Physics 105 Physics for Decision Makers: The Global Energy Crisis

1. Fall 2011 Physics 105 Physics for Decision Makers:The Global Energy Crisis Lecture 8 Thermodynamics II

2. Energy Audit • Energy Audit Project: Download assignment info sheet from Elms • Group project – • When you meet -> Each person should have a role – one person needs to be responsible for organization. • Project Due Dates:Assignment #1 - Sept 30 – only one person needs to turn something in, but the whole group needs to be involvedAssignment #2 - Oct 7Assignment #3 - Oct 21Assignment #4 - Oct 28Assignment #5 (Presentations in Discussion) Oct 29 - Nov 1. Page 2

3. Food Supply and Climate • CNN today - Why is 'food security' sparking unrest • Russian Drought wiped out 25% of their wheat crop • This caused them to stop grain exports • Bread prices went up dramatically 30% in some countries • Riots ensued in Mozambique • Decisions we make about usingbiofuels can affect people around the world Page 3

4. What is Thermodynamics? • Guided Tours • Thermodynamics: Activities:

5. Page 5

6. The Zeroth Law of Thermodynamics • Temperature - if two objects are in thermal equilibrium with a third object (like a thermometer) then they are in thermal equilibrium with each other • Another way of saying it is that temperature is a measurable quantity and it tells us about the energy content of an object • this law asserts that we can define a temperature function, or more informally, that we can 'construct a thermometer'

7. Thermal Equilibrium • if Q=0 then we are in “thermal equilibrium” • TA = TB Q A B

8. Heat Transfer Three methods Conduction •Transfer Of Energy Through Matter •Air Is A Poor Conductor - Metal is a good one •Only important at the earth's surface Convection •Transfer of energy by movement of mass •Can only take place in liquids & gases - e.g. Air •Convection on a global scale creates worldwide atmospheric circulation Radiation Proportional to the 4th power of the temperature How we get energy from the sun Why it gets cold on a clear night

9. Fig. 2-2, p. 30

11. Radiation

12. The First Law of Thermodynamics – Energy Conservation Many statements: Energy is conserved Heat is a form of energy The energy of an isolated system (e.g. the universe) is constant Energy is conserved during any change in state. Specifically: Heat absorbed by a system + work done on the system = change in internal energy of the system Mathematically: Q+W=DU Q is heat, W is work and U is internal energy

13. The internal energy of a system does NOT depend on which of the following: The temperature of the system The amount of material in the system The type of material The amount heat that has been put into the system It depends on all of them

14. First Law - energy is conserved Where did energy to power the light come from?

15. Heat Capacity The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius (or Kelvin) Heat Capacity = Q/T Heat Capacity of a lake depends on the size of the lake (extrinsic) Specific Heat = Heat Capacity/kg - property of the water (intrinsic)

16. Latent Heat Latent Heat of vaporization If we add heat to water - its temperature goes up 1 Kcal - will raise 1kg of water 1oC Until water hits 100oC - then the temperature stops going up What happens? It vaporizes (boils). It takes 539 kcal to boil 1kg of water Eventually once it’s all steam the temp will start going up again 539 kcal/kg is the latent heat of vaporization

17. Latent Heat Latent heat of fusion Take 1kg of ice at -20oC and add heat at a rate of 1kcal/min The Ice will warm up at 2oC/min - (why 2)? Once it hits 0oC - (after 10 min) the ice will start to melt It will take 80 minutes to melt the ice So the latent heat of fusion of ice is 80kcal/kg

18. Does the 1st law prohibit a lake from freezing on a warm day? Yes No Doesn’t apply

19. The 2nd Law of Thermodynamics Many formulations: It is impossible to convert heat completely into work. No perfect engine Can’t just pull heat out of the environment Heat cannot spontaneously flow from a material at lower temperature to a material at higher temperature. No perfect refrigerator In an isolated system, a process can occur only if it increases the total entropy of the system.

20. The Perfect Heat Engine Takes heat out of theenvironment and turns it into work. The solution to all of ourproblems reduce global warming Solve the energy problem 2nd Law T1 Q1 Wout

21. Perfect Refrigerator T1 Hot Q T2 Cold

22. Restatement Zeroth Law – Lunch exists (temperature) First Law – TANSTAAFL (energy conserved) Second Law – (heat flows from hot to cold) Lunch will be expensive

23. Entropy Toss a penny N times… What is probability that you get Heads every time? ans. (1/2)(1/2)(1/2)(1/2)…. = (1/2)N if N =2, p = 0.25 if N= 10, p = .001 if N= 100, p = 8 X 10-31 if N = 104 , p = 5 X 10-3011!!!! We expect about N/2 heads… What is probability that we get within 1% of N/2? if N = 10, p = 0.25 if N=1000, p = .248 if N = 105, p = .998 if N = 108, p = 1 - 3 X 10-2174 !!!!

