Physics Quiz (Conceptual Questions)

1. Two H2 molecules can react with one O2 molecule to produce two H2O molecules. How many moles of hydrogen molecules are needed to react with one mole of oxygen molecules?

1. 2 moles, as that will contain twice as many molecules as the 1 mole of oxygen

1. What does it mean to say that two systems are in thermal equilibrium?

1. They are at the same temperature, and if they are placed in contact, no net heat flows between them. 3. The reading will change. 5. The cold water cools part of the inner surface, making it contract, while the rest remains expanded. The strain is too great for the strength of the material. Pyrex contracts less, so it experiences less strain. 7. In principle, the lid expands more than the jar because metals have higher coefficients of expansion than glass. That should make unscrewing the lid easier. (In practice, getting the lid and jar wet may make gripping them more difficult.) 9. After being heated, the length is ( 1+300α 1+300α1+300α ) ( 1m 1m1m ). After being cooled, the length is (1−300α)(1+300α)(1m) (1−300α)(1+300α)(1m)(1−300α)(1+300α)(1m) . That answer is not 1 m, but it should be. The explanation is that even if α αα is exactly constant, the relation ΔL=αLΔT ΔL=αLΔTΔL=αLΔT is strictly true only in the limit of small ΔT ΔTΔT . Since α αα values are small, the discrepancy is unimportant in practice. 11. Temperature differences cause heat transfer. 13. No, it is stored as thermal energy. A thermodynamic system does not have a well-defined quantity of heat. 15. It raises the boiling point, so the water, which the food gains heat from, is at a higher temperature. 17. Yes, by raising the pressure above 56 atm. 19. work 21. 0°C 0°C0°C (at or near atmospheric pressure) 23. Condensation releases heat, so it speeds up the melting. 25. Because of water's high specific heat, it changes temperature less than land. Also, evaporation reduces temperature rises. The air tends to stay close to equilibrium with the water, so its temperature does not change much where there's a lot of water around, as in San Francisco but not Sacramento. 27. The liquid is oxygen, whose boiling point is above that of nitrogen but whose melting point is below the boiling point of liquid nitrogen. The crystals that sublime are carbon dioxide, which has no liquid phase at atmospheric pressure. The crystals that melt are water, whose melting point is above carbon dioxide's sublimation point. The water came from the instructor's breath. 29. Increasing circulation to the surface will warm the person, as the temperature of the water is warmer than human body temperature. Sweating will cause no evaporative cooling under water or in the humid air immediately above the tub. 31. It spread the heat over the area above the heating elements, evening the temperature there, but does not spread the heat much beyond the heating elements. 33. Heat is conducted from the fire through the fire box to the circulating air and then convected by the air into the room (forced con

1. Consider these scenarios and state whether work is done by the system on the environment (SE) or by the environment on the system (ES): (a) opening a carbonated beverage; (b) filling a flat tire; (c) a sealed empty gas can expands on a hot day, bowing out the walls.

1. a. SE; b. ES; c. ES

11. It is unlikely that a process can be isothermal unless it is a very slow process. Explain why. Is the same true for isobaric and isochoric processes? Explain your answer.

11. Isothermal processes must be slow to make sure that as heat is transferred, the temperature does not change. Even for isobaric and isochoric processes, the system must be in thermal equilibrium with slow changes of thermodynamic variables.

11. Which is more dangerous, a closet where tanks of nitrogen are stored, or one where tanks of carbon dioxide are stored?

11. One where nitrogen is stored, as excess CO 2 CO2CO2 will cause a feeling of suffocating, but excess nitrogen and insufficient oxygen will not.

11. How is heat transfer related to temperature?

11. Temperature differences cause heat transfer. beyond the heating elements. 33. Heat is conducted from the fire through the fire box to the circulating air and then convected by the air into the room (forced convection). 35. The tent is heated by the Sun and transfers heat to you by all three processes, especially radiation. 37. If shielded, it measures the air temperature. If not, it measures the combined effect of air temperature and net radiative heat gain from the Sun. 39. Turn the thermostat down. To have the house at the normal temperature, the heating system must replace all the heat that was lost. For all three mechanisms of heat 41. Air is a good insulator, so there is little conduction, and the heated air rises, so there is little convection downward.

13. One might think that the internal energy of diatomic gases is given by Eint=5RT/2. Do diatomic gases near room temperature have more or less internal energy than that? Hint: Their internal energy includes the total energy added in raising the temperature from the boiling point (very low) to room temperature.

13. Less, because at lower temperatures their heat capacity was only 3RT/2.

13. When heat transfers into a system, is the energy stored as heat? Explain briefly.

13. No, it is stored as thermal energy. A thermodynamic system does not have a well-defined quantity of heat.

13. Most materials expand when heated. One notable exception is water between 0°C and 4°C, which actually decreases in volume with the increase in temperature. Which is greater for water in this temperature region, Cp or CV?

