Entropy
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Various concepts related to entropy, heat energy, and thermodynamics. It includes questions about energy quality, carnot cycle, entropy definition, melting ice, and mixing. Students will learn about the relationship between heat and temperature, the concept of reversible cycles, and the second law of thermodynamics.
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2012/2013
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Entropy
Energy Quality ^
You are offered 1000 J of energy. Would you ratherhave it asA) mechanical workB) frictional workC) heat from an object at 1000 KD) heat from an object at 300 K
Closed Cycle ^
Any closed cycle can beapproximated by a sum ofCarnot cycles.
^
On a PV diagram this is anyreversible cycle.•^
The heat to temperatureratios can be added.
^
Q T 0
Entropy Defined ^
Entropy is defined as theheat flow at an absolutetemperature.
^
The path doesn’t matter, soentropy is a macroscopicstate variable.
Q^ T S^
Mixing ^
A sample of 50.0 kg water at20.
C is mixed with 50.0 kg water at 24
C.
^
Estimate the change in totalentropy.
^
Find the heat transfer. Thereare equal amounts of heat ineach sample.^ ^
Q^ =
mc
T^
= 100. kcal
^
Find the entropy change in eachsample using the averagetemperature.^ ^
SH
=^ Q
/ T^ = -100. kcal/296K = -0.338 kcal/K S
=^ L Q / T
= +100. kcal/294K =
+0.340 kcal/K ^
The difference is the netchange.^ ^
S^
= +0.002 kcal/K