Conceptual Physics: Chapter 16: Heat Transfer
Wood is a better insulator than glass, yet fiberglass is commonly used as an insulator in wooden buildings. Why?
Air is an excellent insulator. The reason that fiberglass is a good insulator is principally because of the vast amount of air spaces trapped in it.
If you hold one end of a piece of metal against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice to your hand? Explain.
Energy "flows" from higher to lower temp, from your hand to the ice. It is the energy, heat, flowing from your hand that produces the sensation of coolness. There is no flow from cold to hot; only from hot to cold.
Since energy is radiated by all objects, why can't we see them in the dark?
Human eyes are insensitive to the infrared radiated by objects at average temperatures.
A number of bodies at different temperatures placed in a closed room share radiant energy and ultimately reach a common temperature. Would this thermal equilibrium be possible if good absorbers were poor emitters and poor absorbers were good emitters? Explain.
If good absorbers were not also good emitters, then thermal equilibrium would not be possible. If a good absorber only absorbed then its temperature would climb above that of poorer absorbers in the vicinity. And if poor absorbers were good emitters, their temps would fall below that of better absorbers.
What is the role of "loose" electrons in heat conductors?
Loose electrons quickly move and transfer energy to other electrons that migrate through the material.
How are the speeds of molecules of air affected as they separate from one another when escaping from the nozzle of a party balloon?
Speeds are decreased with expansion.
What is terrestrial radiation?
Terrestrial radiation is that emitted by Earth's surface.
Why does the pupil of the eye appear black?
The pupil appears black because light that enters the eye usually doesn't exit. With flash cameras, however, some of it does.
When two cups of hot chocolate, one at 50C and the other at 60C, are poured into a bowl, why will the temperature of the mixture be between 50 and 60C?
The temp will be in between because on decreases in temp and the other increases in temp.
Make up a multiple choice Q to test a classmates understanding of the distinction between conduction and convection.
Open-ended.
How does the peak frequency of radiant energy relate to the absolute temperature of the radiating source?
Peak frequency and absolute temperature are directly proportional.
An object that radiates energy at night is in contact with the relatively warm Earth. How does poor conductivity affect the object's temperature relative to air temperature?
Poor conductivity means little heat from the ground and the object can cool by radiation to temps below that of the surrounding air temperature.
Calculate the quantity of heat absorbed by 20 g of water that warms from 30 C to 90 C.
Q= cm change in T = (1 cal/gC)(20 g) (90C-30C) = 1,200 cal.
In what form does radiant energy travel?
Radiant energy travels in electromagnet waves.
What determines whether an object is a net absorber or a net emitter of radiant energy at a given time?
Surrounding temperature determines whether an object is a net emitter or absorber.
What happens to the temperature of something that radiates energy without absorbing the same amount in return.
Temperature will drop when radiation exceeds absorption.
How much radiant energy from the Sun, on average, reaches each square meter at the top of Earth's atmosphere each second?
The Sun pours 1400 J of radiant energy per second per meter squared at the top of the atmosphere.
Why are mittens warmer than gloves on a cold day?
There is more air space in mittens than in gloves, which makes for warmer hands. Also, the fingers in the mittens are next to one another which also keeps hands warmer.
Why are materials such as wood, fur, feathers, and even snow good insulators?
They are poor conductors, which makes them good insulators.
What happens to the volume of air as it rises? What happens to its temperature?
Volume increases as air rises, and correspondingly cools.
At what common temperature will a block of wood and a block of metal both feel neither hot nor cold to the touch?
When the temp of the blocks are the same as the temp of your hand, then no heat transfer occurs. Heat will flow btw your hand and something being touched only if there is a temp difference between them.
Does Newton's law of cooling apply to warming as well as to cooling?
Yes, Newton's law of cooling also applies to warming.
Is it possible for heat to flow between two objects with the same internal energy? Can heat flow from an object with less internal energy to one with more internal energy? Explain your answer.
Yes, in both cases, as long as one has a higher temperature than the other. Differences in temperature, NOT internal energy, dictates heat flow.
Can an object be both a good absorber and a good reflector at the same time? Why, or why not?
A good absorber cannot at the same time be a good reflector because absorption and reflection are opposite processes.
Release a single molecule in an evacuated region and it will fall as fast as, and no differently from, a baseball released in the same region. Explain.
