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Homework Help: Need help with multiple choice questions

  1. Jun 18, 2006 #1
    Ok guys, I’ve got a few multiple choice questions I need help with. Here they are, and thank you in advance!!!


    All I know for this one is……Ql/Qh = Tl / Th Leaning towards B.
    1. In a Carnot cycle
    A. Qh > Ql * Th/Tl
    B. Qh = Ql * Th/Tl
    C. Qh < Ql * Tl/Th
    D. Qh = Ql * Tl/Th

    We are able to choose our own answer too, if none are correct below. I don’t think any of them are correct, so I chose Cp – Cv = R. If one of them is correct, I need to know. I’m thinking of choosing B.
    2. For an ideal gas:
    A. Cp = Cv
    B. Cp > Cv
    C. Cp < Cv
    D. Cv = Cp + R

    And, finally……I have this work problem.

    3. Bill drops a 50 gram (0 deg C) ice cube into a large pond which is at 20 deg C. What is the total entropy change of the total process?

    I found the change in the pond to be -71 J/K. So wouldn't the ice cube be 71 J/K, making the total process be zero??? Or I also figured it to have the ice cube be about 5.4 J/K larger.

    Thanks,
    Brad
     
    Last edited: Jun 18, 2006
  2. jcsd
  3. Jun 19, 2006 #2

    lightgrav

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    Homework Helper

    1) Ever hear of a reciprocal (1/x)? Qh/Ql = Th/Tl , so Qh = ...
    2) So, if Cp - Cv = R , what is Cp ? Is this "R" positive or negative?

    3a) What is the Temperature [K] of the ice as it melts?
    What Q enters it meanwhile? Compute its entropy change (SIGN!)
    3b) What is the Temperature of the pond as the ice melts?
    What Q leaves the pond meanwhile? Compute its entropy change (SIGN!)
     
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