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Laws Of Thermodynamics (Pre-university)

  1. May 15, 2010 #1
    Suppose, in a thermodynamic process, an amount Q of heat is given to the gas and an amount W of work is done by it. The total energy of the gas must increase by Q-W. As a result, the entire gas together with its container may start moving (systematic motion) or the internal energy (random motion of the molecules) of the gas may increase. If the energy doesn't appear as a systematic motion of the gas then this net energy Q-W must go in the form of its internal energy ( denoted by U ). Thus we get:
    U = Q - W​
    The above equation is the statement of the 1st Law of thermodynamics.

    Now, I am not clear about the systematic motion part. First of all, what does 'systematic motion' really mean? I mean, what if energy appears as systematic motion ? does it not then follow 1st Law of Thermodynamics? Isn't The 1st Law true for every process in universe? (even though it is derived assuming gas-container system does not have any systematic motion!) Can anyone please clarify this? (thanks)

    My 2nd question is : Is there any engine which does not work in cyclic process?
    According to Kelvin-Planck Statement: "It is not possible to design a heat engine which works in a cyclic process and whose only result is to take heat from a body at a single temperature and convert it completely into mechanical work."
    1.Why does this statement not hold for any non-cyclic process?
    2.What is the exact meaning of the phrase "single temperature" in the statement?
    3.Can anybody tell me why the Kelvin-Plank statement is true? (I mean is there any proof for 2nd Law or any physical explanation?)
    4. Another form of the 2nd Law is " It is not possible to have a process in which entropy of an isolated system is decreased. " I am unable to relate this form with the Kelvin-Plank statement. Also can anyone explain why entropy cannot be decreased? (may be a foolish question but still cant find a satisfactory explanation!:frown:)

    Any replies would be highly appreciated. Thank you!
  2. jcsd
  3. May 15, 2010 #2


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    Hi saubhik, welcome to PF!

    "Systematic motion" means that the container and the gas are translating and/or rotating. The First Law is generally applied to bodies at rest. In this sense, it isn't as broad as the law of energy conservation, which does apply to all processes in the universe.

    The reason cyclic processes come into play is that, most fundamentally, total entropy in the universe tends to increase. Also, heat transfer carries entropy, while (reversible) work doesn't. Therefore, you can't have a process that takes in heat (thermal energy), provides work, and returns to its starting position (i.e., completes a cycle), because it would consistently decrease the total entropy in the universe.

    It's no problem to take in heat and convert the energy to produce work; you could do that by boiling water and using the steam pressure to push a piston. But that only works once; now the water has changed to steam. To get back to water (i.e., to complete the cycle), you have to condense the steam, which releases heat. Thus, you're not able to completely convert heat to work if you wish to run the cycle repeatedly.

    Does this start to answer your questions?
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