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Does this question make sense?

Discussion in 'MCAT Discussions' started by bluesTank, May 1, 2007.

  1. bluesTank

    bluesTank Zombie

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    This is a physics passage question. It is referring to a simple gas compression experiment with a cylinder.

    If the cylinder has adiabatic walls, then the walls would:

    A) Allow heat to flow through
    B) Do not prevent heat from flowing through
    C) Resist Corrosion
    D) Perform the same function as isothermal walls.

    The answer is D, and the explanation just defines what adiabatic means, which is what I was looking for.
     
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  3. JiSU

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    adiabatic walls won't allow any heat energy be exchanged with the enviroment. If no heat energy is exchanged with the environment, the walls won't change temperature. They will be isothermal.
     
  4. bluesTank

    bluesTank Zombie

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    Isothermal walls DO allow exchange of heat, while adiabatic walls do NOT. So if you compress a gas adiabatically, then no heat can escape, and you have different results then if you do it isothermally.

    Isothermal would be steel walls or something thats highly conductive of heat.

    Adiabatic would be something like styrofoam, or something highly insulating.

    Question still confuses me (if this is a valid question)
     
  5. JiSU

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    That's how my princeton review chemistry chick explained the answer. I didn't get it either. I didn't see a question like that on the MCAT though.. if it makes you feel better. I thought the real test was easier than TPR practice tests and definitely easier than that freaking workbook.
     
  6. killinsound

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    this is how i reasoned D

    i'm a little MCAT rusty (3-4 weeks), but adiabatic means no heat exchange... isothermal is when there is no change in internal energy.

    so if you have an adiabatic wall, you prevent heat from flowing into or out of the wall. q=0

    if you have an isothermal wall, you prevent the internal energy from changing and because the walls aren't getting compressed, or expanding themselves (w =0 ), Q must equal work... and since work is 0, q must be 0 too, so adiabatic and isothermal walls will do the same thing.
     
  7. bluesTank

    bluesTank Zombie

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    Work does not equal zero in an isothermal process. You can still compress a container that has isothermal walls. the reason there is no change in internal energy is that q = -w . So E = q + w = -w + w = 0
     
  8. killinsound

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    w = P delta V... if the walls themselves arent compressing/expanding or there is no pressure, there is no work done on/by the walls.

    i never said work equals zero in an isothermal ALWAYS, but in this situation it does, and therefore, q will equal 0 too.

    i'm really confused why you asked this question if you had such a handle on it
     
  9. bluesTank

    bluesTank Zombie

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    The cylinder is compressing the gas within it, so the volume is changing.

    I am asking because it goes against what I understand it to be, and it's not very often that you come across a problem that the answer just doesn't make sense. So until I can either find that this was a bad question, or that I am just dumb (this often happens in the end), it just bugs me :laugh:

    Thank you for your help
     
  10. Swiperfox

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    D cannot be a correct response unless the passage has information we need to answer this.

    The definition of adiabatic would necessitate that the walls could not allow heat to flow through, thus B must be correct regardless of whether or not D is correct. You cannot have two right answers, so if TPR could somehow support D I'd still say this questions is trash.

    As to the arguments posited about work done on the walls being zero, that is definitely not true. The PdeltaV formula is for work done on a gas, not on walls. cylinder walls would behave like an elastic material, and thus the work done upon them would be equal to the change in strain energy associated with the changing pressure of the cylinder. Since the cylinder is pressurized by a compressing piston, hoop and longitudinal stresses will increase within the cylinder walls. With this increase in stress comes an increase in strain, and thus strain energy.

    Strain energy is sort of analogous to potential energy of a spring with constant K (1/2*K*X^2), but in this case X would be a strain value.
     
  11. Canadian Premed

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    You're wrong, read B again. It says Do not prevent heat from flowing through. So therefore, it means it allows for heat to flow through, which we know is false. It got me the first time I read it too. The only answer that would make the most sense is D.

    To stop heat from flowing in or out, the walls have to resist change to temperature. Therefore, the walls have to be the same throughout the reaction or whatever. So therefore, isothermal (no change in temperature).

    If I'm wrong then sorry, didn't start studying for the MCAT yet. I'm starting next week. Good luck.
     
  12. bluesTank

    bluesTank Zombie

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    I am just going to go talk to my TPR chem teacher about this one, but i'm starting to think it's just a bad problem.

    As to what Isothermal means vs adiabatic. I believe if the walls are adiabatic, a compression would result in a rise in temperature because the compression inputs work, and since energy cannot escape as heat, it simply raises the temp.

    As for isothermal, since the walls allow free heat transfer, instead of raising the temperature, it remains constant by releasing the same amount of heat energy to the surroundings as is put into the system, which is why q = -w, therefore keeping the T constant.

    I will post what my instructor tells me. i don't think this problem has a right answer though...
     
  13. sehnsucht

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    i agree with you on this one. B is a play on words that is easy to miss if you are working fast. For B to be true, it would HAVE to say that the container would PREVENT heat from flowing through which, by definition, is what an adiabatic chamber should do; an adiabatic chamber that does not prevent heat from flowing through wouldn't be an adiabatic chamber now would it?

    D is the correct answer because it isolates the energy in the container from the environment. answer D properly answers the question.
     
  14. zanzounita

    zanzounita Int'l Woman of Mystery

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    This question also bugged me..but the answer must be D due to process of elimination and what the guy above me said :laugh:
     
  15. bluesTank

    bluesTank Zombie

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    But an isothermal container does not isolate energy...
     
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  17. killinsound

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    :laugh: completely right... im a dummy!
     
  18. Swiperfox

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    Haha, I now I know I am tired and need a study break. B and A are the same, for some reason I read B as Prevents heat from flowing, lol.
     
  19. MSTPbound

    MSTPbound student
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    Right. Knowing very little about thermodynamics, you can choose D as the answer. That doesn't mean that D is true... it just means that it's the best answer of the four choices. A and B are the same, so knock them out. Corrosion is a chemical property, so we can eliminate C. That leaves D. We know it has something to do with heat "not changing". Even if you didn't agree that D was "correct", in the the intensity of an MCAT exam, you should stare down A and B for 15 painful seconds at most, be certain they are both wrong, eliminate C, choose D and MOVE ON.
     
  20. Shrike

    Shrike Lanius examinatianus

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    Blues, choice D does seem to be problematic. In an isothermal process, the walls would have to pass heat freely, so it's not crystal clear what the question means by isothermal walls. Yes, there's a way to interpret it that works, but this is non-standard terminology.

    As stated above, you should be able to get D as the answer and move on (because A and B are the same [wrong] answer, and C is just funny), but you're right to think the question's a little off.

    Where is this question: our material, or AAMC's, or someone else's? If it's ours, I'll make sure it gets changed.
     
  21. bluesTank

    bluesTank Zombie

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    It is princeton review (if thats what you mean by our). It is in the science problem book...i forget which passage.
     
  22. sehnsucht

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    for this question, it is as close as you'll get to a correct answer.
     
  23. CATallergy

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    here's my opinion, even though I'm late to the party :(

    A & B are obviously wrong, based on definitions (and also since they are almost the same)

    C is interesting - could an adiabatic surface participate in corrosion? this seems like an atomic-level thermodynamics question, and I don't even know how to approach it. seems too non-MCAT, so I'd rule this out.

    D. This has to be the best choice, but seems problematic. If the wall is isothermal, I think it could have the effect of buffering temperature changes caused by reactions in the cylinder's interior, whereas, an adiabatic surface would not.
     

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