Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Thermodynamics: entropy and engines

  1. Dec 2, 2007 #1
    Entropy and Engines

    1. The problem statement, all variables and given/known data
    A heat engine has the following diagram associated with it. There is a heat source which supplies 1000 J of heat energy during one cycle and remains at a constant temperature of 400 K. A heat sink exhausts 700 J of heat during a cycle at a constant temperature of 300 K. Both are reversible processes. During one cycle the engine itself doesn't change its entropy.

    A) How much work can this engine do in one cycle?
    B) Calculate change of entropy of heat source through one cycle?
    C) Calculate change of entropy of heat sink through one cycle of engine?
    D) Does this engine violate second laws of thermodynamics?

    2. Relevant equations

    Q[tex]_{H}[/tex] = Q[tex]_{L}[/tex] + W
    [tex]\Delta[/tex]S = Q/T

    3. The attempt at a solution

    I just need someone to check my work. Thanks!

    A) 1000 = 700 + W
    W = 300 J

    B) 1000 J/ 400 k = 3 J/K (well, 2.5, but with one sigfig...)

    C) 700 J/ 300 K = 2 J/K (again, 2 and 1/3, but round down...)

    D) No, because heat is lost in order to do work.
    Last edited: Dec 3, 2007
  2. jcsd
  3. Dec 2, 2007 #2
    Anyone? Please? It's a simple problem...
  4. Dec 3, 2007 #3
    And are the sigfigs right?

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook