Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | One often hears about heat transfer by convection in gases and liquids, but not in solids. Why?

| 2 | A heavy drape, hung close to a cold window, reduces heat loss considerably by interfering primarily with one of the three processes of heat transfer. Explain which one.

| 3 | The windchill factor is a term used by weather forecasters. Roughly speaking, it refers to the fact that you feel colder when the wind is blowing than when it is not, even though the air temperature is the same in either case. Which of the three processes for heat transfer plays the principal role in the windchill factor? Explain your reasoning.

| 4 | Often, motorists see the following warning sign on bridges: “Caution–Bridge surface freezes before road surface.” Account for the warning in terms of heat transfer processes. Note that, unlike the road, a bridge has both surfaces exposed to the air.

| 5 | ssm A piece of Styrofoam and a piece of wood are joined together to form a layered slab. The two pieces have the same thickness and cross-sectional area. The exposed surfaces have constant temperatures. The temperature of the exposed Styrofoam surface is greater than the temperature of the exposed wood surface. Is the temperature of the Styrofoam–wood interface closer to the larger or smaller of the two temperatures? Give your reasoning, using Equation 13.1 and data from Table 13-1.

| 6 | One way that heat is transferred from place to place inside the human body is by the flow of blood. Which one of the three heat transfer processes best describes this action of the blood? Justify your answer.

| 7 | Some animals have hair strands that are hollow, air-filled tubes. Other animals have hair strands that are solid. Which kind of hair would be more likely to give an animal an advantage for surviving in very cold climates? Why?

| 8 | A poker used in a fireplace is held at one end, while the other end is in the fire. Why are pokers made of iron rather than copper? Ignore the fact that iron may be cheaper and stronger.

| 9 | In Alaska, a lack of snow allowed the ground to freeze down to a depth of about one meter, causing buried water pipes to freeze and burst. Why did a lack of snow lead to this situation?

| 10 | Concrete walls often contain steel reinforcement bars. Does the steel enhance or degrade the insulating value of the concrete? Explain.

| 11 | Grandma says that it is quicker to bake a potato if you put a nail into it. In fact, she is right. Justify her baking technique in terms of one of the three processes of heat transfer.

| 12 | Several days after a snowstorm, the roof on a house is uniformly covered with snow. On a neighboring house, however, the snow on the roof has completely melted. Which house is probably better insulated? Give your reasoning.

| 13 | ssm One car has a metal body, while another has a plastic body. On a cold winter day these cars are parked side by side. If you put a bare hand on each car, the metal body feels colder. Why?

| 14 | A high-quality pot is designed so heat can enter readily and be distributed evenly, while the rate of energy loss from the pot is kept to a minimum. Many high-quality pots have copper bases and polished stainless steel sides. Based on conduction and radiation principles, explain why this design is better than all-copper or all-steel units.

| 15 | Two objects have the same size and shape. Object A has an emissivity of 0.3, and object B has an emissivity of 0.6. Each radiates the same power. Is the Kelvin temperature of A twice that of B? Give your reasoning.

| 16 | ssm Older homes are often heated by metal devices called “radiators.” These devices are hollow and heated by hot water or steam that is circulated through them. They can generally be found located against the walls in each room. Consider the following curious fact: “radiators” are often painted with a highly reflective silvery paint, which enables them to become hotter than they would be in the absence of the paint and thereby heat the room more effectively. (a) Why does the paint enable a radiator to become hotter? (b) Of the three mechanisms for heat transfer, which one is the primary mechanism by which “radiators” heat a room? Explain.

| 17 | Two strips of material, A and B, are identical, except they have emissivities of 0.4 and 0.7, respectively. The strips are heated to the same temperature and have a red glow. A brighter glow signifies that more energy per second is being radiated. Which strip has the brighter glow? Explain.

| 18 | A pot of water is being heated on an electric stove. The diameter of the pot is smaller than the diameter of the heating element on which the pot rests. The exposed outer edges of the heating element are glowing cherry red. When you lift the pot, you see that the part of the heating element beneath it is not glowing cherry red, indicating that it is cooler than the outer edges. Why are the outer edges hotter?

| 19 | To keep your hands as warm as possible during skiing, should you wear mittens or gloves? (Mittens, except for the thumb, do not have individual finger compartments.) Give a reason for your answer.

| 20 | Two identical hot cups of cocoa are sitting on a table. One has a metal spoon in it and one does not. After five minutes, which cup is cooler? Explain in terms of heat transfer processes.

| 21 | (a) Would a hot solid cube cool more rapidly if it were left intact or cut in half? Explain your answer in terms of one or more of the three heat transfer processes. (b) Using reasoning similar to that used in answering part (a), decide which cools faster after cooking, one pound of wide and flat lasagna noodles or one pound of spaghetti noodles. Assume that both kinds of noodles are made from the same pasta and start out with the same temperature.

| 22 | One day during the winter the sun has been shining all day. Toward sunset a light snow begins to fall. It collects without melting on a cement playground, but it melts immediately upon contact with a black asphalt road adjacent to the playground. Account for the fact that the snow collects in one place but not in the other.

| 23 | If you were stranded in the mountains in cold weather, it would help to minimize energy losses from your body by curling up into the tightest ball possible. Which of the factors in Equation 13.2 are you using to the best advantage by curling into a ball? Why?

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