Both the molecule and the baseball are under the influence of gravity, and both will accelerate downward at g. When other molecules impede downward fall, then the free fall acceleration g isn't maintained.
Which will undergo the greater rate of cooling: a red-hot poker in a warm oven or a red-hot poker in a cold room (or do both cool at the same rate)?
By Newton's law of cooling, the hot poker in the cold room radiates more due to the greater temperature difference between the poker and the room.
Why does the direction of coastal winds change from day to night?
Direction of winds change with changes in land and water temperatures. Air flow reverses as relative temperatures reverse.
Why isn't Millie's hand burned when she holds it above the escape valve of the pressure cooker (see Figure 16.8) ?
Her hand is not in steam, but in a jet of condensed vapor that has expanded and cooled.
Relatively speaking, do high-frequency waves have long wavelengths or short wavelengths?
High-frequency waves have short wavelengths.
To save energy, some ceiling fans are reversible so that they drive air down or pull it up. In which direction should the fan drive the air during winter? in which direction during summer?
In winter you want warm air near the floor, so the fan should push warmer ceiling air downward. In summer you want cooler air near the floor, so the fan should pull air upward.
Does a good insulator prevent heat from escaping or slow its passage?
Insulation delays heat transfer.
One container is filled with argon gas and the other with krypton gas. If both gases have the same temperature, in which container are the atoms moving faster? Why?
As in the explanation of the previous exercise, the molecules of gas with the lesser mass will have the higher average speeds. A look at the periodic table will show that argo (A=18) has less massive atoms than krypton (A=36). The faster atoms are those of argon. This is the case whether or not the gases are in separate containers.
On a very cold sunny day, you wear a black coat and a transparent plastic coat. Which coat should be worn on the outside for maximum warmth?
For maximum warmth, wear the plastic coat on the outside and utilize the greenhouse effect.
On a cold day, why does a metal doorknob feel colder than the wooden door?
The metal doorknob conducts heat better than wood.
Which atoms, on average, move slower in a mixture: U-238 or U-235? How would this affect diffusion through a porous membrane of otherwise identical gases made from these isotopes?
Molecules of gas with greater mass have a smaller average speed. So molecules containing heavier U-238 are slower on the average. This favors the diffusion of the faster gas containing U-235 through a porous membrane (which is how U-235 was separated from U-238 by scientists in the 1940s).
If 70F air feels warm and comfortable to us, why does swimming in 70F water feel cool?
Air at 70F feels comfortable principally because it is a poor conductor. Our warmer skin is slow to transfer heat to the air. Water, however, is a better conductor of heat than air, so our warmer bodies in water more readily transfer heat to the water.
Discuss the purpose of a layer of copper or aluminum on the bottoms of stainless steel cookware.
Copper and aluminum are better conductors than stainless steel, and therefore more quickly establish a uniform temp over the bottom of the pan and transfer heat to the cookware's interior.
What is the explanation for feather beds being warm?
Feathers (and the air they trap) are good insulators and thus conduct body heat very slowly to the surroundings.
In a mixture of hydrogen and oxygen gases at the same temperature, which molecules move faster? Why?
Hydrogen molecules will be the faster moving when mixed with oxygen molecules. They will have the same temperature, which means that they will have the same average KE. KE=1/2 mv^2. Since the mass of hydrogen is considerably less than oxygen, the speed must correspondingly be greater.
Cite a primary difference between waves of solar radiation and waves of terrestrial radiation.
Terrestrial radiation is lower in frequency and intensity than solar radiation.
What would be the consequence of completely eliminating the greenhouse effect?
With no greenhouse effect Earth would be a very cold place, with an average temperature about -18C.
Explain why a firewalker can step quickly without harm on red-hot coals with bare feet.
Wood is a good insulator even when it's red hot, therefore very little thermal energy is transferred to the feet.
In a lab you submerge 100 g of 40 C iron nails in 100g of 20 C water (the specific heat of iron is 0.11 cal/g C). a) Equate the heat gained by the water to the heat lost by the nails, and show that the final temperature of the water becomes 22 C. b) Your lab partner is surprised by the result and says that since the masses of iron and water are equal, the final water temperature should be closer to 30 C, halfway between. What is your explanation?
a) Q gained by water = Q lost by nails, so (cm change in T) water= (cm change in T) nails. (1.0g C) (100G) (T-20C) = (0.11 cal/g C) (100 g) (40 c- T), where T is 22 C. b) although the masses are the same, the specific heats are widely apart, iron being very low and water being incredibly high. It takes as much heat as the iron can release to raise water by about 2C.
