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Thank you for your response. I will point out what you have said, perhaps I can help him learn entropy too, lol.It says that entropy is a measure of the energy that is not available for useful work. That is not correct. These examples will show what I mean:

Example 1: A heat engine operates between 500K and 300K with an efficiency of 10%. For each 1000 Joules of input energy, 900 Joules are not turned into useful work. The change in entropy for 1000 Joules of input heat is:

[tex]\Delta S_1 = -1000/500 + 900/300 = 1.0 J/K[/tex]

Example 2: A heat engine operates between 1000K and 300K with an efficiency of 40%. For each 1000 Joules of input energy, 600 Joules are not turned into useful work. The change in entropy for 1000 Joules of input heat is:

[tex]\Delta S_2 = -1000/1000 + 600/300 = 1.0 J/K[/tex]

Example 3: A Carnot engine operates between 500K and 300K with an efficiency of 1-3/5 = 40%. For 1000 Joules of input energy (heat flow) the amount not available for useful work is 600 J. The change in entropy for 1000 Joules of input heat is:

[tex]\Delta S_3 = -1000/500 + 600/300 = 0 J/K[/tex]

In Examples 1 and 2 the entropy change is the same. But there is 900 Joules of energy not available for work in the first and 600 not available in the second. So how does entropy tell me how much energy is not available for work?

In Example 3, the entropy change is 0 but the same amount of heat (600J) is not available for work as in Example 1, in which the entropy change is 1 J/K. How does entropy tell me how much energy is not available for work?

If Tc = Th, the efficiency would be 0: 1-1=0.

True. But that is not what the article said. It said entropy is a measure of the energy not available for useful work in a thermodynamic process. If the entropy change, 0, is a measure of the energy not available for useful work, then one could not be faulted for thinking that 0 energy is not available for useful work.

You will find that entropy is a difficult concept and not all attempts to explain it are correct. You will just have to read good physics books and stay away from Wikipedia on this topic if you want to eventually understand it.

AM