Reversible and Irreversible Processes - Analysis of Processes

Reversible Process

• Definition see - http://scienceworld.wolfram.com/physics/ReversibleProcess.html
• Example of a Reversible Process - from above link (and other sources)

  

  • A theoretically reversible process can be illustrated by a frictionless piston in a cylinder. The force on the compressed gas is caused by the gravity on a pile of sand on the piston. A reversible expansion could be approximated by successively removing one grain of sand at a time as such an infinitely slow rate that the system is always in thermodynamic equilibrium. (That is all the thermodynamic properties do not change) If the grains of sand were replaced in the same fashion then the system would return to its original state without a change in entropy. Of course this is not possible because the mass of the grains and the time between the removal and replacement of these grains of sand is finite. A reversible process is a concept, like infinity, that we can approach but never actually reach. If any of you can invent an actual 100 % reversible process please come and see me right away. ;)

Irreversible Process

• Example 1 of Irreversible Process (there are many) - conversion of work into heat by friction

• This process cannot be reversed (If you figure out how please come and see me right away. ;) (sorry must be a bit tired of this by now) To reverse this process would violate the Kelvin Plank Statement of the first law of thermodynamics.
• Example 2 of Irreversible Process - conduction

• reversing this process would violate the Clausius Statement of the Second Law of Thermodynamics.

Analysis of Real Processes

• In order to get work from thermal energy we must create a cyclic process. (a heat engine cycle, like the otto cycle)

• Analysis
  • Energy Analysis Qh = Ql + W
  • Draw a pV diagram and the W = the area enclosed in the cycle, (the area under the expansion minus the area under the compression)

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