Which will normally warm faster: a black pot of cold water or a silvered pot of cold water? Which will cool faster?
A black pot both warms faster and cools faster than a silver pot.
Why does a good emitter of heat radiation appear black at room temperature?
A good emitter, by virtue of molecular-or -whatever design, is also a good absorber. A good absorber appears black because radiation that impinges upon it is absorbed; just the opposite of reflection.
From the rules that a good absorber of radiation is a good radiator and a good reflector is a poor absorber, state a rule that relates the reflecting and radiating properties of a surface.
A good reflector is a poor radiator of heat, and a poor reflector is a good radiator of heat.
At a certain location, the solar power per unit area reaching Earth's surface is 200 W/m2, averaged over a 24-hour day. If the average power requirement in your home is 3 kW and you can convert solar power to electric power with 10% efficiency, how large a collector area will you need to meet all your household energy requirements from solar energy? Will a collector fit in your yard or on your roof?
Because of the 20-percent conversion efficiency, each square meter of collector will supply 40 watt of electric power on average. So to meet the 3kW requirement you will need (3000W)/(40 W/m2) = 75 m2 of collector area. This is the area of a square about 9 m or 28 ft on a side. It would fit in a typical yard, but is a little larger than a typical roof.
What does the high specific heat of water have to do with convection currents in the air at the seashore?
Because of the high specific heat of water, sunshine warms water much less than it warms land. As a result, air is warmed over the land and rises. Cooler air from above the cool water takes its place and convection currents are formed. If land and water were heated equally by the Sun, such convection currents (and the winds they produce) wouldn't be established.
Why is the insulation in an attic commonly thicker than the insulation in the walls of a house?
Because warm air rises, there's a higher temperature at the ceiling than at the walls. With a greater difference in the inside and outside temperatures, thicker insulation is needed to slow the transfer of heat.
If you touch the metal sides in an oven with your bare hand, you're in trouble. But hold your hand briefly in the oven air and you're okay. What does this tell you about the relative conductivities of metal and air?
Conductivity of metals are much greater than conductivity of air.
Air density is normally less above any point in air than below, providing a "migration window" in air. How does this affect the movement of faster-moving molecules in air?
Faster-moving (warm air) molecules migrate upward through the "open window" in the atmosphere, producing upward convection.
Radioactive decay of granite and other rocks in Earth's interior provides sufficient energy to keep the interior molten, to heat lava, and to provide warmth to natural hot springs. This is due to the average release of about 0.03 J per kilogram each year. Show that a 500 C increase in temperature for a thermally insulated chunk of granite takes about 13.3 million years.
From Q= cm change in T = (800 J/kg C)(500 C)= 400,000 J/kg The time required is about (400,000 J/kg)/(0.03 J/kg*yr) = 13 million years.
How does glass act like a one-way valve for a conventional greenhouse? Does the atmosphere similarly act as a one-way valve?
Glass allows high-frequency visible-light radiant energy in, but prevents low-frequency infrared re-radiated energy out. Likewise for the atmosphere acting as a one-way valve.
Visit a snow-covered cemetery and note that the snow does not slope upward against the gravestones. Instead, it forms depressions, as shown. Can you think of a reason for this?
Heat from the relatively warm ground is conducted by the gravestone to melt the snow in contact with the gravestone. Likewise for trees or any materials that are better conductors of that than snow, and that extend into the ground.
Even though metal is a good conductor, frost can be seen on parked cars in the early morning even when the air temperature is above freezing. Provide an explanation.
Heat radiates into the clear night air and the temperature of the car goes down. Normally, heat is conducted to the car by the relatively warmer ground, but the rubber tires prevent the conduction of heat from the ground. So heat radiated away is not easily replaced and the car cools to temperatures below that of the surroundings. in this way frost can form on a below freezing car in the above freezing environment.
Cite three ways in which a Thermos bottle inhibits heat transfer.
Heat transfer by conduction, convection, and radiation is inhibited in a Thermos bottle.
