Thermodynamics: Engineer vs. Physics (1 Viewer)

[Edwin_C]

i said that the 1st law dealt with mechanical energies, such as kinetic, work, heat, for mainly closed systems (i actually said only, but i meant almost only...edwin caught that one)

the 2nd law brings in enthalpy for reactions processes...while he contends that entropy is the dominating factor, and 3rd law we were in almost argeement upon being related to gibbs free energy, although he contends that it defines gibbs free energy

no, actually the first law brings that in. the second law brings in entropy to the equation. enthalpy is stored energy, correct? the energy "stored" within a molecule is stored in the form of translational, kinetic, and potential (bonds). although all forms of energy are relevant in the reaction process, i'm only going to take potential into consideration. with the molecular orbital theory in mind, some bonds are more "violent" then others. they store more energy but can't be broken unless certain criteria are met (the 3rd law would come in here). once the bonds are broken, energy is released in the form of heat.

the first law applies once the reaction occurs, the second law says why it occurs.

the first law states- energy is neither created nor destroyed it just changes form.

the second law states that entropy is constantly increasing universally, but locally it's equal to or greater then 0. increase in entropy is attributed by a decrease in potential. if a reaction occurs, and the energy from a bond is released then the potential of that molecule has been reduced. if a reaction occurs and energy is absorbed by the bond (phase changes) then the molecule has more ways in which it can "move" (decrease in order).

very well put Andrew. one thing tho, enthalpy is diabatic too.

 

[Andrew Testa]

very well put Andrew. one thing tho, enthalpy is diabatic too.

I very nearly was as well, but fortunately a change in diet got my insulin response working properly.

Thanks Edwin.

 

[Edwin_C]

I very nearly was as well, but fortunately a change in diet got my insulin response working properly.

hahahhahahahah . that really made my day.

 

[Cary_H]

I'll have to dig out my Reed's texts to brush up on Thermo.

The 1st Law is Conservation of Energy.

Had to edit the rest of this post 'cause it didn't come up as it originally typed. Sorry

 

[Mike Wladyka]

Wow...you guys are smart!! i am beginning to lose the debate, however....

Statements of second law (chemical engineering thermo book)

1. NO apparatus can operate in such a way that its only effect is to convert heat absorbed by a system completely into work done by the system.

2. No process is possible which consists solely in the transfer of heat from one temperature level to a higher one

okay, so these have both been said...but this is my take...enthalpy is stored energy you are correct on that. I say it is for reaction processes because, for reactions the enthalpy change is usually much greater than energies from potential, kinetic, temperature changes, etc...thus making reaction enthalpy more important than these others...now on to the other terms

Heat: enthalpy changes much more with temperature than pressure...so if you are adding/subtracting heat to the system, the enthalpy will be important

Work: work accomplishes 2 things in most systems...1 it moves mass, and 2. it will add heat to the system (thus enthalpy)

Now for the 2 statements.

in both statements, enthalpy is key...why? because they deal with the transfer of heat into work or vice versa...however as Andrew stated there is not a direct conversion between the two...enthalpy is the missing term.
i say the enthalpy in the second law is mainly for reaction processes because for a reaction the two things involved are heat and work...and the transfers of energies between heat, work, and enthalpy is very important...thus the 2 statements show that enthalpy is key...

i hope that you can follow that logic, as last night was wings and beer night...

this is so much fun...i am enjoying this,
keep it up guys...i was almost convinced

mike

 

[Andrew Testa]

OK, let's define terms. But first, let it be known that I loathe trying to write equations in ASCII, and anything more complicated that what's below will get a reference to a properly formated equation online. With that said,

Enthalpy = U + pV

U = energy (heat, temperature)
p = pressure
V = Volume

In a process where properties of enthalpy are used to change one of the constituents, such as lowering temperature by increasing volume, our formula is

U1 + p1V1 = U2 + p2V2

So, while temperature is altered, and work is done, there are no terms for external heat being applied (Q). There is only heat before and heat after the process. Nothing is added or removed, the energy is redistributed. This is simple energy conservation, and governed by the first law.

