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Homework Help: The First Law of Thermodynamics

  1. Jun 19, 2014 #1
    I am working on this problem:

    Q: During an isothermal expansion, a confined ideal gas does -150 J of work against its surroundings. which of the following describes the heat transfer during this process? (a) 150 J of heat was added to the gas; (b) 150 J of heat was removed from the gas; (c) 300 J of heat was added to the gas; (d) 300 J of heat was removed from the gas.

    The answer is (a). The explanation on the book is that "... since ΔU = Q + W by the first law of thermodynamics, and since ΔU = 0 due to isothermal process, therefore Q = -W. Since W = -150 J, it must be true that Q = +150 J."

    My question is that all textbooks write the law as ΔQ = ΔU + W, why does this book writes it differently? Any help would be very much appreciated. Thanks for your time.
  2. jcsd
  3. Jun 19, 2014 #2
    Hi rputra. Welcome to physics forums.

    Some books write the first law as Q-W=ΔU, where W is the work done by the system on the surroundings. Other books write the first law as Q+W=ΔU, where W is the work done by the surroundings on the system. You just need to determine which basis is being used by the particular book.

  4. Jun 19, 2014 #3

    George Jones

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    Adding specific examples of what Chestermiller wrote:
  5. Jun 20, 2014 #4
    Thank you all for your generous responses. Since this is my first use, I am wondering if there is any way of selecting one of them as the best answer, sort of rewarding your time and effort. Thank you again for your time.
  6. Jun 20, 2014 #5
    If you want to do something like that, just go to the response (or responses) you like best and click the Thanks button on each of them.

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