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: Finding the efficiency of an engine given energy input, energy wasted.

  1. Nov 7, 2012 #1
    1. The problem statement, all variables and given/known data
    An inventor claims to have developed an engine which takes in 1.1x108J of heat energy at 400K, and rejects 5.0x107J of heat energy at 200K. Would you advise investing in this project? Explain your answer.

    2. Relevant equations
    W = Q1 – Q2
    η = W/Q1 = 1 – Q2/Q1

    3. The attempt at a solution

    Which means the inventor is claiming 54% efficiency. In reality such an engine would be in the region of 30% efficiency or less.Therefore I would not suggest investing in this project."

    I think my answer seems too short, which means I'm probably doing it wrong. Also, the temperatures probably have some relevance to the solution so I'm probably using the wrong equation(s).
  2. jcsd
  3. Nov 7, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  4. Nov 7, 2012 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Or simply consider the net entropy change of the system.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook