1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Carnot engine efficiency help

  1. Apr 7, 2008 #1
    1. The problem statement, all variables and given/known data
    Which of the following statements about a Carnot engine is true?
    a) Any Carnot engine has an efficiency of 100%
    b) Irreversible energies have the greatest possible efficiency.
    c) Heat can spontaneously flow from a cold reservoir to a hotter reservoir.
    d) If a process is reversible, the efficiency of an engine is maximum.
    e) All energies are reversible.

    2. Relevant equations


    3. The attempt at a solution

    Well, I think the answer is E. I canceled out choice b because no where in my text book did it mention irreversible energies having high efficiency. I don't think it's choice a because the third law of thermodynamics says reaching a temperature of absolute zero is impossible and makes it impossible for a Carnot cycle engine to achieve 100% efficiency. So if I'm right, why is E right? If I'm wrong, then why would it be one of the other choices? I pretty much just used process of elimination.
  2. jcsd
  3. Apr 7, 2008 #2
    i suggest you to check much better the differences beetween rev and irrev process.
  4. Apr 24, 2008 #3
    i've got a better idea. research what a carnot engine is. it's a lot faster than checking the differences between rev and irrev processes. it will actually help you solve the question
  5. Apr 24, 2008 #4
    i think the answer is that all engines are reversible. the one that states that "if a process is reversible, the efficiency of an engine is maximum" wouldn't be accurate because ALL processes have to be reversible to obtain maximum efficiency, not just one.
  6. Apr 26, 2008 #5
    I would say d is the correct answer here, a is completely false due to the second law of thermodynamcs(never can all heat be converted into pure work),b is obviously not true, c would not have a positive entropy change andhence does not occur spontaneously, e doesnt make sense.....hence my choice d, however the statements are rather ambiguous
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook