**General comments**

** I think we are spending too much time with PRS
questions that are too easy. The reading is difficult, so I can tell I'm not
getting enough from lecture. Perhaps either more difficult PRS questions or
less time with PRS questions and more time lecturing on the material in greater
detail.** (1 student). There is no doubt that I am still trying
to get the balance correct, but I would ask you to consider the following: Most
of the people are still getting the wrong answer to most of the questions. So
I am not sure that harder questions are the right answer (the homework helps
to address this). It is also true that it is difficult to think up the questions,
and the first time through, some will be good and others not. In my opinion,
I thought this lecture had a little better balance with the blackborad vs. the
active learning. Although, I would still like to move in the direction you suggest
(a little more time covering material). Limiting the amount of time spent covering
the mud helped. The short problems worked well, but the class seemed to get
more engaged with the long problem at the end. It was nice to see, usually at
the end of a lecture everyone is asleep.

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

(67 respondents)

1)* For adiabatic why does
Q=0?* (1 student) That is the definition of the word "adiabatic"--
no heat transfer. There is frequently some misunderstanding surrounding the
words "temperature" and "heat", and consequently confusion
about the difference between isothermal and adiabatic. Temperature is a property
of the system, it units are Kelvin. Heat is energy transfer by virtue of a temperature
difference, its units are Joules. Temperature and heat are not the same thing.
Note also that a body cannot contain "heat", it contains energy. Heat
is what we call the energy when it crosses the system boundary. It is possible
to have an isothermal process with heat addition (you just need to take energy
out via work at the same rate), and it is possible to have an adiabatic process
where the temperature changes (compress a thermally-insulated cylinder).

2)* I am having trouble applying
the concepts discussed in lecture to the actual problems (i.e. question 4 in
class).* (1 student) Practice makes perfect. There are a lot of
good examples worked out in S, B, &VW, and you will get practice on the
homework.

3)* What are the types of
work that a gas can do and have done on it?* (1 student) Gas typically
does work (or has work done on it) through exerting pressure forces on surfaces,
like those we have been considering in the piston-cylinder problems. Gas can
also exert shear forces on surfaces (think of trying to move your hand quickly
through a bucket of honey. Air is less viscous than honey, but the forces are
still there. In fact viscous forces make up an important part of aircraft drag--you
will learn more about this in Unified Fluids.) One can also take a chunk of
gas and raise or lower it in a potential field (push-pull work), or give the
chunk of gas translational kinetic energy (also resulting from push- pull work).
Of course due to gas's low density, changes in kinetic and potential energy
are often small compared to changes in internal energy. This is why when working
with gases we will often use DU=Q-W
rather than DE=Q-W.

4)* When are things a function
of the state of the system and when are they not?* (1 student)
Whether or not something is a function of the state of a system or path dependent
depends on its behavior in the world around us. The equations we are working
with were developed over many years to model the world around us, and they do
a pretty good job. So some parameters (internal energy, temperature, pressure,
density, enthalpy) are a function of the thermodynamic state of the system and
others are not (heat and work).

5) * Does the gas moved by
the fan gain kinetic energy?* (1 student) and

6)* In the battery
problem why is the battery work negative?* (1 student) The
battery is having work done on it (energy is flowing into the system) when it
is being charged. Our sign convention is that work
done by the system is positive and work done on the system is negative.

7) * How can two path dependent functions guarantee
a path independent result?* (1 student) They can't! That
is one of the amazing things to me about the First Law of Thermodynamics (that
the difference of two path dependent functions ends up in a property which does
not depend on anything but the state of the system).

8) * I don't understand the First Law equation for
quasi-static processes. Why does putting pdv in make it applicable only to quasi-static?
*(7 students) One of the things I will do in recitation tomorrow
is give you a bit of an equation roadmap. In short, we define work from the
perspective of what the system does to the surroundings. Thus we distinguish
between psys and pext, and we define work to be the integral of pext(dv). When
the process is quasi-static psys is approximately equal to pext so in textbooks
and in my notes the subscript is dropped and "p" is used, hence dw=pdv
for a quasi-static process where "p" is the same as "psys".
So when the First Law, de=dq-dw,
is written as de=dq-pdv it means that w= the integral
of pdv, or in other words, the system pressure is equal to the external pressure---quasi-static.

9) * I was unclear on your explanation of the PRS
question on the cyclic process. Why is answer 3 not possible in question
4? If two *(independent)

10) * I don't understand how the difference between
pext and psys has a bearing on the work done. Why does it always work with pext?*
(1 student) See response 8) above and T2 mud response
#2. If it is still unclear, please contact me or one of the TA's or talk
it through with one of your classmates.

11) * Still unclear on the path-dependent, path-independent
stuff. Is the change in energy of a system (DU and
DE) always path-independent?* (2 students)
Yes. Energy is a function of the state of the system, not how you get there.

12) * How many different ways are there to write
the First Law?* (1 student) Hundreds, perhaps thousands (no kidding).
We will use about 20 different forms in this class (no kidding). This is why
it is important to be clear on the nomenclature, and to recognize various terms
as indicators of the assumptions that have been made. In recitation tomorrow,
I will give you a bit of a First Law equation roadmap to help with this.

13) * Don't understand why work is path independent
if the process is adiabatic.* (4 students) This idea is expressed
in one of the corollaries of the First
Law. Internal energy is a function of the thermodynamic state of the system.
It does not depend on the process by which the system arrived at that state.
If the process is adiabatic, then the First Law becomes DU=W.
So work is equal to something that does not depend on path.

14) * The differential forms of the First Law were
not well explained. When would I use the differential form?* (2
students) We will talk more about these in recitation tomorrow. The differential
forms of the First Law are just a useful, convenient way to write them that
sometimes make manipulating the equations easier. For example, sometimes it
is easier to work with du=dq-pdv rather than Du=q-
the integral of (pdv) even though both are identical expressions of the First
Law for a quasi-static process for a system where the changes in kinetic and
potential energy can be neglected.

15) * When Q or W is added or taken away does that
change U, KE, or PE, or all three?* (1 student) Heat and work
can change all three---it depends. For example, I can take a steel ball and
throw it (i.e. do work on it) thus increasing its kinetic and possibly potential
energy. I can take the same steel ball and squeeze it (i.e. do work on it) and
thus increase its internal energy. I could also apply a voltage across the ball,
allow a current to flow (i.e. do electrical work on it) and increase its internal
energy.

16) * You mentioned that DU
is one of the properties that define the state of the system. Is DE
the other or are there others as well?* (1 student) Internal energy,
U, is a thermodynamic property and is a function of the thermodynamic state
of the system (like pressure, temperature, density, specific volume, enthalpy,
entropy). Knowing any two independent properties (e.g. density and specific
volume will not work because one is simply the inverse of the other) fully specifies
the thermodynamic state of the system. You can think of kinetic energy and potential
energy as functions of the dynamic state of the system.

17) * Please work through the answer to the last
PRS question*. (19 students) I will work through this in detail
in recitation tomorrow. Here
is my answer. Note that the wording of the last part of the question is
unclear. It can lead you either to say the work done by the gas is 598J or the
net work done by the system is -1202J.

18) * No mud* (14 students).
Uh oh, my percentage is dropping in this category.