Thermodagnabbits

Discussion in 'Off-Topic' started by thingmaker3, Feb 4, 2006.

  1. thingmaker3

    Thread Starter Retired Moderator

    May 16, 2005
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    There are, perhaps, Things that we construction workers were Not Meant To Know. If so, then I have been dabbling in that which will surely destroy my sanity...

    I've been wading through Introduction to Metallurgical Thermodynamics by David R. Gaskell, and I'm having some trouble with the big words...

    I can follow the math so far, but the definitions are lacking. That is - the definitions are only presented as formulas. "Entropy" is defined as "summation from zero to kelvin temp of all (Heat Capactity times the derivitive of the natural log of the kelvin temperature)."

    He uses the symbols, of course, but I still don't know what I'm reading about. I have in front of me the tools to calculate the Gibbs Free Energy and the enthalpy and such - but I've no clue what these things are...

    Is there an online source that provides, in words, actual definitions for these things?

    (And please don't send me to one of those sites that defines The First Law as "you can't get something for nothing." Those really don't help. Once I know the actual definitions - in Enlgihs words - I might find the humor in such statements.

    I am especially interested in "enthalpy" and "driving force," as these seem to have some practical application to real world things that I am currently interested in.

    (And speaking of real world things, why do so many discussions of thermodagnabbits revolve around imagniary ideal gasses and non-existant perfect adiabatic containers? "If this stuff actually existed, here's how it would behave...") :wacko:
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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  3. Dave

    Retired Moderator

    Nov 17, 2003
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    It makes me chuckle that you cannot find a suitable definition for the word entropy - I feel the same, deciding that its better to learn the meaning of that word from scratch, each I come across it!

    Wikipedia will provide you with some on-line assistance and you could also look over at answers.com. However I would also recommend a trek down to your local library and have a look inside the Encyclopedia Brittanica (obviously should your library have these books). I find that the Brittanica is the first place I reference when there is something I am struggling to understand - that is except the word "entropy"!

    Dave
     
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I haven't a clue about the subject, but empathise with your problem. Can I pass on a tip? I always keep the Google toolbar at the top of the screen, because in addition to having Autofill, Spellcheck and other useful goodies, it has a space to enter a Google search without leaving the environment you are in.

    I tried entering 'define enthalpy' and it came up with immediate answers. Clicked on 'back' and I was back at the forum again.
     
  5. thingmaker3

    Thread Starter Retired Moderator

    May 16, 2005
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    Thank you all for your help. I've not been to Wikipedia in some time, as I was dissapointed with the early effort. They've really come a long way since then!!

    So, enthalpy can't be measured - only changes in enthalpy can be measured. That's a non-intuitive concept for me, but I think I can handle it.

    I've been playing a bit with the equations now that I know what they represent. I should have a handle on this soon. :)
     
  6. haditya

    Senior Member

    Jan 19, 2004
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    my thermodynamics is at best raw but , enthalpy i think is the heat change in a system subject to a certain thermodynamic operation.. entropy is the measure of disorder in a system
     
  7. haditya

    Senior Member

    Jan 19, 2004
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    my thermodynamics is at best raw but , enthalpy i think is the heat change in a system subject to a certain thermodynamic operation.. entropy is the measure of disorder in a system
     
  8. gbm46

    Active Member

    May 6, 2007
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    hmm I believe entropy is defined as being zero in a perfect crystal at 0k, and that the total entropy in the universe can never decrease. I know this pretty old but I'm pretty interested in thermodynamics - it all seems to make sense.

    As for enthalpy, it is a path dependant function like heat and work, and internal energy.

    Hope this regurgitation of pointless knowledge interests someone and is accurate.

    I believe second year uni is where everything turns real, im terrified
     
  9. recca02

    Senior Member

    Apr 2, 2007
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    enthalpy is also sometimes described as total heat content of a system (consider steam flowing thru turbines)
    thus it is its capacity to do work(turning turbine blades) or raise its internal energy (like give atoms more vibrational energy) this internal energy raise may also be accompanied by
    temperature rise.
     
  10. gbm46

    Active Member

    May 6, 2007
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    oo yeah, and ideal gaswise internal energy and enthalpy depend only on temperature
     
  11. Dave

    Retired Moderator

    Nov 17, 2003
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    Considering 0K is unattainable, this obviously means that zero-entropy is a concept?

    Ahh, the beautiful vision of the end of time where all existance will be a existance of uniform radiation everywhere... :(

    The Second Law has a lot to answer for!

    Dave
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    To address the overlooked point about ideal gasses and adiabatic containers. It is simply that the "real" world is so much more complex, that we employ simplification to get close. Once we can see how to get close we can make allowances for deviations from ideality. As in most fields of endeavor it helps to learn how to crawl before we try to tun a marathon.
     
  13. gbm46

    Active Member

    May 6, 2007
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    Yes a purely human constrution that is very useful in analyzing thermodynamics. Entropy values can be derived from knowing temperatures/pressures/volumes and so forth.

    This thing about entropy never decreasing is basically the same as saying that something will never become spontaneously more ordered than it was initially (without an external influence). For example: water turning to ice, throwing a whole bunch of dominoes the air and having them all land on there side in a perfect line. Stuff like that, common sense. Entropy can decrease for a system but only at the expense of an entropy increase for the surroundings that is at least equal to the decrease. A lot of things remain at constant entropy like rock in mountains for instance - the rock would tend toward moving chaotically (increase it's entropy) due to it's potential gravitational energy, but it doesnt because it is bonded to the rock surrounding it. It is a fairly broad concept.
     
  14. recca02

    Senior Member

    Apr 2, 2007
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    pretty nifty way of describing entropy there,
    just wanted to add that increase in entropy is one reason why process arent exactly reversible.
    consider a steam turning the blades of a turbine at the cost of enthalpy provided by fuel(its a dirty job but someone has to do it). the efficiency of no machine is cent percent so total heat provided is not utilized in turning the blades, where does this heat (enthalpy) go then? it is lost in overcoming friction which in turn increases the entropy of the steam (this friction is converted into heat which is also absorbed by steam).
     
  15. Dave

    Retired Moderator

    Nov 17, 2003
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    What about dropping buttered-toast? Whatever angle you hold the toast at it will always land butter-side-down!! Is this an example of something becoming more ordered?! :D

    Dave