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Conservation of mass relation for a closed system

  1. Apr 30, 2017 #1

    Source: Çengel/Cimbala.

    Above part is confusing for me from some points. First of it what can a closed system refer to? I just understand a piston/cylinder arrangement with an ideal gas in it. But this concept must be more comprehensive than I have it in my mind.

    Second, does "closed system undergoing a change" mean that only it's temperature, pressure, specific volume or density is changing?

    Thank you.
  2. jcsd
  3. Apr 30, 2017 #2


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    Staff: Mentor

    Something that cannot exchange matter with the environment (if this would be physics, we would also have to forbid any exchange of energy).
    It does not matter what changes, the mass stays constant in every case.
  4. May 1, 2017 #3
    Its average temperature and pressure are changing, although there may be variations of temperature and pressure with spatial location within the system. The average specific volume of a closed system is, of course, constant, but the specific volume may be varying with spatial position within the system. The term "closed system undergoing a change" means that work is being done on the system at its boundary and/or heat is being exchanged with the surroundings at its boundary.
  5. May 1, 2017 #4
    Cannot it be the work not entering or going out its bounderies but be an electrical work inside it?

    Thank you.
  6. May 1, 2017 #5
    Can you please provide a more detailed example of this?
  7. May 1, 2017 #6
    I know the term electrical work but I do not know what exactly it is. I think the situation might be a wire having an electrical current inside the closed system.

    What do you think?

    Thank you.
  8. May 1, 2017 #7
    Are you envisioning an electrical resistor inside the system connected by wires to the outside, with electrical current flowing through the wires so that the resistor is giving off heat?
  9. May 1, 2017 #8
    Yes. And I think this would be the only case.

    Thank you.
  10. May 1, 2017 #9
    You could also have electrical wires running to a fan. Would you include that too?
  11. May 1, 2017 #10
    Yes, this is also the form of work not related to boundaries but I didn't remember it.

    Thank you.
  12. May 1, 2017 #11
    Who says it's not related to the boundaries? If you include the resistor or the fan as part of your system, then the electrical wires are passing through the exterior boundary of the system, with one wire carrying current into the system at a higher voltage, and the other wire carrying current out of the system at a lower voltage. So the electrical work is being done at the boundary of your system.

    If you do not include the fan as part of your system, then the rotating blades of the fan constitute an interior boundary at which work is being done by the blades exerting forces and displacements on the material in your system. So here again, the work is taking place at the boundary.

    If you do not include the resistor as part of your system, then the surface of the resistor constitutes an interior boundary at which heat is being transferred to the material in your system. So, in this case, it is heat that is being exchanged at the boundary.
  13. May 1, 2017 #12
    I am not very familiar with topics of electricity. Aren't there only one wire and it is the wire having current through, glowing and giving heat and there is only one voltage instead of lower and higher?

    Thank you.
  14. May 1, 2017 #13
    Like you said, you are not very familiar with electricity.
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