13. Typically Cp is greater than CV because when expansion occurs under constant pressure, it does work on the surroundings. Therefore, heat can go into internal energy and work. Under constant volume, all heat goes into internal energy. In this example, water contracts upon heating, so if we add heat at constant pressure, work is done on the water by surroundings and therefore, Cp is less than CV .

15. A pressure cooker contains water and steam in equilibrium at a pressure greater than atmospheric pressure. How does this greater pressure increase cooking speed?

15. It raises the boiling point, so the water, which the food gains heat from, is at a higher temperature.

15. Is it possible for γ to be smaller than unity?

15. No, it is always greater than 1.

15. One cylinder contains helium gas and another contains krypton gas at the same temperature. Mark each of these statements true, false, or impossible to determine from the given information. (a) The rms speeds of atoms in the two gases are the same. (b) The average kinetic energies of atoms in the two gases are the same. (c) The internal energies of 1 mole of gas in each cylinder are the same. (d) The pressures in the two cylinders are the same 16. Repeat the previous question if one gas is still helium but the other is changed to fluorine, F2.

15. a. false; b. true; c. true; d. true

17. An ideal gas is at a temperature of 300 K. To double the average speed of its molecules, what does the temperature need to be changed to?

17. 1200 K

17. There is no change in the internal energy of an ideal gas undergoing an isothermal process since the internal energy depends only on the temperature. Is it therefore correct to say that an isothermal process is the same as an adiabatic process for an ideal gas? Explain your answer.

17. An adiabatic process has a change in temperature but no heat flow. The isothermal process has no change in temperature but has heat flow.

18. What is the distinction between gas and vapor? 19. Heat transfer can cause temperature and phase changes. What else can cause these changes? 20. How does the latent heat of fusion of water help slow the decrease of air temperatures, perhaps preventing temperatures from falling significantly below 0°C, 0°C,0°C, in the vicinity of large bodies of water?

17. Yes, by raising the pressure above 56 atm.

19. Heat transfer can cause temperature and phase changes. What else can cause these changes?

19. work

21. What is the temperature of ice right after it is formed by freezing water?

21. 0°C 0°C0°C (at or near atmospheric pressure)

23. What effect does condensation on a glass of ice water have on the rate at which the ice melts? Will the condensation speed up the melting process or slow it down?

23. Condensation releases heat, so it speeds up the melting.

25. In winter, it is often warmer in San Francisco than in Sacramento, 150 km inland. In summer, it is nearly always hotter in Sacramento. Explain how the bodies of water surrounding San Francisco moderate its extreme temperatures.

25. Because of water's high specific heat, it changes temperature less than land. Also, evaporation reduces temperature rises. The air tends to stay close to equilibrium with the water, so its temperature does not change much where there's a lot of water around, as in San Francisco but not Sacramento.

27. In a physics classroom demonstration, an instructor inflates a balloon by mouth and then cools it in liquid nitrogen. When cold, the shrunken balloon has a small amount of light blue liquid in it, as well as some snow-like crystals. As it warms up, the liquid boils, and part of the crystals sublime, with some crystals lingering for a while and then producing a liquid. Identify the blue liquid and the two solids in the cold balloon. Justify your identifications using data from Table 1.4.

27. The liquid is oxygen, whose boiling point is above that of nitrogen but whose melting point is below the boiling point of liquid nitrogen. The crystals that sublime are carbon dioxide, which has no liquid phase at atmospheric pressure. The crystals that melt are water, whose melting point is above carbon dioxide's sublimation point. The water came from the instructor's breath

29. When our bodies get too warm, they respond by sweating and increasing blood circulation to the surface to transfer thermal energy away from the core. What effect will those processes have on a person in a 40.0-°C 40.0-°C40.0-°C hot tub?

29. Increasing circulation to the surface will warm the person, as the temperature of the water is warmer than human body temperature. Sweating will cause no evaporative cooling under water or in the humid air immediately above the tub.

3. When a liquid is vaporized, its change in internal energy is not equal to the heat added. Why?

3. Some of the energy goes into changing the phase of the liquid to gas.

3. If a thermometer is allowed to come to equilibrium with the air, and a glass of water is not in equilibrium with the air, what will happen to the thermometer reading when it is placed in the water?

3. The reading will change.

3. A constant-volume gas thermometer contains a fixed amount of gas. What property of the gas is measured to indicate its temperature?

3. pressure

31. Some electric stoves have a flat ceramic surface with heating elements hidden beneath. A pot placed over a heating element will be heated, while the surface only a few centimeters away is safe to touch. Why is ceramic, with a conductivity less than that of a metal but greater than that of a good insulator, an ideal choice for the stove top?

31. It spread the heat over the area above the heating elements, evening the temperature there, but does not spread the heat much beyond the heating elements.

33. One way to make a fireplace more energy-efficient is to have room air circulate around the outside of the fire box and back into the room. Detail the methods of heat transfer involved.

