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Chapter 21: Q. 1 (page 594)

A heat engine does 200Jof work per cycle while exhausting 400Jof waste heat. What is the engine’s thermal efficiency?

Short Answer

Expert verified

The thermal efficiency is13≈33%

Step by step solution

01

Definition of efficiency   

For a heat engine, thermal efficiency is the ratio of net heat output (for heating) to the energy input.

.

02

Mathematical expression for efficiency

η=outputenergyinputenergy×100...................(1)

03

Step :3  Sum of work done and waste heat

Output energy = 200 J .

Exhausted energy = 400 J .

Input energy = output energy + exhausted energy = 200 J + 400 J

input energy = 600 J

Substitute 200 J for output energy and 600 J for input energy

in equation (1)

η=200J200J+400J

=13≈33%

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Most popular questions from this chapter

A nuclear power plant generates 3000MWof heat energy from nuclear reactions in the reactor’s core. This energy is used to boil water and produce high-pressure steam at 300°C. The steam spins a turbine, which produces 1000MWof electric power, then the steam is condensed and the water is cooled to 25°Cbefore starting the cycle again.

a. What is the maximum possible thermal efficiency of the power plant?

b. What is the plant’s actual efficiency?

c. Cooling water from a river flows through the condenser (the low-temperature heat exchanger) at the rate of 1.2*108L/h ( ≈30 million gallons per hour). If the river water enters the condenser at 18°C, what is its exit temperature?

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 Carnot engine operates between temperatures of 5°Cand 500°C. The output is used to run a Carnot refrigerator operating between -5°C and 25°C. How many joules of heat energy does the refrigerator exhaust into the room for each joule of heat energy used by the heat engine?

What are (a)Woutand QHand (b)the thermal efficiency for the heat engine shown inFIGUREEX21.13?

At what pressure ratio tyyuudoes a Brayton cycle using a monatomic gas have an efficiency of 50 ?
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