Thermodynamics: More Entropy

Transcription

1 Thermodynamics: More Entropy

2 From Warmup Yay for only having to read one section! I thought the entropy statement of the second law made a lot more sense than the other two. Just probability. I haven't seen it since 8th grade. We assume that all microstates are equally likely, so probability amounts to counting how many microstates in a given macrostate. Is all the math for determining probability as important as the equations for determining entropy? You should know both. I'm not really getting the connection between the flow of energy and the changes in entropy. Especially when the math starts coming into play. Think about shuffling cards. If you split the deck into black and red cards and then shuffle them. Why will there be a net flow of red cards into the black deck?

3 Microstates vs. Macrostates Left microstate: part of the royal flush macrostate Right microstate: part of the garbage macrostate The most common macrostates are those with the most microstates.

4 From Warmup Using ideas from both the reading and from the last lecture, explain why heat flows from hot to cold when the process of energy exchange between two objects is "random". (How can you get directed motion of heat, when energy is being exchanged both ways?!) The probability of heat flowing from hot to cold is so much greater than the opposite (cold to hot) that it is never observed that heat flows from cold to hot in nature. Thus, entropy never decreases in any natural process. Building on this idea: each unit of energy has equal probability to move between objects. Since there are more units in the hot object, it is more probable that the entire object has a net loss of energy and that the cool object will have a net gain. This is what we perceive as heat. If you put a drop of dye in some water, it will spread out (diffuse). Why?

5 Calculating Entropy Entropy is a state variable It doesn t matter what path you use to calculate it. Always use a path that is internally (not totally) reversible This means it is a path on a P-V diagram Only really matters for adiabatic free expansion. Examples (In groups) Change in entropy for an adiabatic compression? Isothermal Compression? Isochoric process? Isobaric process?

6 From Warmup When two systems A and B can exchange energy, the entropy of system A *always* decreases when system A gives energy to system B. If that's so, why would energy ever spontaneously flow from system A to sytem B? I'm not sure I understand the question. The energy flows to B because the entropy of B increases more than A decreases which results in a net gain of entropy for the combined system of A and B. If the entropy is always increasing how do we get things like stars and planets and life to spontaneously form?

7 Dice You roll two dice. What are the microstates? (1,1), (1,2), (1,3), (1,4), (1,5), (1,6), (2,1), (2,2), (2,3) How many microstates are there? How many microstates if we roll 5 dice? What is the most likely macrostate?

8 Many Dice You roll 10^23 dice with your left hand. How many microstates are there? You roll 10^23 dice with your right hand. How many microstates are there? How many microstates are there in the COMBINED system? This is getting ridiculous

9 Solution: Use Logarithm Entropy: S = (constant) x log (# microstates) Log = Natural Logarithm Constant has units J/K. Much more manageable numbers Log(10^23) = 53 Combining two systems: 2 nd Law: System moves to macrostates with more microstates

10 The second law of thermodynamics System in macrostate with most microstates This is a statistical law shouldn t we see unlikely stuff occassionally? Flip 10^23 coins. What is the probability of getting all heads? Do this once a second for the age of the universe (10^15 sec), what is the probability of getting all heads at least once? Warmup review: What is the probability that all the molecules in a mole of gas are in the left 99% of the container?

11 Entropy and Information Theory Entropy is the number of microstates in a macrostate Entropy is how much information you would need to identify which microstate the system is in. The second law states that the lost information about the state of the universe always grows. (and can never be recovered!) Clicker Poll: Is the 2 nd Law of Thermodynamics a Fundamental Law of Physics A. Yes B. No

12 Some Philosophical Questions Thermodynamics is driven by Entropy. Entropy depends on the definition of the macrostates. Who, or what, determines the macrostate? Is it entirely subjective? A matter of perspective? Could we choose different macrostates and get a different theory of thermodynamics? Defining different macrostates for the same system: Dice Sum of the numbers Difference of the numbers Number of 1 s Etc.

13 Maxwell s Demon o Maxwell imagined a microscopic demon that could violate the 2 nd law o Imagine a gas in a container with a partition and a door. o The demon lets fast moving molecules into the right half and vice versa. o Therefore (Maxwell concludes), the 2 nd law is a statistical consequence of our being big (macroscopic)

14 Maxwell s Demon Resolution o Maxwell s demon must process information about the speeds of approaching molecules o What does it do with this information? Record it? Erase it? o It can t record information indefinitely eventually it must erase some of its information. o Erasing information increases the entropy! o The increase in entropy caused by erasing the information is always more than the entropy decrease caused by sorting the molecules o Maxwell s demon cannot violate the second law!

15 Macro/Micro theories in Science Science is hierarchical Branches of science related as macro/micro theories Understanding the relation between macro/micro theories remains one of the fundamental problems in science. Information Physics helps explain the relationship between these fields Microtheory Kinetic/Atomic Theory String Theory Quantum Mechanics Chemistry Biology Microeconomics Weather Macrotheory Thermodynamics Quantum Field Theory Chemistry Biology Psychology Macroeconomics Climate