**General comments**

** Yeah MIT for doing classroom repairs
during lecture.** (1 student) Yes. One might think that the first
thing they would check is the class schedule... but it is nice to get the boards
fixed and they did help us get rolling again with the duct tape, etc. So in
the end it worked out.

The above, combined with my forgetting to bring the 3x5 index cards took some of the polish off the lecture, but I think the content was good. Judging from the muddiest part of the lecture cards, most people were comfortable with the First Law written as a rate equation (SFEE). Remember that there are three common areas of difficulty for the SFEE. The first is units and signs (as demonstrated in the PRS question responses). The other two we will talk about in T9 and T10.

** Time to return with cards: 6min 23s.**(1
student) Are you kidding? Our TA's are much better than that. For the record
it was 1:45 ... and all for a coaster.

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

(59 respondents)

1) ** Can mass flow in and out do work or transport
heat into and out of the system?** (1 student) Great
question. The mass flow in and out carries with it some amount of internal energy
(not heat). As we have defined it heat is the transfer of energy at a system
boundary by virtue of a temperature difference only. In this case energy is
transferred by the flow into and out of the control volume. So the answer to
the heat part of your question is "no". The answer to the work part
of your question is "yes" and this is the topic of tomorrow's lecture
(T9). In preparation for this, please read the section of the notes related
to shaft work and flow work.

2) * No mud, just unclear how
to get u for a gas entering/exiting an engine.* (1 student) and

3) **Physically, what is the
difference between ***
and eout-ein = q-w. and related questions on units* (2 students)
Physically there is no difference. The two equations are the same with the exception
of the choice of variables used (intrinsic versus extrinsic). Sometimes one
is more convenient than the other. The second equation is wholly expressed in
terms of intrinsic variables (J/kg), so in order to find Watts out of a particular
system, one would have to multiply by the mass flow (kg/s) for that system.

4) * c=velocity, not just the
speed of light?* (1 student) In a never-ending search for a symbol
we haven't used yet, I have chosen c for velocity of the flow (we already use
v for specific volume, V for volume, and u for internal energy---which are the
common ones used for velocity in the Fluids lectures).

5) * Why isn't "gh" used in the SFEE in the reading notes?*
(1 student) Good observation. I left it off for the same reason we typically
leave it off in other versions of the First Law we have worked with. For gases,
changes in potential energy are typically small relative to changes in other
forms of energy. You all showed this on T5 part
a. You have to lift a gas an long way (14km) to change its potential energy
an equivalent amount to the work done in a compression of 5x. In the example
we did in class and in the derivation on the board I included it so you
would feel comfortable using it for problems where the change in potential energy
is important (like water flows).

6) * Units in [u + (c^2)/2
+ gh], the latter two were neglected and u taken to be J/kg anyway. Don't understand
it.* (1 student) All the units are consistent. c^2 and gh have
units of m^2/s^2 which is the same as J/kg. (A Joule is a kg-m^2/s^2).

7) * What is e?*
(1 student). The same e we defined in Chapter
4.

8)* The derivation of the
new version of the First Law (for mass flow) was a little rushed.*
(1 student) Please review the derivation in the
notes, and read through the appropriate section of S, B & VW. If there
are parts that are still unclear contact me or a TA to work through them.

9) * When we say d/dt = 0,
does that mean both dEc.v./dt = 0 and dm/dt = 0?* (1 student)
Yes.

10) * Working with equation*
(1 student) This was the first time through. You will get much more practice.
Just remember, be careful with signs and units.

11) * I am still having difficulty
trying to figure out the sign convention for heat added.* (1 student)
Heat added to the system is positive. Heat removed is negative. In the example
given in class the heat was removed from the system so it was negative.

12) * Why does steady flow
imply no change in E as well as mdotout=mdotin?* (1 student) If
the mass within the control volume does not change over time, then what comes
in must equal what comes out. The same is true for the energy. If the energy
within the control volume does not change over time then all of the energy flows
to and from the system (ein, eout, q and w) must all balance to zero.

13) * If the mass is constant
in a closed system, why do we rarely find the mass? I mean, we always use specific
volume.* (1 student) Quite often it is more convenient to work
in intrinsic variables rather than extrinsic variables. But the concepts behind
the equations are the same either way.

14) * How can the entering
and exiting stream heights be so different? (-10m & 300m) or is it just
the example?* (1 student). Basically, it was just an
example, but a difference in heights like this would be relevant for the
flow over a hydroelectric dam (like the Hoover Dam).

15) * How does [u + (c^2)/2
+ gh] sum up to be e? *( 2 students). It expresses the
total energy per kg as the sum of the internal energy, the kinetic energy and
the potential energy (all per kg).

16) * From the lecture notes, why is the flow work
p2v2-p1v1?* (1 student). I am pleased you read the material. This
is the topic of our lecture tomorrow (T9).

17)* In the calculation I get that the rate of work
is approximately equal to 18kW instead of 20kW. Are the calculations right?*
(1 student). Yes they are. The correct answer
is 20005 W ( I just checked it again to make sure).

18) * No mud* (34 students). All-time
high for this semester.