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Chapter 21: Q. 1 (page 593)

In going fromito localid="1648790171714" fin each of the three processes of FIGURE Q21.1, is work done by the system localid="1648790168795" W<0,Ws>0is work done on the system , or is no net work done on the system localid="1648790176001" W>0,Ws<0or is no net work done?

Short Answer

Expert verified

a. Work done isW<0,Ws>0.

b. Work done isW>0,Ws<0.

c. Work done isW>0,Ws<0.

Step by step solution

01

Work done (part a)

(a) Area under curve is positive. Work is done on the system.

02

Work done (part b)

(b) Since system is compresses, hence work is done on the system.

03

Work done (part c)

(c) Work is done on the system overall as clear from the graph since more work is done on the system than by the system.

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Most popular questions from this chapter

FIGURE CP21.70shows two insulated compartments separated by a thin wall. The left side contains 0.060molof helium at an initial temperature of 600Kand the right side contains 0.030molof helium at an initial temperature of 300K. The compartment on the right is attached to a vertical cylinder, above which the air pressure is 1.0atm. A 10-cm-diameter,2.0kg piston can slide without friction up and down the cylinder. Neither the cylinder diameter nor the volumes of the compartments are known.

a. What is the final temperature?

b. How much heat is transferred from the left side to the right side?

c. How high is the piston lifted due to this heat transfer?

d. What fraction of the heat is converted into work?

A heat engine using a diatomic gas follows the cycle shown in FIGURE P21.56. Its temperature at point 1is 20°C.

a. Determine Ws,Q, and ΔEthfor each of the three processes in this cycle. Display your results in a table.

b. What is the thermal efficiency of this heat engine?

c. What is the power output of the engine if it runs at 500rpm?

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 heat engine using 1.0 mol of a monatomic gas follows the cycle shown in FIGURE P21.55. 3750 J of heat energy is transferred to the gas during process 1 -> 2.
a. Determine Ws, Q, and ΔEth for each of the four processes in this cycle. Display your results in a table.
b. What is the thermal efficiency of this heat engine?

A heat engine using 1.0molof a monatomic gas follows the cycle shown in FIGURE P21.55. 3750Jof heat energy is transferred to the gas during process 1→2.

a. Determine Ws,Q, and ΔEthfor each of the four processes in this cycle. Display your results in a table.

b. What is the thermal efficiency of this heat engine?
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