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Basic doubt in thermodynamics 2nd law

  1. Jun 12, 2012 #1
    I ve read that according to 2nd law,in a closed system where there's no inflow and outflow of energy,the potential energy of such system continuously decreases...
    So where does/ what is this energy converted to.?
     
  2. jcsd
  3. Jun 12, 2012 #2
    I think you might have misquoted and/or misunderstood the 2nd law! The Second Law of Thermodynamics has been written in many different forms, but they all say something like this:

    The entropy of a macrostate of a closed system never decreases.

    I don't know of any law that says the potential energy of a closed system must decrease. For example, consider an ideal box of ideal gas molecules in thermal equilibrium at constant temperature. Thermodynamics says the total potential energy of that system is constant.
     
  4. Jun 12, 2012 #3
    What do you mean by this?

    Are you trying to describe a closed system which means there is no inflow or outflow of mass? Energy may, however be exchanged.

    Or are you adding the extra restriction to make the system isolated?

    Either way if your intention is to restrict energy exchange so that the total energy of the system is constant, are you trying to describe the idea that lead to the 'heat death of the universe' where potential energy is converted to kinetic energy at maximum entropy?
     
  5. Jun 20, 2012 #4
    It has been stated so--
    "In all energy exchanges if no energy energy enters or leaves the system,the potential energy of state is always less than that of initial state.This is commonly referred as entropy"
    Is this right.?
     
  6. Jun 20, 2012 #5

    mfb

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    What do you mean by this?
    What is exchanged, if nothing is exchanged?

    An isolated system can increase its potential energy. Think of a fast-moving ball, rolling up a hill (with gravity) and coming to a rest there. Earth+Ball increased the potential energy, while the system "earth+ball" could be isolated from the rest of the universe in this hypothetical experiment.
     
  7. Jun 20, 2012 #6
    Whoever said this has absolutely no idea what he/she is talking about.

    1. The quote defines "energy exchange" as an interaction of an isolated system with itself in a way that does not exchange energy. That's not a useful definition.
    2. As written, the quote says the potential energy of a system's state is always less than that of its initial state. That means the state's initial potential energy is less than itself, which means -1 = 0.
    3. Let's assume the quote meant to say "the potential energy of [a final] state is always less than that of [an] initial state." That is false. (The ball rolling up a hill is, IMO, a clear and simple counterexample.)
    4. Nothing in the quote has any relation to entropy.

    Here are some common definitions of entropy:

    Old thermodynamic entropy (Boltzmann)

    New thermodynamic entropy (Gibbs)

    Information entropy (Shannon)

    Quantum entropy (von Neumann)
     
  8. Jun 20, 2012 #7
    Umm..but just google what I ve quoted,u'll find many links where the 2nd law of thermodynamics is explained so..That's the reason I felt some inconsistancy with general accepted law...so this manifests that all the links which stated so are wrong.! Isn't it..?
     
  9. Jun 20, 2012 #8
    one the link gives following example--
    A watchspring-driven
    watch will run until the potential energy
    in the spring is converted, and not
    again until energy is reapplied to the
    spring to rewind it. A car that has run
    out of gas will not run again until you walk 10 miles to a gas station and refuel
    the car. Once the potential energy
    locked in carbohydrates is converted
    into kinetic energy (energy in use or
    motion), the organism will get no more
    until energy is input again. In the process of energy transfer, some
    energy will dissipate as heat. Entropy is
    a measure of disorder: cells are NOT
    disordered and so have low entropy.
    The flow of energy maintains order and
    life. Entropy wins when organisms cease to take in energy and die.
    THANKS FOR INFO THOUGH,i'll read them out now..
     
  10. Jun 21, 2012 #9

    mfb

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    These are just examples where potential or chemical energy is converted into heat, which is usually not reversible in everyday applications.
    The important part here is the heat (which usually has high entropy per energy, compared to other energy forms).
     
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