Need Help with Thermodynamics Course

Jedi Master · Post by **Jedi Master** » 06 Apr 2006, 10:29

I took so many math classes I got a minor in it by default, so I had a double minor with that and history (because I took so many classes as electives, just because I thought they were interesting!) The math equivalent to thermo is vectors and matrices. I don't know if it was the actual material or the fact that my professor barely spoke English, but it sucked.
If I'd taken 3-4 more math classes I'd have had a double major, but I was burning out on physics and just basically cruised my last year or so. Went from almost a 4.0 average at the end of the soph year to barely over a 3.0 at graduation. Still graduated with honors, though, for all THAT was worth. :roll:

Gator · Post by **Gator** » 06 Apr 2006, 15:22

Thermo was one of the classes where I actually had to apply myself. I worked every practice problem provided in excruciating detail -- it was enough to earn the best score in my class/section (200 or so students) in 1994. For me it was an important class since thermodynamics is at the heart of aerospace engineering.

PanzerMeyer · Post by **PanzerMeyer** » 06 Apr 2006, 15:39

Well....I just feel as dumb as a rock compared to Gator now!

Seriously though, I absolutely hated Math throughout my entire public education and university years. Most of my math teachers were extremely boring and I just never saw the point to graphing a polynomial or finding out if this triangle was scalene or isoceles. I think that's why I was always so good at history/geography and other social sciences. I saw how those subjects applied directly to people and real world events.

Gator · Post by **Gator** » 06 Apr 2006, 15:57

PanzerMeyer wrote:Well....I just feel as dumb as a rock compared to Gator now!

I forget who said it earlier in this very thread - each of us has our strong areas and our weak areas. I struggled through Chemistry and Literature (lit profs are pr!cks

). Thermo was just one of those things that "clicked." Plus, that was nigh on 12 years ago - very little of what I studied in school has even been directly applied in my career. I stand by my theory that school is only a way to learn how to navigate beuracracies (sp) and figure out where to locate the information you really need to solve the real problems later.

PanzerMeyer · Post by **PanzerMeyer** » 06 Apr 2006, 16:01

[/quote]

I struggled through Chemistry and Literature (lit profs are pr!cks

). [/quote]

LOL!! Are they pricks or just simply pretentious?

Gator · Post by **Gator** » 06 Apr 2006, 16:03

perhaps both. The one I had in mind was truly a phallus. Funny thing is - he was relatively well known as a "Star Wars Scholar" which I learned much later.

Hammer · Post by **Hammer** » 06 Apr 2006, 16:06

well, i just interviewed a guy that did his master's thesis on self-assembled magnetic nanowire arrays. talk about fascinating stuff and the things it could be applied to. they are mostly looking at it for medicinal purposes...

but most of my knowledge is in affecting real world stuff and applying it to people as well:

proper sight picture
breath control
smooth trigger pull
field expedient breaching charges
directing others in bounding overwatch during assaults
creating interlocking fields of fire
call for and adjust fire
utilize graphics to depict battle plan
write 5 paragraph operations order
create battle plan
lead a platoon during combat operations

basically, visit death and destruction upon the enemy.

PanzerMeyer · Post by **PanzerMeyer** » 06 Apr 2006, 16:12

Ok, that settles it! If the Chinese ever invade the USA and take us over, I nominate Steel to lead our WOF squad of guerrila resistance fighters!

Grifter · Post by **Grifter** » 06 Apr 2006, 16:32

I second that. :lol:

KODIAK · Post by **KODIAK** » 07 Apr 2006, 04:52

. . . . . . . . self-assembled magnetic nanowire arrays

Hey! HEY!! Can we just stick to maths & arithmetic here!? :shock:

:lol:

Hammer · Post by **Hammer** » 07 Apr 2006, 09:57

that is what i said: :shock:

but he explained it and it was pretty cool...

KODIAK · Post by **KODIAK** » 07 Apr 2006, 10:15

Sounds like a pretty cool topic though - I'm going to have to google this! Now I'm intrigued.

