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Why are real life process irreversible

  1. Aug 10, 2011 #1
    From what I've read all processes of a thermodynamic system are irreversible because some energy will be lost to the surroundings but I don't understand why that can't be reversed by getting the surroundings to put that energy back into the system. Lets say I have a gas canister with a piston and theres a pile of sand weighing the piston down and compressing the gas to a certain degree. If I quickly take away half of the sand the piston will move up then fluctuate a bit until thermodynamic equilibrium is restored. In real life the friction between the piston and the canister will cause some energy to be dissipated into the surroundings so if I put the sand back on the piston the energy of the gas won't be exactly the same as it was at the start. Whats stopping me from adding more energy to the gas though? Sorry if this seems like a stupid question, thermodynamics isn't a strong point of mine.
  2. jcsd
  3. Aug 10, 2011 #2
    If you're adding more energy then the system is no longer closed.
  4. Aug 10, 2011 #3


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    I don't think your example is adequate to understand this. You are doing work to move the sand, so the energy in the gas can easily be added back. However, this is NOT a reversible process as the reactions in your body that allow you to move the sand cannot be reversed.

    For example, lets look at a campfire. The O2 in the air is reacting with the Carbon in the wood and turning into CO2 and releasing energy. The ambient air is at best now equal in temperature to the CO2. Why can't this be reversed? Because there is no way for the energy to flow from the air back into the CO2 to split it up! Heat will not flow from a colder substance to a warmer one so the CO2 and the air stay equal in temp.

    See here: http://en.wikipedia.org/wiki/Irreversibility

    I'm not sure if that is 100% correct in regards to thermodynamics, but I believe the general idea is right.
  5. Aug 11, 2011 #4


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    Last edited by a moderator: Apr 26, 2017
  6. Aug 11, 2011 #5

    Andrew Mason

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    What the output work of the process is used for is immaterial to whether the process is reversible. A reversible thermodynamic process can do work that is dissipated as heat. And a non-reversible process can do work that is completely conserved.

    A reversible process is one whose direction can be reversed at any time by an infinitessimal change in conditions: ie. it is arbitrarily close to equilibrium at all times. Reversibility does not depend on what you do with the work output.

    If you store the output of a Carnot engine operating between two heat reservoirs (say, by lifting a weight), and reverse the direction of the Carnot cycle (by changing the conditions by an infinitessimal amount) you create a Carnot refrigerator that can be driven by the stored work to return the system and surroundings (the reservoirs) to their original thermodynamic states. If the engine uses an irreversible process, you will never be able to get back to the original state using just the stored work output of the engine.

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