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#### Published on 30 Apr 2010 | over 8 years ago

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Second Law of Thermodynamics

The second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium; and that the entropy change dS of a system undergoing any infinitesimal reversible process is given by δq / T, where δq is the heat supplied to the system and T is the absolute temperature of The second law of thermodynamics
The second law of thermodynamics comes in more than one form, but let's state in a way that makes it obviously true, based on what you've observed from simply being alive.
The second law states that heat flows naturally from regions of higher temperature to regions of lower temperature, but that it will not flow naturally the other way.
Heat can be made to flow from a colder region to a hotter region, which is exactly what happens in an air conditioner, but heat only does this when it is forced. On the other hand, heat flows from hot to cold spontaneously.
Heat engines
We'll move on to look at heat engines, which are devices that use heat to do work. A basic heat engine consists of a gas confined by a piston in a cylinder. If the gas is heated, it expands, moving the piston. This wouldn't be a particularly practical engine, though, because once the gas reaches equilibrium the motion would stop. A practical engine goes through cycles; the piston has to move back and forth. Once the gas is heated, moving the piston up, it can be cooled and the piston will move back down. A cycle of heating and cooling will move the piston up and down.
A necessary component of a heat engine, then, is that two temperatures are involved. At one stage the system is heated, at another it is cooled.
In a full cycle of a heat engine, three things happen:
1.Heat is added. This is at a relatively high temperature, so the heat can be called QH.
2.Some of the energy from that input heat is used to perform work (W).
3.The rest of the heat is removed at a relatively cold temperature (QC).

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