Ice cubes float in a glass of iced team. Why would cooling be less if the cubes were instead on the bottom of the drink?
If ice cubes were at the bottom they wouldn't be in contact with the warmest part of the tea at the surface, so cooling would be less. Ice cubes are preferable at the surface to decrease the temperature of the warmer part of the tea.
If the composition of the upper atmosphere were changed so that it permitted a greater amount of terrestrial radiation to escape, what effect would this have on Earth's climate?
If the upper atmosphere permitted the escape of more terrestrial radiation than it does presently, more energy would escape and the Earth's climate would be cooler.
Many tongues have been injured by licking a piece of metal on a very cold day. Why would no harm result if a clean piece of wood were licked on the same day?
In touching the tongue to a very cold metal, enough heat can be quickly conducted away from the tongue to bring the saliva to sub-zero temperature where it freezes, locking the tongue to the metal. In the case of relatively nonconducting wood, much less heat is conducted from the tongue and freezing does not take place fast enough for sudden sticking to occur.
Why is it incorrect to say that, when a hot object warms a cold one, the increase in temperature of the cold one is equal to the decrease in temperature of the hot one? When is this statement correct?
It is correct to say that the increase in thermal energy of one object equals the decrease in thermal energy of the other - not temperature. The statement is correct when the hot and warm objects are the same material and same mass.
Why is it incorrect to say that, when a hot object warms a cold object, temperature flows between them?
It is thermal energy that flows - heat. It is therefore correct to say that thermal energy flows between the objects.
Consider two equal-size rooms connected by an open door. One room is maintained at a higher temperature than the other one. Which room contains more air molecules?
More molecules are in the cooler room. The greater number of slower-moving molecules there produce air pressure at the door equal to the fewer number of faster-moving molecules in the warmer room.
Wrap a fur coat around a thermometer. Will its temperature rise?
No, the coat is not a source of heat, but merely keeps the thermal energy of the wearer from leaving rapidly.
Ceiling fans can make you feel cooler in a warm room. Do ceiling fans reduce room temperature?
No. However ceiling fans can create a "wind chill" effect that can make you feel up to five degrees cooler. Ceiling fans do not reduce the temperature in the room, but merely circulate air, making you feel cooler.
Show that 5000 cal are required to increase the temperature of 50 g of water from 0 C to 100 C. The specific heat capacity for water is 1 cal/gC.
Q= cm change in T = (1 cal/gC)(50 g) (100C-0C) = 5,000 cal.
In which form of heat transfer is a medium not required?
Radiation requires no medium for transfer.
When an air molecule is hit by an approaching, faster-moving molecule, does its rebound speed increase or decrease? How about when it hits a receding molecule?
Rebound speed increases; rebound speed decreases.
How are the speeds of molecules of air affected when the air is compressed by the action of a tire pump?
Speeds are increased with compression.
Text a letter to grandma and share your knowledge about why air temperature is cooler on clear nights and warmer on cloudy nights.
Tell grandma how clouds re-radiate terrestrial energy back to Earth's surface. Examples abound about all objects both emitting and absorbing energy.
Since all objects emit energy to their surroundings, why don't the temperatures of all objects continuously decrease?
Temperatures don't continuously decrease because all objects are also absorbing radiant energy.
In a 25 C room, hot coffee in a vacuum flask cools from 75C to 50C in 8 hours. Explain why your prediction is that its temperature after another 8 hours will be 37.5C.
The coffee decreases 25C in temperature in 8 hours. Newton's law of cooling tells us that its rate of cooling is proportional to the temperature difference. So when the temp diff is half as great, the rate of cooling will be half as great. Hence the coffee will lose 12.5 degrees in another 8 hours, half as much as the first 8 hours, cooling from 50 C to 37.5 C.
Does wood have a low conductivity if it is very hot- that is, in the stage of smoldering, red-hot coals? Could you safely walk across a bed of red-hot wooden coals with bare feet? Although the coals are hot, does much heat conduct from them to your feet if you step quickly? Could you do the same on red-hot iron coals? Explain.
The conductivity of wood is relatively low whatever the temperature - even in the stage of red-hot coals. You can safely walk barefoot across red hot wooden coals if you step quickly b/c very little heat is conducted to your feet. B/c of the poor conductivity of the coals, energy from within the coals does not readily replace the energy that transfers to your feet.