Sabre · Post by **Sabre** » 09 Apr 2006, 09:50

Okay, how about this... I'll try something specific and see what you guys think. I'm working on Energy Analysis of a closed system. Conservation of energy principle and all that. Lots of piston cylinder devices being used with various gases inside. the basic principle used throught is deltaU=Q-W. Every time I think I know what should be considered work and what should be considered internal energy (U), etc. I am proven wrong.

In one example we have a room where we are disregarding any heat transfer through walls and doors. We are given a constant room volume, an initial Pressure and Temperature, the Wattage of a running fan and an amount of Time that the fan is running for. We are asked if the room cools down,which of course it will not because no heat is being removed and the work from the fan will add heat to the room.

Q=0 and we are left with deltaU = -W
delta U = Wattage * Time = kJ
that is one thing that confuses me because I would have thought the work by the fan would = Work or negative work whatever the case might be.

The problem boils down to
delta U = mass * Cv * (T2-T1) and we solve for T2
(mass and Cv were calculated for air from data given)

What happened to work? I'm confused.

KODIAK · Post by **KODIAK** » 09 Apr 2006, 17:36

I understand the comment - no heat loss through the walls or doors. This is simply to eliminate the complexity of calculating losses over and above your basic calculation. As if the whole experiment was being conducted in a vacuum (controlled environment). Perhaps . . .

But I am also confused by the suggestion that the room will cool down, given there are no losses, and energy is being introduced in the way of a heater with a given time of operation (obviously providing a rise in temperature!) :shock:

Sabre · Post by **Sabre** » 09 Apr 2006, 18:11

Well, that is the solution, actually, that the heat increases. The question was: does the room cool down? The answer was no.

The part I didn't understand was, how is it that the work done by the fan is counted as internal energy and not as work in the basic formula U=Q-W?

When I look at the solution it appears to me that work just somehow gets eliminated, so I know I'm missing something, I just don't know what I'm looking at in the wrong way. It seems to be a pattern in my thinking that is messing me up in many problems.

KODIAK · Post by **KODIAK** » 11 Apr 2006, 06:33

It looks to me simply like they are not necessarily categorising each source of heat or leakage. To that end you are left to do that, and that is where the pitfall is IMHO. I can't remember if there is a general rule that states for example: Any source not categorised by the question's author should be automatically categorised as . .say 'internal'.
Can anyone who's more current on this subject suggest anything? It's been a few years for me, am afraid.

Gator · Post by **Gator** » 12 Apr 2006, 08:03

Sabre,

Thermo "problems" are solutions to an equilibrium state. In the problem in this case we have a room. The walls don't move and no energy crosses them. To solve these types of problems, we must choose a control volume for our analysis -- typically, you would want a control volume where no energy crosses. Logically, we would choose our "control volume" to be the walls based on the problem statement that no energy crosses them.

"Work" would occur in the case where say one of the walls was a "piston" that moved (such as the haunted house ride at disney), changing the volume of the room and therefore "doing work" on the room.

In the case of the sample problem you post, the fan is inside the room and is therefore considered to be internal energy in this case.

I'm not sure I explained it well, so if it's not clear, let me know and I'll try again.

-Brian/Gator

Sabre · Post by **Sabre** » 14 Apr 2006, 17:39

Gator,

We are getting very close, I see...
I can totally see how the fan becomes internal energy, since it is within the system barrier. Makes total sense.
So then the energy put off by the fan = U. The general formula here for U is m*Cv*deltaT
Great I can solve for U. But in the energy balance formula, since Q=0 we end up with U=-W, is that right?
If that's true then should my answer be negative?

For some reason it's the logic stuff in this section that kills me. We have moved further on and it's not as bad to me.

Jedi Master · Post by **Jedi Master** » 14 Apr 2006, 17:55

Like I said before, my brain doesn't work the way thermo wants it to.

Gator · Post by **Gator** » 14 Apr 2006, 18:22

Well,

We're talking heat capacity at contant volume ...

deltaU=Cv x (T2-T1) or [deltaU = m x cv x (T2-T1)] right? where deltaU is the internal energy, which is the fan wattage multiplied by how long it was running ... I'm assuming we know Cv? That will give us deltaT, the change in temperature ... which will tell us if the temperature increases or decreases...

I believe that is correct -- I'm a little rusty on the notations/formulae ...

-B

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