Dismiss Notice
Hey Guest! Check out the 3 MCAT Study Plan Options listed in the 'stickies' area at the top of the forums (BoomBoom, SN2ed, and MCATJelly). Let us know which you like best.

Also, we now offer a MCAT Test-Prep Exhibitions Forum where you can ask questions directly from the test-prep services.
Dismiss Notice
SDN members see fewer ads and full resolution images. Join our non-profit community!

1st law of thermodynamics, confusing

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

  1. tiger85

    Joined:
    Feb 17, 2007
    Messages:
    3
    Likes Received:
    0
    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.
     
  2. Thread continues after this sponsor message. SDN Members do not see this ad.

  3. soccerpunk60

    soccerpunk60 Junior Member

    Joined:
    Aug 6, 2006
    Messages:
    223
    Likes Received:
    0
    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

    Physician Faculty Moderator Emeritus

    Joined:
    Aug 5, 2006
    Messages:
    1,241
    Likes Received:
    39
    Status:
    Attending Physician
    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

    Physician Faculty Moderator Emeritus

    Joined:
    Aug 5, 2006
    Messages:
    1,241
    Likes Received:
    39
    Status:
    Attending Physician
    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

    Physician

    Joined:
    Aug 30, 2006
    Messages:
    907
    Likes Received:
    3
    Status:
    Resident [Any Field]

    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

    Physician Faculty Moderator Emeritus

    Joined:
    Aug 5, 2006
    Messages:
    1,241
    Likes Received:
    39
    Status:
    Attending Physician
    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

    Physician

    Joined:
    Aug 30, 2006
    Messages:
    907
    Likes Received:
    3
    Status:
    Resident [Any Field]
    i guess... if you want to memorize everything.
     
  9. DiverDoc

    DiverDoc KCUMB 2012

    Joined:
    Oct 30, 2005
    Messages:
    1,476
    Likes Received:
    0
    Status:
    Attending Physician
    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

    Physician Faculty Moderator Emeritus

    Joined:
    Aug 5, 2006
    Messages:
    1,241
    Likes Received:
    39
    Status:
    Attending Physician
    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

    Physician

    Joined:
    Aug 30, 2006
    Messages:
    907
    Likes Received:
    3
    Status:
    Resident [Any Field]


    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

    Joined:
    Oct 1, 2006
    Messages:
    520
    Likes Received:
    0
    Status:
    Post Doc
    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

    Physician Faculty Moderator Emeritus

    Joined:
    Aug 5, 2006
    Messages:
    1,241
    Likes Received:
    39
    Status:
    Attending Physician
    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

    Joined:
    Sep 18, 2006
    Messages:
    692
    Likes Received:
    0
    Status:
    Pre-Medical
    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

    Joined:
    Dec 30, 2006
    Messages:
    693
    Likes Received:
    0
    Status:
    Post Doc
    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. Thread continues after this sponsor message. SDN Members do not see this ad.

  17. Nowaythisnameis

    Joined:
    Mar 2, 2007
    Messages:
    32
    Likes Received:
    0
    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
     

Share This Page