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Old May 22nd 2008, 01:28 AM   #1
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Second law of thermodynamics

Is it practical or in theory to violate the second law of thermodynamics under any circumference?



Some people suggest that when we try to build up something store energy, the loss in useful energy in more than the useful energy stored,
is it a must to say so?

Has reliable experiment been carried out to verify that?


Why those so call "useless energy" cannot be used for external work?
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Old May 22nd 2008, 08:48 AM   #2
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Originally Posted by werehk View Post
Is it practical or in theory to violate the second law of thermodynamics under any circumference?



Some people suggest that when we try to build up something store energy, the loss in useful energy in more than the useful energy stored,
is it a must to say so?

Has reliable experiment been carried out to verify that?


Why those so call "useless energy" cannot be used for external work?
The second law may be broken "locally" in the sense that we are not considering the total system. For example we can dig up oil, convert it to gasoline, and run a car engine with it. By all appearances we are getting more energy from the substance than we put into it (which is nothing at all, really.) So the second law is "broken." However if you look to see how that oil got the chemical potential energy to do all this in the first place you will see that the original source of the fuel is from the Sun which increases the entropy of the Universe vastly more than any entropy breakage we are getting from using the oil. So the second law isn't really broken after all.

I don't believe that any experiment has been carried out (recently) to prove or disprove the second law, however I have never heard of any experimental results contradicting it (and I'm sure it would be trumpeted loudly if someone had) and I'm sure when the concept was being developed there were numerous experiments carried out.

Finally, the reason that the "useless energy" can't be used to do external work is that it cannot be extracted from the system.

For example, consider a simple circuit with a source and a resistor. Realistically this resistor wastes energy: it heats up and we can't get it all back. Why not? Couldn't we put an engine on it to absorb that heat and put it to work for us and bypass the second law? There are two problems with this. The first is that the best engine we could put into the system to convert the heat energy into something useful is a Carnot engine and whereas the Carnot engine has the highest efficiency of any engine it still does not have a 100% efficiency, so some energy is lost. The second reason is that some of the wasted energy from the resistor goes into a disorganization of the structure of the resistor. In time these minor flaws will cause a structural defect in the resistor and it will break down. Because this is so the system cannot run forever and thus does not violate the second law.

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