MisConceptual Questions Chapter 15

A heat engine operates between a high temperature of about 600°C and a low temperature of about 300°C. What is the maximum theoretical efficiency for this engine? (a) 100%. (b) 66%. (c) 50%. (d) 34%. (e) Cannot be determined from the given information.

(d) 34%.

On a very hot day, could you cool your kitchen by leaving the refrigerator door open? (a) Yes, but it would be very expensive. (b) Yes, but only if the humidity is below 50%. (c) No, the refrigerator would exhaust the same amount of heat into the room as it takes out of the room. (d) No, the heat exhausted by the refrigerator into the room is more than the heat the refrigerator takes out of the room.

(d) No, the heat exhausted by the refrigerator into the room is more than the heat the refrigerator takes out of the room. A common misconception in this situation is not realizing that a heat cycle running in reverse, like a refrigerator, must have a high-temperature exhaust. Furthermore, that high-temp exhaust is the sum of the heat removed from the inside of the refrigerator, plus the work done by the refrigerator's compressor.

Which of the following possibilities could increase the efficiency of a heat engine or an internal combustion engine? -Increase the temperature of the hot part of the system and reduce the temperature of the exhaust. -Increase the temperatures of both the hot part and the exhaust part of the system by the same amount. -Decrease the temperatures of both the hot part and the exhaust part of the system by the same amount. -Decrease the temperature of the hot part and increase the temperature of the exhaust part by the same amount. -None of the above; only redesigning the engine or using better gas could improve the engines efficiency.

-Increase the temperature of the hot part of the system and reduce the temperature of the exhaust. -Decrease the temperatures of both the hot part and the exhaust part of the system by the same amount. The maximum efficiency of an engine is given by equation e ideal = 1−TL/TH

Which statement is true regarding the entropy change of an ice cube that melts? (a) Since melting occurs at the melting point temperature, there is no temperature change so there is no entropy change. (b) Entropy increases. (c) Entropy decreases.

c. Entropy increases. Heat must be added to the ice cube to melt it. The change in entropy is the ratio of the heat added to the temperature of the ice cube. Since heat is absorbed in the process, the entropy increases.