Thermodynamics: Entropy and Heat Energy, Slides of Physics

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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

Second Law III ^

The second law of thermodynamics can be describedin terms of entropy: The entropy of an isolated system never decreases.It only stays the same for reversible processes.

^

env

sys^

S

S

S