Exam 19: Heat Engines and Refrigerators

A Carnot engine operating between a reservoir of liquid mercury at its melting point (233 K) and a colder reservoir extracts 10.0 J of heat from the mercury and does 8.0 J of work during each cycle. What is the temperature of the colder reservoir?

A hot piece of iron is thrown into the ocean and its temperature eventually stabilizes. Which of the following statements concerning this process is correct? (There may be more than one correct choice.)

A real (non-Carnot) heat engine, operating between heat reservoirs at temperatures of 650 K and 270 K, performs 4.3 kJ of net work and rejects 8.0 kJ of heat in a single cycle. The thermal efficiency of this heat engine is closest to

The second law of thermodynamics leads us to conclude that

A system consists of two very large thermal reservoirs in contact with each other, one at temperature 300°C and the other at temperature 200°C. When 600 J of heat transfers from the 300°C reservoir to the 200°C reservoir, what is the change in entropy of this system?

A 810-g quantity of ethanol, in the liquid state at its melting point of -114°C, is frozen at atmospheric pressure. The heat of fusion of ethanol is 1.04 × 10⁵ J/kg, the molecular mass is 46.1 g/mol, and the ideal gas constant is R = 8.314 J/(mol∙K). The change in the entropy of the ethanol as it freezes is closest to

A heat engine takes 2.0 moles of an ideal gas through the reversible cycle abca, on the pV diagram shown in the figure. The path bc is an isothermal process. The temperature at c is 820 K, and the volumes at a and c are 0.010 m³ and 0.16 m³, respectively. The molar heat capacity at constant volume, of the gas, is 37 J/mol·K, and the ideal gas constant is R = 8.314 J/(mol∙K). The thermal efficiency of the engine is closest to

A coal-fired plant generates 600 MW of electric power. The plant uses 4.8 × 10⁶ kg of coal each day. The heat produced by the combustion of coal is 3.3 × 10⁷ J/kg. The steam that drives the turbines is at a temperature of 300°C, and the exhaust water is at 37°C. (a) What is the overall efficiency of the plant for generating electric power? (b) What is the maximum efficiency that this plant could possibly have using the same temperature extremes that it presently uses? (c) How much thermal energy is exhausted each day by this plant?

An ideal reversible refrigerator keeps its inside compartment at 9.0°C. What is the high temperature, T_h, needed to give this refrigerator a coefficient of performance of 3.7?

A perfect Carnot engine operates between the temperatures of 300K and 700K, drawing 60 kJ of heat from the 700K reservoir in each cycle. How much heat is dumped into the 300K reservoir in each cycle?