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 engine question

  1. Nov 17, 2005 #1
    Ok, my question is as follows:

    An inventor claims to have developed an engine that takes in 10^8 J (Q_in) at a temperature of 400 K (T_2), and rejects 4x10^7 J (Q_out) to a reservoir of Temperatue of 200 K (T_1). The engine delivers 15 kilowatt hours of mechanical work (which = 3600 sec/hour *15 * 10^3 watts = 5.4x10^6 Joules). Would you advise investing money to put this engine on the market?

    the way i approached it was to calculate the max efficiency that a carnot engine would have, which is:

    efficiency = 1 - T_1/T_2 = 1 - 200/400 = 0.5 or 50%​

    Now, using the expression,
    Efficiency = Work output/Heat input​
    for the hypothetical engine gives. This gave me an efficiency of roughly 54%, and by Carnot's theorem, no engine can be more efficient than a carnot engine, or:

    Efficiency(carnot) > Efficiency(hypothetical)​
    and in this case, it doesnt hold, i.e.:
    50 % > 54 % is not true.​

    Is this the correct way to do this problem?
  2. jcsd
  3. Nov 17, 2005 #2

    Physics Monkey

    User Avatar
    Science Advisor
    Homework Helper

    I didn't check your number crunching, but you've approached the problem correctly.
  4. Nov 17, 2005 #3
    Thanks very much
  5. Nov 18, 2005 #4

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    So what is your answer and why?

    My reason for not investing (besides the fact that the claim cannot be true as you have shown) would be: where are you going to find a reservoir to output at -73C?

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