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Chapter 21: Q. 67 (page 598)
In Problems through you are given the equation(s) used to solve a problem. For each of these, you are to
a. Write a realistic problem for which this is the correct equation(s).
b. Finish the solution of the problem.
a. The heat will be wanted to identify.
b. The heat value is.
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An air conditioner removes of heat from a house and exhausts to the hot outdoors.
a. How much power does the air conditioner’s compressor require?
b. What is the air conditioner’s coefficient of performance?
A heat engine running backward is called a refrigerator if its purpose is to extract heat from a cold reservoir. The same engine running backward is called a heat pump if its purpose is to exhaust warm air into the hot reservoir. Heat pumps are widely used for home heating. You can think of a heat pump as a refrigerator that is cooling the already cold outdoors and, with its exhaust heat , warming the indoors. Perhaps this seems a little silly, but consider the following. Electricity can be directly used to heat a home by passing an electric current through a heating coil. This is a direct, conversion of work to heat. That is, of electric power (generated by doing work at the rate of at the power plant) produces heat energy inside the home at a rate of . Suppose that the neighbor’s home has a heat pump with a coefficient of performance of , a realistic value. Note that “what you get” with a heat pump is heat delivered, , so a heat pump’s coefficient of performance is defined as.
a. How much electric power does the heat pump use to deliver of heat energy to the house?
b. An average price for electricity is about per dollar. A furnace or heat pump will run typically hours per month during the winter. What does one month’s heating cost in the home with a electric heater and in the home of the neighbor who uses a heat pump?
The power output of a car engine running at is . How much (a) work is done and (b) heat is exhausted per cycle if the engine's thermal efficiency is ? Give your answers in.
A Carnot heat engine with thermal efficiency is run backward as a Carnot refrigerator. What is the refrigerator’s coefficient of performance?
In Problems 67 through 70 you are given the equation(s) used to solve a problem. For each of these, you are to
a. Write a realistic problem for which this is the correct equation(s).
b. Draw a pVdiagram.
c. Finish the solution of the problem.
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