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Chapter 21: Q 3 (page 593)

Could you have a heat engine with 畏>1? Explain.

Short Answer

Expert verified

A heat engine withn>1is No.

Step by step solution

01

Step :1 Introduction 

A heat engine is a machine that turns heat into mechanical energy. It receives heat from a reservoir, then performs some work, such as moving a piston or raising a weight, before releasing heat energy into the sink.

02

Step ;2 Explanation 

Let us derive an expression for the efficiency of a heat engine. We can define heat engine efficiency as:

畏=WQ1

W

= Work done by the engine

Q1

= Heat taken from the source

03

Step :3 Substitution 

Since畏=Wout/QinandWout<Qinalways, hence畏<1always.

No

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Most popular questions from this chapter

A heat engine using 120mgof helium as the working substance follows the cycle shown in FIGURE P21.61.

a. Determine the pressure, temperature, and volume of the gas at points 1,2, and 3.

b. What is the engine's thermal efficiency?

c. What is the maximum possible efficiency of a heat engine that operates between TmaxandTmin?

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 螖Eth 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?

FIGURE P21.60is the pVdiagram of Example 21.2, but now the device is operated in reverse.

a. During which processes is heat transferred into the gas?

b. Is thisQH, heat extracted from a hot reservoir, or QC, heat extracted from a cold reservoir? Explain.

c. Determine the values ofQHandQC.

Hint: The calculations have been done in Example 21.2and do not need to be repeated. Instead, you need to determine which processes now contribute to QHand which to QC.

d. Is the area inside the curve Winor Wout? What is its value?

e. The device is now being operated in a ccw cycle. Is it a refrigerator? Explain.

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鈫�2.

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

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

What are (a) the heat extracted from the cold reservoir and (b) the coefficient of performance for the refrigerator shown in Figure Ex-21.20?
See all solutions

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