33. Heat is conducted from the fire through the fire box to the circulating air and then convected by the air into the room (forced convection).

35. When watching a circus during the day in a large, dark-colored tent, you sense significant heat transfer from the tent. Explain why this occurs.

35. The tent is heated by the Sun and transfers heat to you by all three processes, especially radiation.

37. Why are thermometers that are used in weather stations shielded from the sunshine? What does a thermometer measure if it is shielded from the sunshine? What does it measure if it is not?

37. If shielded, it measures the air temperature. If not, it measures the combined effect of air temperature and net radiative heat gain from the Sun.

39. Your house will be empty for a while in cold weather, and you want to save energy and money. Should you turn the thermostat down to the lowest level that will protect the house from damage such as freezing pipes, or leave it at the normal temperature? (If you don't like coming back to a cold house, imagine that a timer controls the heating system so the house will be warm when you get back.) Explain your answer.

39. Turn the thermostat down. To have the house at the normal temperature, the heating system must replace all the heat that was lost. For all three mechanisms of heat transfer, the greater the temperature difference between inside and outside, the more heat is lost and must be replaced. So the house should be at the lowest temperature that does not allow freezing damage.

41. Broiling is a method of cooking by radiation, which produces somewhat different results from cooking by conduction or convection. A gas flame or electric heating element produces a very high temperature close to the food and above it. Why is radiation the dominant heat-transfer method in this situation?

41. Air is a good insulator, so there is little conduction, and the heated air rises, so there is little convection downward.

5. Pouring cold water into hot glass or ceramic cookware can easily break it. What causes the breaking? Explain why Pyrex®, a glass with a small coefficient of linear expansion, is less susceptible.

5. The cold water cools part of the inner surface, making it contract, while the rest remains expanded. The strain is too great for the strength of the material. Pyrex contracts less, so it experiences less strain.

5. In the last chapter, free convection was explained as the result of buoyant forces on hot fluids. Explain the upward motion of air in flames based on the ideal gas law

5. The flame contains hot gas (heated by combustion). The pressure is still atmospheric pressure, in mechanical equilibrium with the air around it (or roughly so). The density of the hot gas is proportional to its number density N/V (neglecting the difference in composition between the gas in the flame and the surrounding air). At higher temperature than the surrounding air, the ideal gas law says that N/V=p/ k B T N/V=p/kBTN/V=p/kBT is less than that of the surrounding air. Therefore the hot air has lower density than the surrounding air and is lifted by the buoyant force.

5. Is it possible for the temperature of a system to remain constant when heat flows into or out of it? If so, give examples

5. Yes, as long as the work done equals the heat added there will be no change in internal energy and thereby no change in temperature. When water freezes or when ice melts while removing or adding heat, respectively, the temperature remains constant.

7. Does adding heat to a system always increase its internal energy?

7. If more work is done on the system than heat added, the internal energy of the system will actually decrease.

7. Does it really help to run hot water over a tight metal lid on a glass jar before trying to open it? Explain your answer.

7. In principle, the lid expands more than the jar because metals have higher coefficients of expansion than glass. That should make unscrewing the lid easier. (In practice, getting the lid and jar wet may make gripping them more difficult.)

7. If one kind of molecule has double the radius of another and eight times the mass, how do their mean free paths under the same conditions compare? How do their mean free times compare?

7. The mean free path is inversely proportional to the square of the radius, so it decreases by a factor of 4. The mean free time is proportional to the mean free path and inversely proportional to the rms speed, which in turn is inversely proportional to the square root of the mass. That gives a factor of 8 - √ 88 in the numerator, so the mean free time decreases by a factor of 2 - √ .

9. Calculate the length of a 1-meter rod of a material with thermal expansion coefficient α when the temperature is raised from 300 K to 600 K. Taking your answer as the new initial length, find the length after the rod is cooled back down to 300 K. Is your answer 1 meter? Should it be? How can you account for the result you got?

9. After being heated, the length is (1+300α) (1m). After being cooled, the length is (1−300α)(1+300α)(1m) . That answer is not 1 m, but it should be. The explanation is that even if α is exactly constant, the relation ΔL=αLΔT is strictly true only in the limit of small ΔT. Since α values are small, the discrepancy is unimportant in practice.

9. Why do the atmospheres of Jupiter, Saturn, Uranus, and Neptune, which are much more massive and farther from the Sun than Earth is, contain large amounts of hydrogen and helium?

9. Since they're more massive, their gravity is stronger, so the escape velocity from them is higher. Since they're farther from the Sun, they're colder, so the speeds of atmospheric molecules including hydrogen and helium are lower. The combination of those facts means that relatively few hydrogen and helium molecules have escaped from the outer planets.

9. When a gas expands isothermally, it does work. What is the source of energy needed to do this work?

9. The system must be in contact with a heat source that allows heat to flow into the system.