Temperature and heat(theory questions) Flashcards
What does it mean to say that two systems are in thermal equilibrium?
They are at the same temperature, and if they are placed in contact, no net heat flows between them.
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?
The reading will change.
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.
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.
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.
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.)
Calculate the length of a 1-meter rod of a material with thermal expansion coefficient alpha 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?
After being heated, the length is (1 + 300 alpha) (1 m). After being cooled, the length is (1 - 300 alpha) (1 + 300 alpha) (1 m). That answer is not 1 m, but it should be. The explanation is that even if alpha is exactly constant, the relation delta_L = alpha L delta_T is strictly true only in the limit of small delta_T. Since alpha values are small, the discrepancy is unimportant in practice.
How is heat transfer related to temperature?
Temperature differences cause heat transfer.
When heat transfers into a system, is the energy stored as heat? Explain briefly.
No, it is stored as thermal energy. A thermodynamic system does not have a well-defined quantity of heat.
A pressure cooker contains water and steam in equilibrium at a pressure greater than atmospheric pressure. How does this greater pressure increase cooking speed?
It raises the boiling point, so the water, which the food gains heat from, is at a higher temperature.
Can carbon dioxide be liquefied at room temperature (20°C)? If so, how? If not, why not? (See the phase diagram in the preceding problem.)
Yes, by raising the pressure above 56 atm.
Heat transfer can cause temperature and phase changes. What else can cause these changes?
Work
What is the temperature of ice right after it is formed by freezing water?
0°C (at or near atmospheric pressure)
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?
Condensation releases heat, so it speeds up the melting.
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.
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.
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 the table (Melting Point (°C), Boiling Point (°C) for: Nitrogen -210.0, -195.8; Oxygen -218.8, -183.0; Water 0.00, 100.0).
The liquid is oxygen, whose boiling point (-183.0°C) is above that of nitrogen (-195.8°C) but whose melting point (-218.8°C) 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 (0.00°C) is above carbon dioxide’s sublimation point. The water came from the instructor’s breath.
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?
It spreads the heat over the area above the heating elements, evening the temperature there, but does not spread the heat much beyond the heating elements.
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 hot tub?
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.
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.
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).
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.
The tent is heated by the Sun and transfers heat to you by all three processes, especially radiation.
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.
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.
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?
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.
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?
Air is a good insulator, so there is little conduction, and the heated air rises, so there is little convection downward.
The height of the Washington Monument is measured to be 170.00 m on a day when the temperature is 35.0°C. What will its height be on a day when the temperature falls to -10.0°C? Although the monument is made of limestone, assume that its coefficient of thermal expansion is the same as that of marble (alpha = 2.5 x 10^-6 /°C). Give your answer to five significant figures.
L = L_0 + delta_L = L_0 (1 + alpha delta_T) = 170 m [1 + (2.5 x 10^-6 /°C) (-45.0°C)] = 169.98 m. (Answer rounded to five significant figures to show the slight difference in height.)
What is the change in length of a 3.00-cm-long column of mercury if its temperature changes from 37.0°C to 40.0°C, assuming the mercury is constrained to a cylinder but unconstrained in length? Your answer will show why thermometers contain bulbs at the bottom instead of simple columns of liquid.
We use beta instead of alpha since this is a volume expansion with constant surface area. Therefore: delta_L = alpha L delta_T = (6.0 x 10^-5 /°C) (0.0300 m) (3.00°C) = 5.4 x 10^-6 m.
(a) Suppose a meter stick made of steel and one made of aluminum are the same length at 0°C. What is their difference in length at 22.0°C? (b) Repeat the calculation for two 30.0-m-long surveyor’s tapes. (alpha_al = 2.5 x 10^-5 /°C, alpha_steel = 1.2 x 10^-5 /°C)
a. delta_L_Al - delta_L_steel = (alpha_Al - alpha_steel) L_0 delta_T = (2.5 x 10^-5 /°C - 1.2 x 10^-5 /°C) (1.00 m) (22°C) = 2.9 x 10^-4 m; b. By the same method with L_0 = 30.0 m, we have delta_L = 8.6 x 10^-3 m.