The second law only comes into play if external energy is added to the system. In refrigeration processes, this is in the heat pump that warms the cold refrigerant.The throttling process, where the warm, high pressure refrigerant is cooled using pressure and volume changes, requires no added energy and is the enthalpic process shown above.

enthalpy is key...why? because they deal with the transfer of heat into work or vice versa...however as Andrew stated there is not a direct conversion between the two...enthalpy is the missing term

This is most definitely NOT true. enthalpy has nothing to do with the incomplete conversion of heat into work. There is no missing term. That heat is lost, lost forever, and can never be recovered, reused, or accessed in any way, shape, or form. But work IS completely converted into energy, so again there is no room for any enthalpic process.

Enthalpic reactions are all first law processes. The engines and pumps which drive the process add heat to produce work to keep the cycle going. These engines are governed by the second law, since they lose heat in the process of producing work to pump the reactants. The enthalpic reaction itself, however, is only governed by the first law.

Wings and beer is an acceptable excuse for most transgressions, BTW.

Andy

 

[Edwin_C]

in both statements, enthalpy is key...why? because they deal with the transfer of heat into work or vice versa...however as Andrew stated there is not a direct conversion between the two...enthalpy is the missing term.
i say the enthalpy in the second law is mainly for reaction processes because for a reaction the two things involved are heat and work...and the transfers of energies between heat, work, and enthalpy is very important...thus the 2 statements show that enthalpy is key...

enthalpy only applies to reaction processes. enthalpy is the amount of heat that the system has to absorb/release in order to make the reaction happen.

 

[Mike Wladyka]

Correction: I did not mean that the losses between changing heat to work was enthalpy...although i did sound that way...thanks for accepting the beer/wings excuse on that one


Andrew and Edwin C,

I stand corrected, everything you guys say makes a lot of sense...it finally fills in the holes of my thermo thinking...i have always had little holes in my thermo knowledge and i had just filled them with what i thought made sense, so i could easily pass thermo classes, which were fairly easy for me (i hate to criticize profs, but i think some tend to dumb things down, and use rules that work for problem solving only...instead of explaining theory) . My set rules seemed to always work during all of my problem solving, ie tests and homeworks, but i see now that they kept me from realizing the obviousness of the truth (sorry, about the matrix qoute but since it is a home theater forum i thought i would throw it in). Congrats on a debate won...I am thankful for it...i realize that i know very little in the grand scheme, and i am always thankful for explanations that answer all questions...that is why i love debating so much...

i was hoping for this to continue but i ran out of points to try and make...

Thanks guys...How about starting up a debate/discussion on something else? Why is speaker sensitivity so low? What causes it to be 90db, why isn't it 98 or 99db?

 

[Andrew Testa]

i realize that i know very little in the grand scheme

Who does?

I'm glad I could help. I am always grateful when my understanding of something is clarified, excepting when said clarification is prefaced with the phrase, "You dipshit,".

Andy

 

[Andrew Testa]

Why is speaker sensitivity so low? What causes it to be 90db, why isn't it 98 or 99db?

Second Law of Thermodynamics, maybe?

A speaker is a motor. you add energy (current) and produce work (motion of the voice coil). The more efficient the motor, the greater the percentage of energy converted to work. The more work produced, the greater the cone travel, and the higher the resulting wave amplitude. Very efficient motors produce high decibel ratings for a given input.

Lower efficiency speakers have more energy loss (heat, VC resistance, back EMF, basket resonance, spider and surround stiffness, cone resonance, etc) and less work being applied to moving the cone. This results in lower amplitude output, thus a lower decibel rating for a given input.

Got any questions that DON'T involve the second law?

Andy

 

[Mike Wladyka]

Who does? I have met a great many people who think they do and it is ironical that they usually are the ones to use "You Dipshit"

 

[Edwin_C]

if you want to start up a debate, it has to be done on something that's not definite in nature. something that could be questioned as ethical/logical.

i realize that i know very little in the grand scheme

that's the most important thing. life is an endless learning cycle. with every question answered, new ones arise. there'll never be a point where you know a lot.

