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1st law of thermodynamics, confusing

Discussion in 'MCAT Discussions' started by tiger85, Mar 25, 2007.

  1. tiger85

    tiger85 7+ Year Member

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    Feb 17, 2007
    I am preparing for MCAT from TPR and Kaplan and they both different eqation for the 1st law of Thermodynamics.

    In TPR

    Change in Internal E = Q - W

    and IN kaplan it says

    Change in Internal E = Q + W

    I also noticed that they both different signs for the work done by the system on its surroundings

    In TPR W is Negative and in Kaplan W is positive.

    This is pretty confusing I think TPR is right because I think work done by the system that means the energy is lost so it has to be negative (just like enthalpy is negative)

    Any suggestion on my opinion.
     
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  3. soccerpunk60

    soccerpunk60 Junior Member 5+ Year Member

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    Aug 6, 2006
    I don't know what you're reading, but I'm using kaplan and it gives the equation for internal energy as :

    U = Q - W

    U as the internal energy of the system
     
  4. RPedigo

    RPedigo Physician Faculty Moderator Emeritus 10+ Year Member

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    Aug 5, 2006
    Los Angeles, California
    MDApps:
    No, the change in internal energy is

    ΔU = q + w

    And can be written as ΔU = q - PΔV

    q = heat
    q = work = -PΔV for PV-related work-- the MCAT would not ask you about non-PV-related work. If they do, they would first explain it in the passage.

    ΔE and ΔU, for the purposes of the MCAT, can be used interchangeably. ΔU can be really used in lieu of ΔE when you're talking about a system at rest, with no external fields.
     
  5. RPedigo

    RPedigo Physician Faculty Moderator Emeritus 10+ Year Member

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    MDApps:
    I will also add, if it really says this, that is a pretty egregious error on their part, and I hope it's been rectified since.
     
  6. killinsound

    killinsound Physician 10+ Year Member

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    Aug 30, 2006

    why would it be egregious? it's all relative.

    just know it intuitively...

    if you are compressing something, you are putting energy into the system so you need to add the "work", if you are expanding you're taking energy out of the system you are subtracting it.
     
  7. RPedigo

    RPedigo Physician Faculty Moderator Emeritus 10+ Year Member

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    MDApps:
    Because no matter how you put it, w = -PΔV

    If you plug that into their above and incorrect equation, you'll get that:

    ΔU = q + PΔV, which gives the incorrect answer.
     
  8. killinsound

    killinsound Physician 10+ Year Member

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    Aug 30, 2006
    i guess... if you want to memorize everything.
     
  9. DiverDoc

    DiverDoc KCUMB 2012 10+ Year Member

    You just need to be careful and know how to correctly designate whether work is being done BY the system as in U=q-w or on the system for sign values and you will still get the correct answer.
     
  10. RPedigo

    RPedigo Physician Faculty Moderator Emeritus 10+ Year Member

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    MDApps:
    I agree with both of you in that intuition is the better approach, but it doesn't change the fact that it is a very significant error on their part, because it's the formula that much of thermodynamics is based off of. I personally didn't consciously memorize the equation, but my research is in the realm of physical chemistry, so I happen to know it.
     
  11. killinsound

    killinsound Physician 10+ Year Member

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    Aug 30, 2006


    agreed.

    i think kaplan defines work as P delta V which is "incorrect", but nonetheless serves their purpose and works fine for the mcat...

    kinda makes you wonder if that's the reason they left out the enthalpy equation.
     
  12. MSTPbound

    MSTPbound student Moderator Emeritus 2+ Year Member

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    Oct 1, 2006
    for the first law. Everyone means the same thing, but depending on the field they express it differently, e.g. from the perspective of the system vs. perspective of the environment.

    If you check your Chemistry textbook against your Physics text, you may or may not see the same expression for the 1st law... and if you do, grab an engineering text, and I'm CERTAIN you won't see identical expressions, even if they are equivalent.

    I came across a nice blurb on Wiki about this, so I'll just link it up here... for those interested in REALLY understanding vs. "plugging and chugging," you might enjoy this explanation:

    http://en.wikipedia.org/wiki/First_law_of_thermodynamics#Sign_convention
     
  13. RPedigo

    RPedigo Physician Faculty Moderator Emeritus 10+ Year Member

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    MDApps:
    Ah, I did not realize that they also defined work as +PdV. Seems a bit counter-intuitive as a definition, but whatever works for them.
     
  14. paranoid_eyes

    paranoid_eyes 2+ Year Member

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    Sep 18, 2006
    E = q - w? That's funny, in my gen chem text it said E = q + w?? Anyways, I always remembered the first law as, "the change internal energy (E) of an isolated system is zero." therefore, in an isolated system, q = -w.
     
  15. arsenewenger

    arsenewenger 2+ Year Member

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    Dec 30, 2006
    Washington
    Both eqns are right. It all depends on what is going on in the system.
    Expansion... system does work on the surrounding and system therefore loses energy resulting in E = Q-W(i.e. when PV work is done by the system)

    Compression...surrounding does work on the system and system therefore gains enegy resulting in E= Q+ W(i.e. PV work is done on the system)
     
  16. Nowaythisnameis

    Nowaythisnameis 2+ Year Member

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    Mar 2, 2007
    lol you guys all don't know what ur talking about.
    for the equations the op posted, both are correct. but the OP forgot to add the delta in front of E and Q. so the change in energy is always equal to the sum of the heat plus the work done on the system or by the system. if the work done is by the system, then it would be -W, and if work is done on the system, it would be +W. both books are correct, u just need to realize whether the system or the surrounding is doing work
     

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