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SheeshKebabi OP t1_j1z2dj2 wrote

Reply to comment by r3dl3g in ELI5 What is Thermodynamics? by SheeshKebabi

Ah, I see, thanks. I always keep hearing the terms entropy and enthalpy used. Do they mean the same thing, or what's the difference between them?

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r3dl3g t1_j1z2v0q wrote

Enthalpy is actually closer to energy, and basically amounts to a slightly different methodology for bookkeeping of the total energy contained within a system. The only difference between enthalpy and energy is that enthalpy takes what we call "flow work" into account, or the additional energy extractable/storable in a medium via the combination of pressure and changes in volume.

Enthalpy ends up being used in place of energy in a lot of situations where pressure-driven flow is important (e.g. turbines).

Entropy is a separate thermodynamic quality that more or less describes the "desire" of concentrated energy/enthalpy to disperse.

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DavidRFZ t1_j1zerwx wrote

Yes, just to add that Entropy has different units. Entropy is usually multiplied by absolute temperature to compare to heat.

The concept of “Free Energy” (Gibbs) is Enthalpy minus Absolute Temperature times Entropy. The free energy helps determine whether or not you are at equilibrium. The effect of entropy is higher at higher temperatures.

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etoleb123 t1_j20p5cb wrote

Enthalpy is well described above.

With Entropy, think of it as a measure of the “quality” of the energy—specifically, how useless has the energy become? High entropy = more useless. This is important because energy is not created or destroyed, but it does change and in those changes it becomes less useful.

For example, when you burn the gas in your car, you get ~35% efficiency—that is, only 35% turns into kinetic energy of the car. The other 65% isn’t lost…it is just dissipated in the atmosphere as heat and cannot easily be used any more. So its entropy has increased.

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