Lecture T5: First Law of Thermodynamics


General comments

I thought the lecture went pretty well. In class experiments/demos are always fun. We will do another one on Monday. It is clear from the PRS responses that there is confusion regarding the various forms of the First Law of Thermodynamics. This is natural, I have introduced a lot of new terms, etc. However, since it is a common point of confusion, be careful when you do the homework to use the appropriate form of the equation for the job at hand.

I left my PRS clicker @ home. Does that mean that I wasn't in class today according to your records? (1 student) Yes it does. If you tell me who you are I can correct the records to reflect your attendance.

Note about the TA session yesterday. It had the potential to be a lot more useful than it was. If we were told about it beforehand, we could have prepared questions to ask. (1 student). You are right. For the record, U5 and U6 are titled "The Language of Engineering" and will cover a lot of the basics that are relevant for all of the disciplines as well as items identified in the web-based diagnostic as most in need of remediation. U7 will be a mid-term feedback and discussion session, and U8 will be an end-of-term feedback and discussion session.

There was no lecture, just one example (1 student). I will assume this is a criticism and not a plaudit. I will note that by my watch, we spent 5 minutes on the example itself and 12 minutes on thinking about what information we need. Prior to that we did 3 PRS questions, discussed the applicability of the various equations, described enthalpy, reviewed the necessity for knowing a quantity like specific heat, etc.


Responses to 'Muddiest Part of the Lecture Cards'

(60 respondents)

1) Unclear on all the various versions of the First Law, when they are applicable, etc. (18 students) I have started to put together a list of all the First Law equations and the associated assumptions. When I complete it, I will put it up on the web.

2) We didn't derive the du=cvdT and dh=cpdT equations (1 student). Right. And I don't intend to derive these in class. I expect you to work through these on your own. If you have questions on the derivations that are presented in the notes, please let me know. I will however, spend some more time on talking about the uses of these equations, limitations, etc.

3) I am not sure what you mean by quasi-static (1 student). Read this again. Here is an alternative explanation from Prof. Greitzer's class notes for 16.050: A system in thermodynamic equilibrium satisfies: a) mechanical equilibrium (no unbalanced forces) b) thermal equilibrium (no temperature differences) c) chemical equilibrium. For a finite, unbalanced force, the system can pass through non-equilibrium states. We wish to describe processes using thermodynamic coordinates, so we cannot treat situations in which such imbalances exist. An extremely useful idealization, however, is that only "infinitesimal" unbalanced forces exist, so that the process can be viewed as taking place in a series of "quasi-equilibrium" states. (The term quasi can be taken to mean "as if"; you will see it used in a number of contexts such as quasi-one-dimensional, quasi-steady, etc.) For this to be true the process must be slow in relation to the time needed for the system to come to equilibrium internally. For a gas at conditions of interest to us, a given molecule can undergo roughly 10E10 molecular collisions per second, so that, if ten collisions are needed to come to equilibrium, the equilibration time is on the order of 10E-9 seconds. This is generally much shorter than the time scales associated with the bulk properties of the flow (say the time needed for a fluid particle to move some significant fraction of the lighten of the device of interest). Over a large range of parameters, therefore, it is a very good approximation to view the thermodynamic processes as consisting of such a succession of equilibrium states.

4) Enthalpy.I don't quite see how enthalpy is applicable yet (4 students). For now take it as just another property of the system that I would like you to begin to get more comfortable with, think about its physical meaning, and if you desire, read ahead to see how we will use it with the Steady Flow Energy Equation.

5) The calculation for the last experiment was unclear to me (9 students). Here is a write-up of my estimation procedure. If two properties determine the state of the system, why couldn't we do the experiment with the tennis ball by only measuring two properties before the ball was thrown and the same two properties after? (1 student) Great question. We could. I was looking for a means of estimating the temperature rise based on observable quantities (though you can argue about how observable the density is).

6) Cv, Cp ----> ? :) (10 students) First, read the section of the notes again to familiarize yourself. Here are the most important points: 1) cv and cp tell us how many Joules of heat transfer are required to change the temperature of a material by 1 Kelvin via a constant volume or constant pressure process, respectively. 2) These are properties of the material we are dealing with (that is they are different for different gases, solids, etc.). 3) In general, u is a function of two properties of the system (as is h), however, if u=u(T) only and h=h(T) only then du=cvdT and dh=cpdT. If this gas also obeys pv=RT (a thermally perfect gas), then the gas is called an ideal gas. 4) cv and cp allow us to relate changes in h and u to changes in T. As to which to use when, if you want to calculate the change in internal energy use cv, if you want to calculate the change in enthalpy, use cp. 5) Generally, cv and cp are functions of temperature (that is, the specific heat of a material changes with temperature), but for most of what we do in this class we will assume they are constant. For air for example, du=cvdT with cv=716.5 J/kg-K implies that if the temperature of 1 kg of airs changes by 1 K, then its internal energy has changed by 716.5 J. Similarly, since cp=1003.5 J/kg-K for air, its enthalpy (u+pv) would change by 1003.5 J.

7) No mud, fun example, etc. (17 students).