The four laws of Thermodynamics. WOD are underlined.

Transcription

1 The four laws of Thermodynamics WOD are underlined.

2 The four laws of Thermodynamics The 0 th Law (discovered 4 th ) The 1 st Law (discovered 2 nd ) The 2 nd Law (discovered 1 st ) The 3 rd Law (discovered 3 rd )

3 The 0 th Law If: Objects A and B are the same temperature Objects B and C are the same temperature Then: Objects A and C are the same temperature Just the transitive property of mathematics.

4 1 st Law of Thermodynamics The total sum of all energy in an isolated system will never increase or decrease. Law of conservation of energy. Energy cannot be created or destroyed, only transfer forms.

5 2 nd Law of Therm. If two objects are not the same temperature then: Heat will always flow from high to low temperatures. o Hot object will decrease in temperature and cold object will increase in temperature until they are both the same temperature.

6 2 nd Law of Therm. Entropy is randomness, i.e., disorder, spread out, lack of structure, messiness, etc. Entropy must increase (unless controlled by an intelligence). Means that: Machines cannot be 100% efficient. Heat flows from hot to cold. Examples: What happens to these if left alone? a.) A stack of lumber in the forest. b.)your bedroom.

7 Anyone know how the steam engines on trains of the 1800 s worked? Please describe it?

8 Talk about the steam engines on trains in the 1800 s. Cold water Heated water Fire 2.) Water Tank. Cold water enters, heated by fire box, exits to steam turbine. Heat lost to non insulated water tank and pipes. Heat Loss 1.) Fire box. Some heat rises up to heat water, but most escapes to air around box. 3.) Steam Turbine More heat wasted on Friction at joints. Jake Burkholder 2012

9 Talk about the steam engines on trains in the 1800 s. Heat Loss? Where does the lost heat go to? What does it do? Jake Burkholder 2012

10 Talk about the steam engines on trains in the 1800 s. Heat Loss: goes where? Heat is lost to air. It increases the temperature of the air molecules, which means the speed of the air molecules increase, which makes them more random, i.e., it increases the entropy of air. Jake Burkholder 2012

11 Talk about the steam engines on trains in the 1800 s. So how efficient are engines? Efficiency is Work Out = Work Out Work In Q In if you get 1000J of work produced going out and put 4000J of heat in, you have a 1000J/4000J = 25% efficient engine. Jake Burkholder 2012

12 Talk about the steam engines on trains in the 1800 s. Cold water Heated water Fire Heat Loss 1.) Fire box. If only 2/7 of heat goes up to water, best efficiency is 28% (=2/7). More of the heat is lost from the hot water tank and due to friction in the turbine. These 1800 train engines were only 5 to 10% efficient. Modern car engines are about 30% efficient. They best that modern cars can achieve is about 37% efficient. Jake Burkholder 2012

13 Talk about the steam engines on trains in the 1800 s. Cold water Heated water Fire Heat Loss 2 nd Law of Thermo leads to: Machines cannot be 100% efficient which means that 1.) you cannot build a perpetual motion machine and 2.) You cannot create energy, (i.e., efficiency cannot exceed 100%) Jake Burkholder 2012

14 New Idea S is for Entropy S = Q/T Internal Energy / Temperature (absolute) Joule/Kelvin

15 What is going to happen? This system starts with same pressure and temperature on both sides. Then heat is applied to the left side. Which way will the boundary move? (What happens to the pressure on the left side when heated?)

16 What is going to happen? The boundary will move, and temperatures and pressure will even out. 2 Actions: Increasing left pressure will push the boundary towards the right until pressures are the same. At the same time, the unequal temps will affect each other until they are equal.

17 Entropy is a measure of disorder. In this low entropy state, there will be a net force, since there is more movement to the right than to the left.

18 Operating principle of a steam engine. Some of the heat in the hot gas was turned into work done on the cold gas.

19 Question Can I get any more work out of these gasses?

20 Question I would need to find a third gas sample at a lower temp to get more useful work. Or supply my own energy to create another temperature difference.

21 Entropy is a measure of how evenly spread out the energy is. Entropy is a measure of energy that is no longer useful, or workable. Work is change in energy. It won t change anymore if it s spread out evenly. (Work won t change anymore if the temperature is spread out evenly.)

22 The amount of work done or the efficiency of an engine is determined by the difference in the temperatures. Hotter engine and cooler water makes for a greater temperature difference and more work done. This means: 1.) more work will be generated from an engine and 2.) the engine will run more efficiently. (estimated temps.) When the temperature differences are greatest. Car Engine Metal Oil Water Air 2000F 800 F 300F 180F 70F The greater these temperature differences, the more energy from the car engines. Discuss why cars overheat in winter or summer. Jake Burkholder 2012

23 More examples of entropy Entropy is randomness. Which is more random? A or B? A B

24 More examples of entropy 2 nd Law of Therm. says that nature always goes from order to disorder. A B

25 More examples of entropy In nature, do you move from A to B or B to A? A B

26 More examples of entropy 2 nd law says that nature always, always moves from A to B and never from B to A. A B

27 More examples of entropy

28 More examples of entropy On the large scale, the ice looks more disordered. On the small scale, the solid phase severely limits where the molecules could be. The ice crystal molecules are much more ordered than the free moving liquid water molecules.

