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Ch 21: Heat Engines and Refrigerators
Chapter 21, Problem 21

What are (a) the heat extracted from the cold reservoir and (b) the coefficient of performance for the refrigerator shown in FIGURE EX21.21?

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Heat Transfer

Heat transfer refers to the movement of thermal energy from one object or system to another due to a temperature difference. In the context of refrigeration, heat is extracted from the cold reservoir, which is the area being cooled, and transferred to the hot reservoir, typically the surrounding environment. Understanding the principles of heat transfer is essential for analyzing the efficiency and operation of refrigeration systems.
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Refrigerator Coefficient of Performance (COP)

The Coefficient of Performance (COP) of a refrigerator is a measure of its efficiency, defined as the ratio of the heat removed from the cold reservoir to the work input required to remove that heat. A higher COP indicates a more efficient refrigerator, as it means more heat is extracted for each unit of work done. This concept is crucial for evaluating the performance of refrigeration systems and optimizing their design.
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Thermodynamic Cycles

Thermodynamic cycles describe the series of processes that a refrigerant undergoes in a refrigeration system, including compression, condensation, expansion, and evaporation. These cycles are fundamental to understanding how refrigerators operate, as they illustrate how energy is transferred and transformed within the system. Familiarity with these cycles helps in analyzing the performance and efficiency of refrigeration systems.
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Related Practice
Textbook Question
A Carnot heat engine operates between reservoirs at 182℃ and 0℃. If the engine extracts 25 J of energy from the hot reservoir per cycle, how many cycles will it take to lift a 10 kg mass a height of 10 m?
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Textbook Question
A typical coal-fired power plant burns 300 metric tons of coal every hour to generate 750 MW of electricity. 1 metric ton = 1000 kg. The density of coal is 1500 kg/m³ and its heat of combustion is 28 MJ/kg. Assume that all heat is transferred from the fuel to the boiler and that all the work done in spinning the turbine is transformed into electric energy. a. Suppose the coal is piled up in a 10 m ✕ 10 m room. How tall must the pile be to operate the plant for one day?
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Textbook Question
An air conditioner removes 5.0 x 10⁵ J/min of heat from a house and exhausts 8.0 x 10⁵ J/min to the hot outdoors. b. What is the air conditioner's coefficient of performance?
280
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Textbook Question
A freezer with a coefficient of performance 30% that of a Carnot refrigerator keeps the inside temperature at -22℃ in a 25℃ room. 3.0 L of water at 20℃ are placed in the freezer. How long does it take for the water to freeze if the freezer's compressor does work at the rate of 200 W while the water is freezing?
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Textbook Question
The heat engine shown in FIGURE P21.62 uses 2.0 mol of a monatomic gas as the working substance. c. What is the engine's thermal efficiency?
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Textbook Question
A heat engine with 0.20 mol of a monatomic ideal gas initially fills a 2000 cm³ cylinder at 600 K. The gas goes through the following closed cycle: Isothermal expansion to 4000 cm³. Isochoric cooling to 300 K. Isothermal compression to 2000 cm³. Isochoric heating to 600 K. How much work does this engine do per cycle and what is its thermal efficiency?
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