24. I flip a coin 10 times - the first 9 come up heads..What is the probability that the next toss is also heads? • .001 • 1 - 9/10 = 0.1 • (9/10)(1/2) = 0.45 • 0.5 • 9/10 = 0.9

25. Entropy - microstates Why is all heads so rare and ~50/50 so common? Let’s call each possible outcome a “microstate” There is only one microstate that is all heads HHHHHHHHHH There are many microstates that are ~ 50/50 HHHHHTTTTT HHHHTHTTTT HHHTHHTTTT HHTHHHTTTT TTTTTHHHHH THTHTHTHTH … Each microstate is equally likely.

26. Entropy and Particles in a box Particles start all on left What happens? What are the odds that later we findall of them on the left? Just like the coin tossing! After mixing, chance of all on the left is (1/2)N It would take work push all the molecules back to the left side

27. Entropy and the macrostate We can define macroscopic properties “more-or-less uniform distribution between right and left” When a physical system is allowed to evolve in isolation, some single macroscopic outcome is overwhelmingly more probable than any other Second Law: If a system of many particles is permitted to change, it will evolve to the macrostate made of the largest number of microstates, and stay there.

28. Entropy We define a quantity called entropy Entropy = S = kBln (no. of microstates) With this definition, it can be shown that: Sfinal - Sinitial = energy input from heating/T = Q/T Second Law: S > Q/T -

29. Definition of Entropy Change in entropy S=DQ/T Take an example - Q=30J Scold=+30J/283oK= +0.106 Swarm=-30J/333ok= -0.09 Stotal= +.07J/ok Entropy of the entire systemincreases 10o Q=30J 60o

30. The 2nd Law of Thermodynamics Most systems are microscopically reversible Particles in a box The 2nd Law tells us the “direction of time”

31. Reversible and Irreversible Processes

32. Reversible and Irreversible Processes

33. Reversible and Irreversible Processes

34. Reversible and Irreversible Processes

35. Reversible and Irreversible Processes

36. Reversible and Irreversible Processes

37. Reversible and Irreversible Processes

38. Reversible and Irreversible Processes

39. Heat Engines Heat engines take heat from a hot reservoir and does work and expels heat to the cold reservoir Note that the first law says Q1=W+Q2 The 2nd law tells us the amount of work we can get from a temperature difference Efficiency = (work output)/(energy in)

40. Carnot Engine The Carnot Engine is an idealized engine that works in a reversible way What is a reversible engine? A refrigerator By adding work we can take heat from the cold reservoir and deposits it to the hot reservoir Again - 1st law works W+Q1 =Q2 Notice more heat is delivered than work done!

41. Heat Pump Cools in summer Heats in winter What is temperature of the air blowing out in summer and winter?

42. Heat Pump Winter Summer Outside Outside Page 42

43. Carnot Efficiency The efficiency (work/energy in) of a Carnot Engine T1 -T2)/T1 When is the efficiency high? When T2 is low Example T1= 500oC = 773K T2= 0oC = 273K T1 -T2)/T1= 500/773= 65% This says W=65% Q2= 35% So if we take 100J from T1 we get 65J of work Redo if T1 is 100oC = 373k T1 -T2)/T1= 100/373= 26% so our 100J of energy only gives us 26J of work

44. Proof that no engine can have efficiency greater than Carnot Assume Engine 1 is higher efficiency than Carnot Operating between T1 & T2 it produces W1. W1 is greater than Wc would produce for the same Q1. (i.e. Q2 is less) Since the Carnot is reversible run it backwards powered by W1. This will pump more heat out of the cold reservoir than engine 1 put in as Qc >Q2 Net result - a perfect refrigerator - which violates the 2nd law since we are moving heat from cold to hot with no external work (lake freezing) 1 1

45. What is the net effect of putting a refrigerator in a room opening the door an turning it on? The room cools down a little The room cools down a lot The room heats up a little The room heats up a lot

46. System Properties Extensive quantities (depend on amount of material) U = Internal energy V = Volume Ni = # of Moles Heat Capacity The intensive quantities (do NOT depend on amount of material) Pressure Temperature Specific Heat

47. Real Heat Engines Real heat engines always are less efficient than Carnot engines In all heat engines there is waste heat from the 2nd law Electric motors are not governed by the 2nd – although generally the production of electricity is… In real engines there is additional waste heat The Carnot engine gives us a goal

48. Real Engine T1 Q1 Wout Qwaste Q2 T2

49. Car Engines efficiency of about 25% The efficiency may be as high as 37% at the optimum operating point. Most internal combustion engines waste about 35% of the energy in gasoline as heat lost to the cooling system and another 35% through the exhaust. The rest, about 5%, is lost to friction

50. Car Engine