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.
The heat in and out are of opposite sign.Quantifying Quality
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/KMelting Ice
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/KMixing
The entropy of an isolated system never decreases. It only stays the same for reversible processes.