The heat of volcanoes and natural hot springs comes from trace amounts of radioactive minerals in common rock in Earth's interior. Why isn't the same kind of rock at Earth's surface warm to the touch?
The energy given off by a rock at the Earth's surface transfers to the surroundings practically as fast as it is generated. Hence there isn't the buildup of energy that occurs in the Earth's interior.
Solid uranium can be converted chemically to uranium fluoride, UF6, which can be cooked up to a dense vapor that diffuses through a porous barrier. Which is likely to diffuse at a greater rate: a gas with isotope U-235 or U-238?
The faster molecules with U-235 will diffuse faster than the slower molecules with heavier U-238.
Turn a common incandescent lamp on and off quickly while holding your hand a few inches from the bulb. You feel its heat, but when you touch the bulb, it isn't hot. Explain this in terms of radiant energy and the bulb's transparency.
The heat you feel is radiant energy, which passes through the glass, without heating the glass.
Discuss in terms of physics why restaurants serve bakes potatoes wrapped in aluminum foil.
The main reason for serving potatoes wrapped in aluminum foil is to increase the time that the potatoes remain hot after being removed from the oven. Heat transfer by radiation is minimized as radiation from the potatoes is internally reflected, and heat transfer by convection is minimized as circulating air cannot make contact with the shielded potatoes. The foil also serves to retain moisture.
A snow-making machine used for ski areas blows a mixture of compressed air and water through a nozzle. the temperature of the mixture may initially be well above the freezing temperature of water, yet crystals of snow are formed as the mixture is ejected from the nozzle. Explain how this happens.
The mixture expands when it is ejected from the nozzle, and therefore cools. At the freezing temperature of 0C, ice forms.
In a still room, smoke from a candle will sometimes rise only so far, not reaching the ceiling. Explain why.
The smoke, like hot air, is less dense than the surroundings and is buoyed upward. It cools with contact with the surrounding air and becomes more dense. When its density matches that of the surrounding air, its buoyancy and weight balance and rising ceases.
If you were caught in freezing weather with only your own body heat as a source, discuss why you'd be warmer in an Arctic igloo or in a wooden shack.
The snow and ice of the igloo is a better insulator than wood. You would be warmer in the igloo than the wooden shack.
Wrap a piece of paper around a thick metal bar and place it in a flame. Notice that the paper will not catch fire. Can you explain this in terms of the conductivity of the metal bar? Paper generally will not ignite until its temperature reaches about 230 C.
This activity shows that metal is a good conductor of heat. Paper in a flame by itself easily reaches ingnition temperature and catches fire. But this ignition temperature isn't reached when the paper is wrapped around a thick metal bar that absorbs energy from the flame, which then isn't absorbed by the paper.
When there is morning frost on the grass in an open park, why is frost unlikely to be found on the ground directly beneath park benches?
Under open skies, the ground radiates upward but the sky radiates almost nothing back down. Under the benches, downward radiation of the benches decreases the net radiation from the ground, resulting in warmer ground and likely, no frost.
Distinguish between weather and climate.
Weather is the state of the atmosphere at a particular time and place. Climate is the weather pattern over broader regions and longer times.
Why is whitewash sometimes applied to the glass of florists' greenhouses in the summer?
When it is desirable to reduce the radiation that comes into a greenhouse, whitewash is applied to the glass to simply reflect much of the incoming radiation. Energy reflected is energy not absorbed.
If we warm a volume of air, it expands. Does it then follow that if we expand a volume of air, it warms? Explain.
When we warm a volume of air, we add energy to it. When we expand a volume of air, we normally take energy out of it (because the expanding air does work on its surroundings). So the conditions are quite different and the results will be different. Expanding a volume of air actually lowers its temperature.
Wood conducts heat very poorly - it has a very low conductivity. Does wood still have a low conductivity if it is hot? Could you quickly and safely grasp the wooden handle of a pan from a hot oven with your bare hand? Although the pan handle is hot, is much heat conducted from it to your hand if you grasp it briefly? Why would it be a poor idea to do the same with an iron handle? Explain.
Wood is a poor conductor whatever the temperature, so you can safely grab a pan by its wooden handle for a short time. Very little heat will be conducted to your hand. Touching the iron part of the pan is another story, for then heat is readily conducted to your hand.
