Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | For the highest accuracy, would you choose an aluminum or a steel tape rule for year-round outdoor use? Why?

| 2 | The first international standard of length was a metal bar kept at the International Bureau of Weights and Measures. One meter of length was defined to be the distance between two fine lines engraved near the ends of the bar. Why was it important that the bar be kept at a constant temperature?

| 3 | Conceptual Example 7 provides background for this question. A circular hole is cut through a flat aluminum plate. A spherical brass ball has a diameter that is slightly smaller than the diameter of the hole. The plate and the ball have the same temperature at all times. Should the plate and ball both be heated or both be cooled to prevent the ball from falling through the hole? Give your reasoning.

| 4 | For added strength, many highways and buildings are constructed with reinforced concrete (concrete that is reinforced with embedded steel rods). Table 12-1 shows that the coefficient of linear expansion for concrete is the same as that for steel. Why is it important that these two coefficients be the same?

| 5 | At a certain temperature, a rod is hung from an aluminum frame, as the drawing shows. A small gap exists between the rod and the floor. The frame and rod are heated uniformly. Explain whether the rod will ever touch the floor, assuming that the rod is made from (a) aluminum and (b) lead.

| 6 | A simple pendulum is made using a long thin metal wire. When the temperature drops, does the period of the pendulum increase, decrease, or remain the same? Account for your answer.

| 7 | One type of cooking pot is made from stainless steel and has a copper coating over the outside of the bottom. At room temperature the bottom of this pot is flat, so that the pot sits flat on a kitchen counter top. When heated, however, this pot can be made to rock back and forth on the counter top with a gentle touch, because the bottom of the pot is bowed outward. With the aid of Table 12-1 account for the outward bowing.

| 8 | ssm A hot steel ring fits snugly over a cold brass cylinder. The temperatures of the ring and cylinder are, respectively, above and below room temperature. Account for the fact that it is nearly impossible to pull the ring off the cylinder once the assembly has reached room temperature.

| 9 | For glass baking dishes, Pyrex glass is used instead of common glass. A cold Pyrex dish taken from the refrigerator can be put directly into a hot oven without cracking from thermal stress. A dish made from common glass would crack. With the aid of Table 12-1, explain why Pyrex is better in this respect than common glass.

| 10 | Suppose liquid mercury and glass both had the same coefficient of volume expansion. Explain why such a mercury-in-glass thermometer would not work.

| 11 | Is the buoyant force provided by warm water (above 4 °C) greater than, less than, or equal to that provided by cold water (also above 4 °C)? Explain your reasoning.

| 12 | When the bulb of a mercury-in-glass thermometer is inserted into boiling water, the mercury column first drops slightly before it begins to rise. Account for this phenomenon. (Hint: Consider what happens initially to the glass.)

| 13 | Two different objects are supplied with equal amounts of heat. Give the reason(s) why their temperature changes would not necessarily be the same.

| 14 | Two objects are made from the same material. They have different masses and temperatures. If the two are placed in contact, which object will experience the greater temperature change? Explain.

| 15 | ssm Two identical mugs contain hot coffee from the same pot. One mug is full, while the other is only one-quarter full. Sitting on the kitchen table, which mug stays warmer longer? Explain.

| 16 | Bicyclists often carry along a water bottle on hot days. When the bottle is wrapped in a wet sock, the water temperature stays near 75 °F for three or four hours, even when the air temperature is near 100 °F. Why does the water stay cool?

| 17 | To help lower the high temperature of a sick patient, an alcohol rub is sometimes used. Isopropyl alcohol is rubbed over the patient’s back, arms, legs, etc., and allowed to evaporate. Why does the procedure work?

| 18 | Your head feels colder under an air-conditioning vent when your hair is wet than when it is dry. Why?

| 19 | Suppose the latent heat of vaporization of H2O were one-tenth its actual value. (a) Other things being equal, would it take the same time, a shorter time, or a longer time for a pot of water on a stove to boil away? (b) Would the evaporative cooling mechanism of the human body be as effective? Account for both answers.

| 20 | Fruit blossoms are permanently damaged when the temperature drops below about −4 °C (a “hard freeze”). Orchard owners sometimes spray a film of water over the blossoms to protect them when a hard freeze is expected. From the point of view of phase changes, give a reason for the protection.

| 21 | A camping stove is used to boil water on a mountain. Does it necessarily follow that the same stove can boil water at lower altitudes, such as at sea level? Provide a reason for your answer.

| 22 | If a bowl of water is placed in a closed container and water vapor is pumped away rapidly enough, the remaining liquid will turn to ice. Explain why the ice appears.

| 23 | Medical instruments are sterilized under the hottest possible temperatures. Explain why they are sterilized in an autoclave, which is a device that is essentially a pressure cooker and heats the instruments in water under a pressure greater than one atmosphere.

| 24 | ssm A bottle of carbonated soda is left outside in subfreezing temperatures, although it remains in the liquid form. When the soda is brought inside and opened, it immediately freezes. Explain why this could happen.

| 25 | A bowl of water is covered tightly and allowed to sit at a constant temperature of 23 °C for a long time. What is the relative humidity in the space between the surface of the water and the cover? Justify your answer.

| 26 | Is it possible for dew to form on Tuesday night and not on Monday night, even though Monday night is the cooler night? Incorporate the idea of the dew point into your answer.

| 27 | Two rooms in a house have the same temperature. One of the rooms contains an indoor swimming pool. On a cold day the windows of one of the rooms are “steamed up.” Which room is it? Explain.

| 28 | A jar is half filled with boiling water. The lid is then screwed on the jar. After the jar has cooled to room temperature, the lid is difficult to remove. Why?

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