**General comments**

The lecture started out well and ended in a train wreck. I began with a presentation of some of the many forms of the first law of thermodynamics. Included with this discussion was a PRS question regarding the applicability of a particular form of the law. It is important that you recognize the various assumptions implicit in the different forms of the equations and that you use the appropriate equations when solving problems. The compendium of equations lists most of the common forms along with their applicability. We then did a second PRS question to motivate the need for specific heats. The point was to show that in order to use the first law to calculate changes in temperature, pressure, etc. of a system, we need to be able to relate changes in internal energy (or enthalpy) to other properties. We do this using the specific heats. All this went well (except for Andy who didn't win the $5 bill) and then the train wreck started. I tried to go through the derivation of the relationships for cv and cp too fast in too little time. We will begin with this tomorrow and I will try to sort out the confusion.

**Responses to 'Muddiest Part of the Lecture Cards'**

(53 respondents out of 66 students)

1) * Unclear on Cv, Cp, the derivation,
etc.* (45 students) Please read the section
of the notes 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

2)** I am still not clear on why you don't need an
ideal gas to use dU=delQ-pdV.** (1 student) All that is required
for this is that the work equal the integral of pdV, it does not require a particular
relationship between p and V--like pV=RT--could be pV^2=RT for example (but
then it wouldn't be an ideal gas).

3)* Is it a fundamental aspect of thermo that you
can have an indeterminate # of necessary variables?* (1 student)
I am not sure what you are asking. Please see me and I can try to help you sort
it out.

4) * In the equations for the first law, when can
we use psys? Is it only for quasi-static processes?* (1 student)
Yes.

5) * When you fold DEblood
sugar = DKE/hAndy, does
this ignore his blood sugaar moving his arm?* (2 students) It
depends how hAndy is defined. The 20% number I quoted
is roughly the efficiency of a human at producing a unit of mechanical work
output -- so I think it would include moving the arm.

6)* I know that adiabatic processes do not mean
that the temperature is constant, but it is difficult to conceptualize. Can
you give some examples of adiabatic processes where DT<0, DT = 0, and DT> 0?*
(1 student) I hope the discussion in the recitation helped with this. If not,
please see me and we can discuss it more.

7) * How is an adiabatic process represented on a
p-v diagram? What makes an adiabatic process definite when both p and T change?*
(1 student) An adiabatic process is represented on a p-v diagram with a curve
pv^g = constant as shown in this
section of Chapter 4. Both T and p change in an isothermal process also,
but just in a different way as you will see in homework
T4.

8) * What is the point of modeling an adiabatic process
when it can never exist?* (1 student) It is a useful idealization
and many processes indeed are closely approximated as adiabatic (we will use
this frequently for example for engines and rockets).