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Homework Help: Problem inspired from thermodynamics

  1. Nov 29, 2012 #1
    1. The problem statement, all variables and given/known data

    While studying thermodynamics, I constructed the following problem, which I can't seem to solve.

    Suppose that [itex]Q_{4} > Q_{3} [/itex] and [itex] |Q_{3}| > |Q_{4}| [/itex]. Then for what values of ΔU is it necessarily the case that [itex]|ΔU-Q_{4}| > |ΔU-Q_{3}|[/itex]

    2. Relevant equations

    3. The attempt at a solution
    I really have no idea what to do; I solved a much simpler textbook style problem and then decided to consider a more general version of the problem, which led me to this. Any ideas? I conjecture that both the Q values are negative, but is this true? How would I prove it?

  2. jcsd
  3. Nov 30, 2012 #2


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    Certainly Q3 < 0, but Q4 need only be smaller in magnitude. You could draw up a truth table to see this, and similarly to deduce the range of ΔU.
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