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Entropy changes: Thermal energy to work and (some) back to Thermal energy 
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#1
Oct411, 02:39 PM

P: 1,011

In classical thermodynamics, if we dissipated the kinetic energy of an object as thermal energy, then we would increase the entropy.
However, let's say we took 90% of some thermal energy in a reservoir, and converted it into work, and 10% of that is converted back into thermal energy after 1 minute is passed. This would mean that 81% of the thermal energy has been converted into work. If we dissipate work as heat, entropy increases. So what happens if we convert heat into work? Shouldn't the opposite occur  a decline of entropy? I think we should have a sum of changes, an entropy increase in excess of a subsequent decrease. Is this the correct view? 


#2
Oct411, 05:55 PM

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P: 8,344

In classical thermodynamics, you always need a temperature difference to do work. Every time you do work, you reduce the temperature difference. When a temperature difference is reduced, entropy increases.



#3
Oct411, 06:53 PM

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P: 6,654

So the second law puts a limit on the efficiency at which you can convert thermal energy into work. That upper limit is the Carnot engine cycle. It can never get better than that. AM 


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