (d) 2500 miles 5. The radioactive 40 K in your body gives you a radiation dose of about 3µSv per year. Assuming the linear hypothesis is correct, estimate to 20% accuracy how many cancers per year this leads to in the USA. (a) 35 (b) less than one (c) 3000 (d) 900

13 6. 14 C has a half-life of a few thousand years, so why is there any of it on earth? (a) it is created by beta decay of 14 N (b) it is created by cosmic rays hitting the atmosphere (c) its half life was longer in the past (d) it is constantly being created by biological processes 7. A skeleton is discovered buried in a field near your house. A sample of bone from the skeleton contains 10 g of 12 C and g of 14 C. How old, to within 20%, is the skeleton? (a) 6000 years (b) less than 100 years (c) 12,000 years (d) 3000 years 8. How was most of the gold on earth originally created from other elements? (a) by a very skilled alchemist using the philosopher s stone (b) in the big bang, 14 billion years ago (c) in supernova explosions of massive stars (d) in collisions between neutron stars 9. How was most of the sodium on earth originally created from other elements? (a) by photosynthesis (b) in supernova explosions of massive stars (c) in the big bang, 14 billion years ago (d) in collisions between neutron stars 10. The half-life of 235 U is 700 million years. A piece of very old rock now contains 0.2 g of 235 U. To 10% accuracy, how much 235 U was there in the rock when it was formed on earth 4 billion years ago? (a) 60 g (b) 6 g (c) 0.3 g (d) 10 g

14 Physics 171, Physics and Society Review Homework 7 Due: 1pm Tues Oct 30, 2018 Regular question 1. In this question we look at whether spontaneous fission in 235 U could start the fission runaway process in a nuclear weapon. (a) The half-life of a 235 U nucleus is 700 million years. Suppose you have a 235 U nucleus in front of you. Estimate the probability that it will decay during the next 1 second. (b) Usually a 235 U nucleus decays by alpha emission, but two times out of a billion decays it will do spontaneous fission instead, producing neutrons. Estimate the probability that the 235 U nucleus in front of you will undergo spontaneous fission in the next second. (c) In a gun-type nuclear weapon, like the one used in the Hiroshima bomb, it takes about a millisecond for the two subcritical pieces of 235 U to combine to form a supercritical 60 kg piece. It is important that while they are combining there should not be any spontaneous fission in the Uranium, since that would produce neutrons that could set off a premature explosion, blowing the two pieces apart again. Estimate the probability that during a 1 ms time period there will be a spontaneous fission of one of the 235 U nuclei in the weapon. Check that the probability is small (much less than 0.1), as required for the weapon to avoid premature explosion. (d) It is also important that after the supercritical mass has been assembled, there should be a spontaneous fission to start the runaway fission process and produce the desired explosion. Is it likely that within one second of the supercritical mass being created, one of the nuclei in it will do a spontaneous fission, producing neutrons that will kick off the runaway fission process? Back up your answer with a numerical estimate. (e) Comment on whether a 235 U nuclear fission bomb needs to include an initiation mechanism to start the chain of fission events. {12 points} Multiple choice questions {2 points each} For most questions there is one correct choice, but in some cases there may be more than one, and to get full credit you must identify all the correct choices. In some cases none of the choices may be correct: in that case write none. The grading scheme for multiple choice questions is given on the course webpage.

15 1. The definition of fissile is: any material in which (a) the nuclei undergo spontaneous fission (b) energy can be produced by both fission and fusion of the nuclei (c) a self-sustaining chain of fission events can occur (d) neutrons are slowed down by collisions with nuclei 2. Uranium-235 is difficult to extract from natural Uranium ore because (a) its chemistry is similar to that of Uranium-238 (b) it decays away in a few hours (c) pure Uranium-235 tends to explode immediately (d) it has the same atomic mass as one of the isotopes of Plutonium 3. In a lump of Uranium that is heavier than the critical mass, (a) each fission event causes more fission events (b) each fission event produces neutrons but no additional fission events (c) the neutrons escape from the Uranium and do not cause fission (d) the chain reaction is maintained by cosmic ray neutrons entering the Uranium 4. The nuclear weapon dropped on Hiroshima generated most of its energy by (a) fusion of 235 U and 238 U (b) fusion of hydrogen and helium (c) fission of 238 U (d) fission of 235 U 5. Most of the energy produced in a nuclear explosion emerges as (a) beta rays (b) alpha rays (c) heat (d) light 6. Putting a neutron reflector near a piece of almost-critical Uranium-235 is (a) not likely to have much effect on the rate of fission events (b) a good safety precaution to stop it from going critical (c) impossible because there is no material that can reflect neutrons (d) likely to reduce the number of fission events per second 7. Fission fragments tend to be highly radioactive because (a) they contain trace amounts of 235 U (b) they have more protons than neutrons (c) they are large, unstable nuclei with more protons than a Uranium nucleus (d) the fission of Uranium produces unstable isotopes of smaller nuclei 8. Strontium-90 ( 90 Sr) is (a) highly radioactive because it decays in days (b) an example of a fission fragment (c) chemically similar to Carbon, and hence readily metabolized (d) a dangerous component of nuclear fallout 9. If a nuclear fission weapon explodes on the ground then the fission fragments (a) are not a concern because they are carried away and dispersed in the upper atmosphere (b) fall back to earth and contaminate the surrounding area (c) are quickly washed away by rain caused by the explosion (d) bounce off the ground and fly up in the air away from the surrounding area