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I have some idea what entropy is, but how much is it? It should be a property in $JK^{-1}$, but what is its size? Is it in $10^{15}$, $10^{20}$ or what? For example for a $100ml$ glass of water at $300 K$. How much for a human body?

Milan Smolík
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  • Maybe somebody will post a reasonable answer but I suspect our understanding of liquids isn't really there (especially for water). But it should in principle be a calculation one could do - see for instance the Sackur-Tetrode equation for the same question about an ideal monatomic gas. – jacob1729 Aug 21 '18 at 21:37
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    Determining the entropy of a glass of water by counting the number of micro states may be difficult. However, another way of determining the entropy is from the equation $T dS = dQ$ or $T dS = C(T) dT$, where C(T) is the heat capacity of the glass of water, and integrating from zero temperature up to room temperature. Unfortunately, it's late and I don't have the energy to do this right now, but it shouldn't be too difficult to get an estimate of the entropy of a glass of water. –  Aug 22 '18 at 07:26
  • If you are using an ideal gas, you can use another approximation I discuss here. – Kyle Kanos Aug 22 '18 at 10:10

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For a glass of water, a nice and easy approach is to use property tables for saturated water.

The entropy of water at 300K is 3254.8 J/kg.K (the value is for a saturated liquid but it is a reasonable estimate for our purpose since the entropy is a strong function of temperature!)

So, for a glass containing 100 ml of water, entropy is about 325.48 J/K (assuming density of water is 1g/ml)

Frédéric Grosshans
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Shah M Hasan
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