 

[Cary_H]

One little error earlier regarding heat pumps. What actually occurs is the refrigerant flashes off as it enters the evaporator, (low side) absorbing heat from whatever medium is passing by. The heated refrigerant in a vapor state is now "pumped" over to the high side where it dumps the heat it absorbed as it condenses due to higher pressures in the condensor, passing this heat to whatever medium is pushed over this coil.
The heat given off by the compressor plays no real part in the whole process. It's measure is a pretty good indicator of the efficiency, or rather the relative inefficiency of the loop.

 

[Edwin_C]

One little error earlier regarding heat pumps. What actually occurs is the refrigerant flashes off as it enters the evaporator, (low side) absorbing heat from whatever medium is passing by. The heated refrigerant in a vapor state is now "pumped" over to the high side where it dumps the heat it absorbed as it condenses due to higher pressures in the condensor, passing this heat to whatever medium is pushed over this coil.
The heat given off by the compressor plays no real part in the whole process. It's measure is a pretty good indicator of the efficiency, or rather the relative inefficiency of the loop.

1. the refrigerant doesn't "flash off."
2. there's no such thing as an "evaporator," the freon liquid runs through an expansion valve that causes it to change phases.
3. the heat isn't dumped as it condenses, the heat is caused by it's condensation (ideal gas law ring a bell?). once the liquid is condensed, it runs through the piping to reduce heat much in the same way as a radiator does so in an engine. the final cooling is done at 2.
4. this isn't a heat pump.
5. who ever mentioned refrigerators/air-conditioners?

 

[Cary_H]

I wanted to keep this as simple as I could.
The refrigerant in the high side is indeed metered into the evaporator. Since it is in a liquid state beforehand in the higher pressure environment of the condenser it will start to absorb heat from the medium we want to cool that is being passed over the coil. Let's call it a heat exchanger. Absorbing this heat removes heat from the medium and this heat will, well, result in the refrigerant changing phase to a gas as you suggest.
In the condensor the gas passes the heat energy it now has, to a cooler medium passed over this side. It gives up enough heat that it reverts back to a, let's call it a liquid state. Yup, the heat is freed up when it condenses, but the heat has to be taken away to keep it all going.
Depending on the device and what each uses to perform this task, the process is the same.
This describes an A/C unit. A heat pump is the same thing. You just reverse the direction of the refrigerant flow. A compressor is a compressor, but it's sole function here, whether you want to heat or cool, is to move the refrigerant and maintain two seperate environments for it to work. Heat absorbed on one side, heat given up on the other. What you do with it is up to the user.

 

[Edwin_C]

This describes an A/C unit. A heat pump is the same thing. You just reverse the direction of the refrigerant flow. A compressor is a compressor, but it's sole function here, whether you want to heat or cool, is to move the refrigerant and maintain two seperate environments for it to work. Heat absorbed on one side, heat given up on the other. What you do with it is up to the user.

ok maybe i'm just not understanding really what you're trying to say. this topic is about the second law of thermodynamics, which has been settled. the way you're describing it, you make it seem as though it follows closely to carnot's engine... which is obviously not true.

really tired right now, i have a few more things to say.. just can't remember exactly what it was hahah.

 

[Mike Wladyka]

I am getting drawn back in, but i have to do some work first...got to make those TIM's

Mike

 

[Andrew Testa]

5. who ever mentioned refrigerators/air-conditioners?

I'm afraid I did, Edwin. However, I was only using it as an example of the difference between the 1st and 2nd laws, and had no intention to build a verbal model of one. My point (to Mike) was that the throttling process, or expansion valve, was a 1st law enthalpic process, but the motor and compressor side was a 2nd law process. That's all. I had no intention to get into the details of refrigerants.

Andy

 

[Edwin_C]

ok when you brought out the issue of the heat pump, i made a mistake and thought you were talking about a heat engine.

I'm afraid I did, Edwin. However, I was only using it as an example of the difference between the 1st and 2nd laws, and had no intention to build a verbal model of one. My point (to Mike) was that the throttling process, or expansion valve, was a 1st law enthalpic process, but the motor and compressor side was a 2nd law process. That's all. I had no intention to get into the details of refrigerants.

Andy

i'm blind hahah.