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Chapter 21: Q. 9 (page 594)

Do the energy-transfer diagrams inFIGUREQ21.9represent possible refrigerators? If not, what is wrong?

Short Answer

Expert verified

a. The figure is not refrigerator ,because this engine dumps heat in the cold reservoir which is not a refrigerator does.

b. In figure K<Kc,so it is refrigerator engine.

c. In figure K>Kc,so it is not refrigerator engine.

Step by step solution

01

Possible of refrigerators for part a

a.

WorkWinis used to compress a coolant and raise its temperature over that of the surrounding environment.

Win=QH−QC

Consider the figure of part a,

20=(10-10)

20≠0

02

Possible of refrigerators for part b

b.

Consider the part b of figure,


The efficiency of normal refrigerator engine is,

K=QCWin

K=10/20=0.5

The efficiency of carnot refrigerator engine is,

Kc=T1T2-T1

Kc=300/300=1

So it's a possibility.

03

 Possible of refrigerators for part c

c.

Consider the figure,


The efficiency of normal refrigerator engine is,

K=QCWin

K=20/10=2

The efficiency of carnot refrigerator engine is,

Kc=T1T2-T1

Kc=300/300=1

So it's a possibility.

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Most popular questions from this chapter

In Problems 65through 68you 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.

0.20=1-QC/QH

Wout=QH-QC=20J

How much work is done per cycle by a gas following thePV trajectory of FIGURE EX21.11?

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 QH, 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, 100%conversion of work to heat. That is, 15kWof electric power (generated by doing work at the rate of 15kJ/sat the power plant) produces heat energy inside the home at a rate of 15kJ/s. Suppose that the neighbor’s home has a heat pump with a coefficient of performance of 5.0, a realistic value. Note that “what you get” with a heat pump is heat delivered, QH, so a heat pump’s coefficient of performance is defined asK=QH/Win.

a. How much electric power inkWdoes the heat pump use to deliver 15kJ/sof heat energy to the house?

b. An average price for electricity is about 40MJper dollar. A furnace or heat pump will run typically 250 hours per month during the winter. What does one month’s heating cost in the home with a 15kW electric heater and in the home of the neighbor who uses a heat pump?

A Carnot heat engine operates between reservoirs at 182°Cand 0°C. If the engine extracts 25Jof energy from the hot reservoir per cycle, how many cycles will it take to lift a 10Kg mass a height of 10m?

Prove that the coefficient of performance of a Carnot refrigerator is KCarnot=TC/TH-TC.
See all solutions

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