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Thermodynamics -- first and second law concepts

  1. Nov 7, 2016 #1
    1. The problem statement, all variables and given/known data
    An inventor claims to have invented four engines, each of which operates between constant-temperature reservoirs at 400 and 300 K. Data on each engine, per cycle of operation, can be found in the table below. Of the first and second laws of thermodynamics, which (if either) does each engine violate?
    Engine |||||||||| 1 ||||||||||| 2 |||||||||| 3 |||||||||| 4
    QH |||||||||||| 200J ||||| 500J||||||| 600J ||||| 100J
    QL||||||||||||| -175J||||| -200J|||||| -200J|||||| -90J
    W |||||||||||||||40J ||||||||400J ||||||||400J|||||||10J
    Of the first and second laws of thermodynamics, which (if either) does each engine violate?
    2. Relevant equations
    should i use the efficiency rule? I know that first law says that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. then Qh=|Qc|+W then one and two violates first law since in both Qh<|Qc|+W But how can i show that three and four violates the second law???
     
    Last edited: Nov 7, 2016
  2. jcsd
  3. Nov 7, 2016 #2
    Calculate the entropy changes for the reservoirs in cases 3 and 4.
     
  4. Nov 7, 2016 #3
    Yeah, Thanks I didn't pay attention to that at the beginning
    also we could relate it to the efficiency for which it must not be greater than 40% (carnot engine)
     
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