Lecture T12: Reversible and Irreversible Processes, Entropy


General comments

I started the lecture with a review of the stagnation properties concepts. This part of the lecture is written down in the mud responses for T11. Please review these if the issues are still unclear. I then introduced the concepts of reversibility and irreversibility. I did this by way of a concept question. All of the reverse processes discussed in this question are allowed by the first law of thermodynamics. However, the processes are only observed to spontaneously occur in one direction. To make them occur in the reverse direction requires the application of work. We then did a second PRS question related to a common irreversible process (a free expansion). Note there are two points that are frequently are misunderstood. The first is the detail of the temperature being constant for an adiabatic free expansion (it really is). The second is that sometimes people forget that the test of reversibility is that both the system and the surroundings are returned to their initial state (it is not enough to put Humpty Dumpty back together again, the whole world has to go back together again too). If this is confusing, think of the example of the chalk. I dropped it and it broke (the forward process). I can pick it up again and put it back together (the reverse process). When I am done the chalk is back to its original state, but I am not. I have used fuel (blood sugar) to do work producing waste heat. Also remember that one of the most important points is that the degree of irreversibility is related to the amount of lost work. Think of the Mars bubble popping and brick contacting example to remind you of this.


Responses to 'Muddiest Part of the Lecture Cards'

(27 respondents out of 66 students attending class)

1) Still a bit confused about stagnation temperature and enthalpy, and related questions (2 students). Please read T11 mud and the materials in the notes and then we can talk about it in recitation. If this doesn't resolved the confustion, please email me to set up a time to discuss it. I am even more confused no as to what static temperature is... Is it the temperature I would measure if I were to place an infinitely thin massless thermometer in the flow??? (1 student) Not if the flow stagnated on the surface of the thermometer. The statis temperature is the temperature you would feel if you would to move along with the flow at the speed of the flow. So the molecules of gas for example (which move along at the speed of the flow), condense when the static temperature drops below the condensation temperature. In the jet engine concept question, it seems that T1 would be lower than atmospheric temperautre because air from the atmosphere must flow to the inlet without q of ws. (1 student) It is lower than the atmospheric temperature any place the flow is moving. Why can't you use a thermometer to measure stagnation temperature? Isn't that what you get when you stick a thermometer in a steady flow? (1 student) Yes. You can't use a thermometer to measure static temperature. When an airplane is moving, is the speed of the flow in the atmosphere relative to the plane the same as the speed at the inlet (of the compressor)? (1 student) In general no. Why do TT and PT decrease as C^2/2 increases fo flow? (1 student). They don't. The static temperature and static pressure decrease.

2) How do you explain "wind chill" in terms of stagnation temperature? (1 student) When the temperature is quoted it is the stagnation temperature (measured using a thermometer, etc.). Why does it feel colder when the wind is blowing? Because when the flow moves over a surface it increases the rate of heat transfer (even though the stagnation temperature is the same). This is called "convective heat transfer" and you will learn much more about it in 16.050. This is why blowing on soup cools it off faster and blowing on icecream warms it faster. Because the rate of heat loss from you body is faster, it feels colder. Although I am not an expert on physiology, I think our bodies respond to fluxes of energy, not to temperature. This is why a piece of metal feels colder than a piece of wood even though they are at the same temperature. One has a higher thermal conductivity.

3) You said entropy is a measure of both efficiency and irreversibility. How are these two things related? (1 student) Entropy is a measure of the difference between the real efficiency and the ideal efficiency. For example in homework problem T7, you will find that the actual efficiency of the power plant is much lower the the ideal efficiency. This difference in efficiency (how much work is extracted for a given amount of heat-added) is related to irreversibilities in the various processes (friction, non-ideal expansions, heat transfer across finite temperature differences, etc.).

4) For the examples in the concept question, would the processes be reversible because they obey the first law, or irreversible because work would be required? (1 student) Both reversible and irreversible processes can obey the first law. The measure of irreversibility is that the surroundings must be changed (typically work being converted to heat) to put the system back too its initial state. Regarding the 1st PRS question, I understand ho they all could be reversed, disregarding entropy. What confused me is when you said we would have to add work to reverse the processes. Does this violate the 1st law or does the work come from heat already in the system? (1 student) Taking work from the surroundings to reverse the process does not violate the first law (or the second law for that matter).

5) How is entropy higher when gas enters a vacuum rather than pushing a piston? (1 student) I am confused how you calculate entropy. (1 student) We will get to this in the next lecture. Please read ahead in the notes.

6) So the second law is a predictor? I'm confused by mmories from 3.091 where energy went into entropy. (1 student) Energy is conserved, always. Thus energy always goes into energy (in one form or another). Entropy is thermodynamic property that is a function of the state of the system. When summed for the universe (system plus surroundings) it can only increase by irreversibility.

7) Are there estimates of, based on observed rates of entropy increase in the universe, how long it will take for the entire universe to become entirely uniform in material and thermodynaamic properties? (1 student) I recommend you read A Brief History of Time by Stephen Hawking.

8) Is a rechargable a perfectly reversible process? (1 student). I assume you are referring to a battery. The answer is no. Waste heat is generate at several parts of the process.

9) If you crack an egg and then cook the inside making scrambled egg, what amount of work could reverse this process? (1 student) Although it is not practically feasible for complex biological structures like an egg, it is possible for simpler structures to recreate them from their fundamental chemical building blocks. Therefore it is at least conceptually feasible for something like an egg. But it would take a lot of work.

13) No mud (9 students). Good.