29 3 rd law of Thermodynamics Absolute zero is a state of zero motion. This means absolutely no entropy. So it can t be reached.

30 Stop here for Phy1 Don t do the rest of the powerpoint

31 Entropy is a measure of disorder. In this high entropy state, there will be no net force, since there is only random particle motion on both sides.

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33 On the large scale, the ice looks more disordered. On the small scale, the solid phase severely limits where the molecules could be. The ice crystal molecules are much more ordered than the free moving liquid water molecules.

34 Entropy is a measure of repeatability I throw two dice. What is the most likely outcome? What is the least likely outcome?

35 Entropy is a measure of repeatability I throw two dice. What is the most likely outcome? What is the least likely outcome? Thirty six possible microstates 36 ways the dice could come up.

36 11 Possible macrostates 11 possible scores or states with meaningful differences. Macrostateswith more microstates have a higher probability of occuring.

37 Since there are more disordered states than ordered states, they are inherently more repeatable. New symbol W for multiplicity W is the number of other microstates that could have the same value.

38 Roll a 2? W = 1 Roll a 4? W = 3 Roll a 7? W = 6

39 An entropic state is more probable Which is more probable?

40 An entropic state is more probable Which is more probable? There are many more possible disordered states than ordered states, so repeatability is a sign of disorder

41 How many ways can you arrange bricks into a neat pile? Not that many. How many ways can you arrange bricks into a total mess? A whole lot.

42 S = k log W k is Boltzmann s constant The more repeatable the state is, the more entropy there is.

43 Restating the second law ΔS 0 For just about every process, entropy will increase. If you re very, very lucky it will stay constant.

44 Restating the second law Order to disorder. Since disordered states have much higher multiplicity, the are much more likely, so nature will drive every system in that direction.

45 Entropy as Time s Arrow Newton s laws work equally well forwards and backwards. Momentum, SHM, equal and opposite reactions, all of these principles are reversible. The increase in entropy isn t. An increase in disorder is the only way to tell forwards from backwards.

46 Time s Arrow Which came first? Which direction are the balls moving?

47 Restatement of 2 nd law States where energy is not spread out evenly will evolve into states where energy is spread out more evenly. Chemistry people? What is Gibbs Energy?

48 Gibbs Free Energy Gibbs Free Energy is energy that is still useful. Opposite of entropy A chemical reaction will occur if the Gibbs would decrease.

49 Gibbs Free Energy A chemical reaction will occur if the Gibbs would decrease. E = Gibbs + S*T E is total energy S*T is entropic energy. Random, thermal motion.

50 Gibbs Free Energy A chemical reaction will occur if the Gibbs would decrease. E = Gibbs + S*T Chemical reactions occur in a way that energy gets spread out more evenly. Chemical reactions increase entropy.

51 Conservation of energy E = Gibbs + S*T E can never increase or decrease. S*T can only increase. What has to happen to Gibbs Energy?

52 Conservation of energy E = Gibbs + S*T E can never increase or decrease. S*T can only increase. What has to happen to Gibbs Energy?

53 Conservation of energy E = Gibbs + S*T When Gibbs runs out there will be no more chemical reactions. As entropy increases, available energy will vanish.

54 Can t you just restack the bricks? Entropy can be lowered by doing work on the system. You can stack the bricks back up by hand. By providing energy, you can drive chemical reactions backwards.

55 Can t you just restack the bricks? Entropy on a larger scale will always increase. The energy you put in had to come from somewhere. Factor in where that work came from, entropy will increase. If you do it yourself, you will perform chemical reactions inside you that increase entropy.

56 Large scale entropy The human body is a good example of low entropy. Think about how much matter (food) has made unusable to develop one and maintain it. Think about a city, and then think about how much waste it produces.

57 Chemical reactions occur all the time. Where does all the energy come from to lower entropy to make life possible come from?

58 Chemical reactions occur all the time. Where does all the energy come from to lower entropy to make life possible come from? Plants use sunlight to do work to lower entropy on sugars that get burned by every living thing on Earth.

59 Now, compare the mass of every living thing on Earth to the amount of useful hydrogen that got converted to entropic helium ash in the suns core to maintain that.

60 Eventually the sun will burn out There won t be enough Gibbs energy, and all processes will stop.

61 Heat Death Eventually, all energy will be entropic. Chemical reactions will burn themselves out. Temperatures will equalize. Matter and energy will be spread out evenly across the universe, and no more chemical or physical processes will occur.

62 3 rd law of Thermodynamics Absolute zero is a state of zero motion. This means absolutely no entropy. So it can t be reached.