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First Law of Thermodynamics

  1. May 21, 2014 #1
    photoups.png

    I used the formula Qin-Qout=Wnet and therefore had:

    100-Qout =20 , so Qout=80 J..

    They got -80J and I got 80J but is my answer still correct?
     
  2. jcsd
  3. May 21, 2014 #2
    Yes, your answer is correct. But recognize that, by sign convention, heat flows in are considered positive and heat flows out are considered negative.

    Chet
     
  4. May 21, 2014 #3
    Thanks.

    The first law of thermodynamics applies only to closed systems (not to open systems), but it also applies to cycles.. I'm confused as to what the relationship between cycles and closed systems is? Are cycles a type of closed system? My lecturer's notes are terribly confusing.
     
  5. May 21, 2014 #4
    A closed system is one in which mass neither enters nor leaves. This is the case for the system you are looking at.

    Chet
     
  6. May 22, 2014 #5
    First law of thermodynamics applies to all processes - both cyclic and non cyclic.

    There are three types of systems in thermodynamics:

    1. Open system - A system that can exchange both matter and energy (in the form of heat and
    work), with surroundings.
    2. Closed system - A system that can not exchange matter but can exchange energy (in the
    form of heat and work), with surroundings.
    3. Isolated system - A system that can neither exchange matter nor energy with surroundings.
    The combination of a system and surroundings always constitute an isolated system.

    The first law of thermodynamics says: The energy of an isolated system remains constant.

    In case of other two types of systems, the energy of the system plus surroundings remains constant; consequently the change in the energy of a system have opposite sign to the change in the energy of surroundings.

    When a system undergoes a cyclic process, it suffers no change whatever and therefore, no change of energy. Consequently, the surroundings also suffer no change of energy, when the system undergoes a cyclic process.

    There are no exceptions for the first law of thermodynamics.

    rkmurty
     
  7. May 23, 2014 #6
    What about work? Does the above rule also apply for work flows? (i.e. is work flow in considered positive and work flow out considered negative)

    Thanks
     
  8. May 23, 2014 #7
    Work done by the system on the surroundings is considered positive.

    Chet
     
  9. May 23, 2014 #8
    In the image I posted in my first post I was told that the engine produces 20J of work so if I use:

    ΣQ = ΣW

    which is Qin + Qout = Win + Wout

    Does Wout = +20J ?

    Thanks
     
  10. May 25, 2014 #9
    There are two sign conventions:

    1. The so called engineering sign convention
    2. The international sign convention.

    The first convention is simple to follow from the point of view of applications. In this convention, we give heat as input to an enginge and expect it to deliver work as out put, and the work is cnsidered positive.

    The first law statement here is: ΔU = Q - W. ΔU is the increase in internal energy of the system, Q is the heat supplied to the system, W is the work delivered by the system.

    The second convention is based on the fact that energy received by the system ( in what ever form) is positive and the energy lost by the system is negative.

    The first law statement here is: ΔU = Q + W. ΔU is the increase in internal energy of the system, Q is the heat supplied to the system, W is the work done on the system.

    rkmurty
     
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