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Materials and Chemical Engineering
Entropy: measure of amount of energy unavailable to do work?
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[QUOTE="Andrew Mason, post: 5551109, member: 15795"] Entropy is a thermodynamic [I]state [/I]function that is defined in terms of a [I]process [/I]between two states. So it is bound to be confusing. There are a variety of ways you can look at it. It provides a quantitative basis for the second law. With the development of atomic and kinetic theory, it was realized (ie. Boltzmann) that the second law is essentially a statistical law. A [URL='https://www.physicsforums.com/insights/grandpa-chets-entropy-recipe/']change in entropy[/URL] of a body is defined in classical thermodynamics as the integral of heat flow divided by temperature along a reversible path between two thermodynamic states: ΔS = ∫dQ[SUB]rev[/SUB]/T. Since heat will not flow between two bodies unless there is a temperature difference and it will only flow from higher to lower temperature, in any real process in which heat flow occurs between two bodies, the entropy increase of the cooler body will always be greater than the entropy loss of the hotter body. They will approach equality as the temperature difference approaches 0. That is just a consequence of ΔS = ∫dQ[SUB]rev[/SUB]/T. If you want to picture what entropy represents, try this: Entropy is a measure of the extent to which a thermodynamic process can be completely reversed without the addition of mechanical work from outside the system. The [URL='https://www.physicsforums.com/insights/grandpa-chets-entropy-recipe/']change in entropy[/URL] is a measure of how far from the original states the system and surroundings will be after that reversal. For example, if you store the mechanical energy output of a Carnot engine and then reverse the cycle using the stored energy to cause heat to flow from the cold reservoir back to the hot, the system and surroundings will be in their original thermodynamic states after all the stored energy is exhausted. Total [URL='https://www.physicsforums.com/insights/grandpa-chets-entropy-recipe/']change in entropy[/URL] is 0 representing 0 difference between the final and initial states of both system and surroundings after reversal. For a real engine, that reversal will always result in the system and surroundings not making it back to their original states (ΔS[SUB]sys[/SUB] + ΔS[SUB]surr[/SUB] > 0). You will never have a process in which the system and surroundings reach and exceed their original states using just the energy gained from the mechanical work generated in the forward process (ΔS[SUB]sys[/SUB] + ΔS[SUB]surr[/SUB] < 0). AM [/QUOTE]
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Entropy: measure of amount of energy unavailable to do work?
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