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4 4 Lecture #4 Chapter 3 & 4 2nd Law of Thermodynamics 1 st Law of Motion

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7 7 Newton’s 1 st Law of Motion A body will stay at rest or in motion at constant velocity and direction unless acted upon by an outside force Inertia is the tendency to resist change Inertia is the measure of an object’s mass

8 8 Automobile Efficiency Losses at cruising speeds (not including engine efficiency)

9 9 Fig. 4-20, p. 118 Heat Engine - OTEC

10 10 2 nd Law of Thermodynamics (Natural decrease in usefulness)

11 11 Observations Processes occur naturally only in one direction → A cup of hot chocolate always cools down It will never heat up by taking energy from the colder room

12 12 Observations (cont’d) Pressurized air will always escape from a container that is punctured More air will never push into the pressurized container

13 13 Observations (cont’d) An object held by a string above the floor will fall when the string is cut The object will never gain energy from the room and rise or stay hovering above the floor

14 14 Observations (cont’d) From these examples we see that processes naturally occur in the direction that creates more uniformity of temperature, pressure, etc (i.e, towards equilibrium with surroundings); and therefore less ability to produce work. → Heat flows from hot to cold (hot objects cool down) A refrigerator transfers heat the opposite way, but it does not occur naturally it requires work into a compressor → Pressurized air escapes when the container is punctured → Objects above the floor fall

15 15 Observations (cont’d) The 1 st Law of Thermodynamics does not determine in what direction a process will naturally occur →For example the 1 st law is not violated if a hot object gains more energy from the cold surroundings as long as the energy gained by hot object is equal to the energy lost by cold object

16 16 Observations (cont’d) For each naturally occurring process there was an opportunity to produce work →When the hot object cooled down we could have heated up steam and sent it through a turbine to make work →When the compressed air escaped we could have forced it through a turbine to make work →When the weight fell to the floor we could have connected the weight to an electrical generator and made work (or raised a slightly lighter weight)

17 17 Observations (cont’d) We need laws of thermodynamics to predict: →The direction a process will naturally take Simple processes are easy to predict Complicated processes are more difficult to predict →The amount of work the naturally occurring process could have made

18 18 The 2 nd Law of Thermodynamics The 2 nd Law of Thermodynamics predicts in what direction processes will naturally occur →The direction that creates more energy at ambient conditions and less ability to produce work This is useful for both: →Simple processes where intuitively we know the direction →For complex processes where we may not know the final outcome

19 19 The 2 nd Law of Thermodynamics The 2 nd Law of Thermodynamics determines: →The maximum possible amount of work that can be produced from a process →The amount of disorder the process has caused

20 20 Summary of the 2 nd Law of Thermodynamics Heat flows naturally from hotter to colder Naturally occurring processes result in more disorder Energy has quality as well as quantity All energy in the form of heat cannot be converted to work →A portion must be transferred to a low temperature sink Entropy is a rating of disorder and randomness →High quality ≈ low entropy →Low quality ≈ high entropy

21 21 Heat Engines (power cycle) Energy from a high temperature source is transferred to the heat engine A portion of the high temperature energy is converted to work The remaining energy is transferred to low temperature sink The efficiency = (work out)/(heat in) is always less than 100% Electrical power plants, automobile and jet engines all operate by these principles

22 22 Heat Engines (power cycle) Usually the heat engine consists of turbines, pistons, etc. →For now it will be modeled as a circle or box with: Energy at high temperature going in Work and energy at low temperature coming out

23 23 Heat Engines – Power Cycles (cont’d) Nicolas Leonard Sadi Carnot (1796 – 1832) determined maximum possible efficiency for a heat engine Biographical comment: →“A quiet, unassuming Frenchman who lived during the turbulent Napoleonic years and had an unspectacular life” One of Carnot’s mottos →“Speak little of what you know, and not at all of what you do not know”

24 24 Carnot Efficiency Max Efficiency = (T h – T c ) / T h For % efficiency multiply by 100 T h = High temperature source T c = Low temperature sink All temperatures must be in absolute units (e.g., K or R)

25 25 Example of Maximum Efficiency of Heat Engine Heat engine receives heat from steam at 300°C and exhausts heat to air at 100°C →What is the maximum efficiency? →T h = 300°C + 273 = 573 K →T c = 100°C + 273 = 373 K →Max Efficiency = (573 K – 373 K) / 573 K = 0.35 = 35%

26 26 Actual Efficiencies of Heat Engines – Power Cycles The efficiencies of all heat engines are less than 100% because: →All heat engines cannot operate greater than Carnot Efficiency even if they were constructed perfectly (no friction, etc.) →Losses such as friction even decrease the efficiency to a value lower than the Carnot efficiency Less efficient processes are used because: →Cheaper →Easier to use

27 27 To Increase Efficiency of Heat Engine – Power Cycle: Increase temperature of heat source Decrease temperature of heat sink C

28 28 Why will heat energy never reach 100%?

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