Download presentation

Presentation is loading. Please wait.

Published byYazmin Geels Modified over 6 years ago

1
Short Version : 19. 2 nd Law of Thermodynamics

2
19.1. Reversibility & Irreversibility Block slowed down by friction: irreversible Bouncing ball: reversible Examples of irreversible processes: Beating an egg, blending yolk & white Cups of cold & hot water in contact Spontaneous process: order disorder ( statistically more probable )

3
19.2. The 2 nd Law of Thermodynamics Heat engine extracts work from heat reservoirs. gasoline & diesel engines fossil-fueled & nuclear power plants jet engines Perfect heat engine: coverts heat to work directly. Heat dumped 2 nd law of thermodynamics ( Kelvin-Planck version ): There is no perfect heat engine. No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work.

4
Efficiency (any engine) (Simple engine) (any cycle) Hero Engine Stirling Engine

5
Carnot Engine 1.isothermal expansion: T = T h, W 1 = Q h > 0 2.Adiabatic expansion: T h T c, W 2 > 0 3.isothermal compression: T = T c, W 3 = Q c < 0 Adiabatic compression : T c T h, W 4 = W 2 < 0 Ideal gas: Adiabatic processes: A B: Heat abs. C D: Heat rejected: B C: Work done D A: Work done

6
Engines, Refrigerators, & the 2 nd Law Carnot’s theorem: 1.All Carnot engines operating between temperatures T h & T c have the same efficiency. 2.No other engine operating between T h & T c can have a greater efficiency. Refrigerator: extracts heat from cool reservoir into a hot one. work required

7
2 nd law of thermodynamics ( Clausius version ): There is no perfect refrigerator. perfect refrigerator: moves heat from cool to hot reservoir without work being done on it. No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature.

8
Perfect refrigerator Perfect heat engine Clausius Kelvin-Planck

9
Carnot engine is most efficient e Carnot = thermodynamic efficiency e Carnot e rev > e irrev Carnot refrigerator, e = 60% Hypothetical engine, e = 70%

10
19.3. Applications of the 2 nd Law Power plant fossil-fuel : T h = 650 K Nuclear : T h = 570 K T c = 310 K Actual values: e fossil ~ 40 %e nuclear ~ 34 %e car ~ 20 % Prob 54 & 55 Heat source Boiler Turbine Generator Electricity Condenser Waste water Cooling water

11
Application: Combined-Cycle Power Plant Turbine engines: high T h ( 1000K 2000K ) & T c ( 800 K ) … not efficient. Steam engines : T c ~ ambient 300K. Combined-cycle : T h ( 1000K 2000K ) & T c ( 300 K ) … e ~ 60%

12
Example 19.2. Combined-Cycle Power Plant The gas turbine in a combined-cycle power plant operates at 1450 C. Its waste heat at 500 C is the input for a conventional steam cycle, with its condenser at 8 C. Find e of the combined-cycle, & compare it with those of the individual components.

13
Refrigerators Coefficient of performance (COP) for refrigerators : COP is high if T h T c. Max. theoretical value (Carnot) 1 st law W = 0 ( COP = ) for moving Q when T h = T c.

14
Example 19.3. Home Freezer A typical home freezer operates between T c = 18 C to T h = 30 C. What’s its maximum possible COP? With this COP, how much electrical energy would it take to freeze 500 g of water initially at 0 C? Table 17.1 2 nd law: only a fraction of Q can become W in heat engines. a little W can move a lot of Q in refrigerators.

15
Heat Pumps Heat pump as AC : Heat pump as heater : Ground temp ~ 10 C year round (US) Heat pump: moves heat from T c to T h.

16
19.4. Entropy & Energy Quality Energy quality Q measures the versatility of different energy forms. 2 nd law: Energy of higher quality can be converted completely into lower quality form. But not vice versa.

17
Entropy lukewarm: can’t do W, Q Carnot cycle (reversible processes): Q h = heat absorbed Q c = heat rejected Q h, Q c = heat absorbed C = any closed path S = entropy[ S ] = J / K Irreversible processes can’t be represented by a path.

18
Entropy lukewarm: can’t do W, Q Carnot cycle (reversible processes): Q h = heat absorbed Q c = heat rejected Q h, Q c = heat absorbed C = any closed path S = entropy[ S ] = J / K Irreversible processes can’t be represented by a path. C = Carnot cycle Contour = sum of Carnot cycles.

19
Entropy change is path-independent. ( S is a thermodynamic variable ) S = 0 over any closed path S 21 + S 12 = 0 S 21 = S 21

20
Entropy in Carnot Cycle Ideal gas ： Adiabatic processes ： Heat absorbed: Heat rejected:

21
Irreversible Heat Transfer Cold & hot water can be mixed reversibly using extra heat baths. Actual mixing, irreversible processes reversible processes T 1 = some medium T. T 2 = some medium T.

22
Adiabatic Free Expansion Adiabatic Q ad.exp. = 0 S can be calculated by any reversible process between the same states. p = const. Can’t do work Q degraded. isothermal

23
Entropy & Availability of Work Before adiabatic expansion, gas can do work isothermally After adiabatic expansion, gas cannot do work, while its entropy increases by In a general irreversible process Coolest T in system

24
Example 19.4. Loss of Q A 2.0 L cylinder contains 5.0 mol of compressed gas at 300 K. If the cylinder is discharged into a 150 L vacuum chamber & its temperature remains at 300 K, how much energy becomes unavailable to do work?

25
A Statistical Interpretation of Entropy Gas of 2 distinguishable molecules occupying 2 sides of a box MicrostatesMacrostatesprobability of macrostate 1/4 2 ¼ = ½ 1/4

26
Gas of 4 distinguishable molecules occupying 2 sides of a box MicrostatesMacrostatesprobability of macrostate 1/16 = 0.06 4 1/16 = ¼ =0.25 1/16 = 0.06 6 1/16 = 3/8 = 0.38

27
Gas of 100 molecules Gas of 10 23 molecules Equal distribution of molecules Statistical definition of entropy : # of micro states

28
Entropy & the 2 nd Law of Thermodynamics 2 nd Law of Thermodynamics : in any closed system S can decrease in an open system by outside work on it. However, S 0 for combined system. S 0 in the universe Universe tends to disorder Life ?

Similar presentations

© 2021 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google