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1st law of thermodynamics

  1. Mar 29, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi everyone, in 2 of my physics books the 1st law of thermodynamics is stated in 2 different ways:
    I am unsure why these are stated in 2 different forms. Here is what I think:

    2. Relevant equations
    1. Eth=W+Q
    2. Eth=Q-W

    3. The attempt at a solution
    2. takes into consideration the fact that when heat is added to a system then the system does work on the surroundings. for example in a cylinder filled with gas and covered with a piston: when heat is added (temperature increases therefore the internal energy also increases, this causes the piston to move up by dx and since there is a force exerted on the system (in the opposite direction dx) work becomes negative. hence, w=-pdV. therefore the first law becomes Q-W.
    1. equation is a more general form the law and it doesn't take into consideration the sign of work.

    Is this the right explanation?
  2. jcsd
  3. Mar 29, 2015 #2


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    Some texts use the symbol W to represent work done by the system on the environment. Other books define W as the work done on the system by the environment. These two ways of defining W will generally have the same magnitude but differ in sign.
  4. Mar 29, 2015 #3
    So I was partially right then! Thank you TSny!
  5. Mar 29, 2015 #4
    It's a difference in the definition of work. The sign is negative when work is stated as "work done by the system" and the sign is positive when work represents "work done on the system".
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