23. The Second Law of Thermodynamics

Heat Engines & PV Diagrams

23. The Second Law of Thermodynamics

Heat Engines & PV Diagrams

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Calculating Work in Heat Engines Using PV Diagrams

07:27

Calculating Work in Heat Engines Using PV Diagrams

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PHYS 40B: Heat Engines and Refrigerators

09:33

PHYS 40B: Heat Engines and Refrigerators

Heat Engines & Efficiency

Physics. Thermodynamics. Efficiency of the heat engine

Efficiency of Heat Engine p-V Graph

Video 3.5: PV diagrams and heat engines

Thermodynamics - PV Diagrams and Heat Engines

PV diagrams and heat engines

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Multiple Choice

The cycle in the figure shows four processes. Process a is an isobaric expansion at

330 kPa

. Process b is a constant volume reduction in pressure to

100 kPa

. Process c returns the gas to its original state where the volume is

100 {cm}^{3}

. If the gas does

15 J

of work each cycle, what is the maximum volume of the gas?

264

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Multiple Choice

The figure shows the cycle of a heat engine that uses a diatomic gas. The temperature at point 1 is

350 K

. What is the thermal efficiency of the engine?

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Multiple Choice

A heat engine follows the cycle shown below. What is the thermal efficiency (%) of this engine?

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

205

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Textbook Question

A Boeing 777 jet engine, the world's largest, has a power output of 82 MW. It burns jet fuel with an energy density of 43 MJ /kg. What is the engine's fuel consumption rate, in kg/s, if its efficiency is 30%?

362

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Textbook Question

The power output of a car engine running at 2400 rpm is 500 kW. How much (a) work is done and (b) heat is exhausted per cycle if the engine's thermal efficiency is 20%? Give your answers in kJ.

246

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Textbook Question

What are Wₒᵤₜ and QH and (b) the thermal efficiency for the heat engine shown in FIGURE EX21.14?

292

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Textbook Question

What are (a) the thermal efficiency and (b) the heat extracted from the hot reservoir for the heat engine shown in FIGURE EX21.16?

149

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Textbook Question

A 15 kW electric generator burns 1.2 gal of diesel fuel per hour. The energy density of diesel fuel is 140 MJ/gal. What is the generator's thermal efficiency?

156

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Textbook Question

A nuclear power plant generates 3000 MW of heat energy from nuclear reactions in the reactor's core. This energy is used to boil water and produce high-pressure steam at 300℃. The steam spins a turbine, which produces 1000 MW of electric power, then the steam is condensed and the water is cooled to 25℃ before starting the cycle again. b. What is the plant's actual efficiency?

288

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Textbook Question

FIGURE P21.57 shows the cycle for a heat engine that uses a gas having r = 1.25. The initial temperature is T₁ = 300 K, and this engine operates at 20 cycles per second. a. What is the power output of the engine?

169

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Textbook Question

The heat engine shown in FIGURE P21.62 uses 2.0 mol of a monatomic gas as the working substance. b. Make a table that shows ∆Eₜₕ, Wₛ, and Q for each of the three processes.

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Textbook Question

The heat engine shown in FIGURE P21.62 uses 2.0 mol of a monatomic gas as the working substance. c. What is the engine's thermal efficiency?

219

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Textbook Question

A heat engine with 0.20 mol of a monatomic ideal gas initially fills a 2000 cm³ cylinder at 600 K. The gas goes through the following closed cycle: Isothermal expansion to 4000 cm³. Isochoric cooling to 300 K. Isothermal compression to 2000 cm³. Isochoric heating to 600 K. How much work does this engine do per cycle and what is its thermal efficiency?

191

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Textbook Question

(a) At a steam power plant, steam engines work in pairs, the heat output of the first one being the approximate heat input of the second. The operating temperatures of the first are 750°C and 440°C, and of the second 415°C and 240°C. If the heat of combustion of coal is 2.8 x 10⁷ J/kg, at what rate must coal be burned if the plant is to put out 950 MW of power? Assume the efficiency of the engines is 65% of the ideal (Carnot) efficiency.

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Textbook Question

(II) A heat pump is used to keep a house warm at 22°C. How much work is required of the pump to deliver 2800 J of heat into the house if the outdoor temperature is

(a) 0°C, Assume a COP of 3.0.

COP = T_H / (T_H - T_L) .

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Textbook Question

(II) A heat pump is used to keep a house warm at 22°C. How much work is required of the pump to deliver 2800 J of heat into the house if the outdoor temperature is

(b) - 15°C? Assume a COP of 3.0.

COP = T_H / (T_H - T_L) .

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Textbook Question

(II) (a) What is the coefficient of performance of an ideal heat pump that extracts heat from 6°C air outside and deposits heat inside a house at 24°C?

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Textbook Question

(II) An ideal heat pump is used to maintain the inside temperature of a house at Tᵢₙ = 22°C when the outside temperature is Tₒᵤₜ . Assume the heat pump does work at a rate of 1700 W. Also assume that the house loses heat via conduction through its walls and other surfaces at a rate given by ( 650 W/C°) (Tᵢₙ - Tₒᵤₜ) .

(b) If the outside temperature is less than you just calculated, what happens?

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Textbook Question

(II) How much less per year would it cost a family to operate a heat pump that has a coefficient of performance of 2.9 than an electric heater that costs $2100 to heat their home for a year? If the conversion to the heat pump costs $15,000, how long would it take the family to break even on heating costs? How much would the family save in 20 years?

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Textbook Question

A car engine whose output power is 145 hp operates at about 15% efficiency. Assume the engine’s water temperature of 85°C is its cold-temperature (exhaust) reservoir and 495°C is its thermal “intake” temperature (the temperature of the exploding gas–air mixture).

(a) What is the ratio of its efficiency relative to its maximum possible (Carnot) efficiency?

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23. The Second Law of Thermodynamics

Heat Engines & PV Diagrams

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