Physics 10 Homework Solutions

Chapter 16

(1) The three common ways in which heat is transferred are conduction, convection, and radia-tion. Conduction is the usual way that heat is conducted through solids and involves hotatoms— those with large vibrational motions— colliding with cold atoms—those with smallvibrational motions—and heating them up, that is, increasing their vibrational motions. Con-vection is the usual way that heat is conducted in fluids and involves the movement of hotatoms from one location to another. Radiation is the way that heat is transferred through avacuum and involve energy carried by electromagnetic radiation.

(2) The poorest conductors of heat are gases. The materials mentioned in this question are goodinsulators because they contain pockets of air. Among the warmest jackets made are thosecontaining goose down, which contains pockets of air. I such a jacket gets wet, though, thegoose down collapses, becoming a much better conductor of heat. You will not stay warm in awet goose down jacket.

(3) Since the product of the frequency of a wave and its wavelength is equal to the speed of thewave is equal to the speed of the wave, increasing the frequency of a wave will cause its wave-length to decrease. Thus, a high-frequency wave will have a short wavelength.

(4) We must make some assumptions about the temperature of the thermometer and that of thefur coat. Clearly, if the fur coat is initially hotter than the thermometer, it will cause the tem-perature of the thermometer to rise when the two come into contact. Similarly, the temperatureof the thermometer will drop if the fur coat is initially colder than the thermometer. If the coatand thermometer have the same temperature initially, no change will occur. Note, though, thatthe same is not true if a person puts the coat on. Suppose the coat starts off at a lower tempera-ture than the person, perhaps because both are out in the snow. The person feels chilly and isprobably shivering. When the person puts on the coat, he or she will initially feel even colder.However, the coat is designed to be an insulator. Thus, the rate at which the person loses heatto the environment is reduced. By the act of living, a person generates heat. Since less of thisheat is now being lost to the environment, both the (inside) of the coat and the person willeventually become warmer than either’s initial temperature.

(5) A body feels cold if its average energy per molecule is less than the average energy permolecule in your hand. This is, in fact, what we mean when we say that the temperature of onebody is less than that of another. Whether a body is hot or cold, then, depends on the presenceor absence of internal energy. It is energy and not its absence that flows. Thus, it is not properto say the cold is conducted. In the case of the nail against the ice, heat is conducted from youhand through the nail to the ice.

(6) The specific heat of water is large, the largest of all common materials. This means that theland will change its temperature by a greater amount than the sea when conditions change.Suppose that by dawn, the land and the sea have come to the same temperature. As the Sunrises, the temperatures of both the land and the sea will rise, but that of the land will risemore. The land becomes hotter than the sea. The air above the land will warm faster than thatover the sea. It will rise, and cool air from the sea will flow in to replace it. This keeps the landnear a large body of water cooler than that found away from such a body of water. The reversehappens at night. At night, the land cools faster than the sea so the hotter air above the searises. This draws the cooler air of the land, tending to keep it warmer at night. The tempera-ture difference in San Francisco between night and day rarely exceeds ten to twenty degrees(EF), while that over the desert southwest is commonly fifty degrees or more.