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

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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.

The heat in and out are of opposite sign.

Quantifying QualityClosed Cycle and heat.

- 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 and heat.

- 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. and heat.

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 IceA sample of 50.0 kg water at 20.0 and heat.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

MixingSecond Law III and heat.

- 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.

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