Homework Assignment 23

E. 2 No, assuming the coat is at the same temperature as the room there will be no change in the thermometer. Coats keep us warm by preventing the loss of body heat.

E. 4. Heat flow requires a temperature difference. If the blocks are at the same temperature as your hand, there will be no heat flow and yhey will feel neither hot nor cold.

E. 6 The aluminum foil slows the rate at which heat is lost by the potato. This increases the time the potato will remain hot. after being removed from the oven. Heat from the potato is reflected internally which minimizes heat loss by radiation. Circulating air does not make contact with the potato reducing heat loss by conduction and convection.

E. 10. Ice and snow are better insulators than wood. Therefore, you would be warmer in the igloo than in a wooden shack.

E. 13. When you hold your fingers close to the side of a candle flame, the air between them and the flame serve as insulation. When you hold your fingers above the flame, convection currents transfer the heat and you can burn your fingers.

E. 19. Molecules in a gas move randomly and there are many collisions in every possible direction. As a result of these collisions, some molecules speed up and some slow down. Hence, some molecules are moving faster than the average speed and some slower. In the absence of a gain or loss of heat the average kinetic energy (T) the same.

E. 32. As the mixture leaves the nozzle it expands and some of it evaporates. Expansion and evaporation are both cooling processes. When the mixture cools 0°C snow and ice are formed.
 
 
 
 
 
 
 
 

Homework Assignment 23

E. 9. Snow is a very poor conductor of heat. A stone, such as granite, is a much better heat conductor. If the ground is relatuvely warm, heat may be conducted by a gravestone and melt the snow in contact with the stone.

E. 12. Heat is conducted through the paper to the water and the water absorbs the energy. The water temperature will not rise above 212° F. This is well below the ignition temperature of paper (about 451° F). If you read Fahrenheit 451, a novel about book burning, you will recognize this temperature.

E. 24. Molecules in a gas are pulled down by gravity. Those molecules moving directly up or down are slowed down or speeded up but their direction is not changed. Molecules in a gas at room temperature move quite rapidly and the distance between collisions is so small that we may treat their motion as linear.

P. 1. Initial temperature is 80° in a room that is 20° C. The temperature difference is 60°. and the temperature of the water changes one degree in 15 seconds.

According to Newton’s law of cooling the rate of cooling is proportional to temperature differance. When the temperature of the water is 50 degrees, the temperature difference is 30°, half of that in the first case. It will take twice as long - 30 seconds- for it to change the same amount one degree.

When the water temperature is 40°, the temperature difference is 20° one-third of the difference in the first case. Cooling one degree will take three times as long or 35 seconds.

P. 3. The collector receives 200 W/m². It is 10% efficient so 20 W/m² is available fur use.

(3000 W)/( 20 W/m²) = 150 m²

This is area is equivalent to about 20 ft by 40 ft . It would fit in an average yard.