Entropy

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# Entropy - PowerPoint PPT Presentation

Entropy. Energy Quality. You are offered 1000 J of energy. Would you rather have it as A) mechanical work B) frictional work C) heat from an object at 1000 K D) heat from an object at 300 K. A Carnot cycle found a relationship between the temperatures and heat.

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## PowerPoint Slideshow about 'Entropy' - ulric-logan

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### Entropy

Energy Quality
• You are offered 1000 J of energy. Would you rather have it as
• A) mechanical work
• B) frictional work
• C) heat from an object at 1000 K
• D) heat from an object at 300 K

The heat in and out are of opposite sign.

Quantifying Quality
Closed Cycle
• Any closed cycle can be approximated by a sum of Carnot cycles.
• On a PV diagram this is any reversible cycle.
• The heat to temperature ratios can be added.
Entropy Defined
• Entropy is defined as the heat flow at an absolute temperature.
• The path doesn’t matter, so entropy is a macroscopic state variable.
The latent heat of ice is 79.7 kcal/kg.

What is the change of entropy for a very slowly melting 1.00 kg piece of ice?

What is the change in entropy for the surroundings?

Find the heat transfer.

Q = mL = 79.9 kcal

Find the entropy change.

DS = Q/T = 0.292 kcal/K

The process is reversible.

DSsurr = -0.292 kcal/K

Melting Ice
A sample of 50.0 kg water at 20.0 C is mixed with 50.0 kg water at 24 C.

Estimate the change in total entropy.

Find the heat transfer. There are equal amounts of heat in each sample.

Q = mcDT = 100. kcal

Find the entropy change in each sample using the average temperature.

DSH = Q/T = -100. kcal/296K = -0.338 kcal/K

DSL = Q/T = +100. kcal/294K = +0.340 kcal/K

The difference is the net change.

DS = +0.002 kcal/K

Mixing
Second Law III
• The second law of thermodynamics can be described in terms of entropy:

The entropy of an isolated system never decreases. It only stays the same for reversible processes.