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r3dl3g t1_j1z1spq wrote

Thermodynamics is the study of the motion and usage of energy, and is a critical aspect of all sorts of other scientific fields like chemistry and fluid dynamics.

Thermodynamics functions on three core laws. The first law simply states that energy must be conserved, and cannot be created or destroyed. The second law states that entropy cannot decrease, or (put more simply) that energy seeks to expand and flow from areas of high energy to areas of low energy. The third law basically just shows that entropy cannot remain constant. Thus, the second and third laws are typically bundled together to state that entropy can only increase over time.

There's also a tautological "zeroth" law, which isn't an actual law, but which is used as a teaching tool so as to sidestep some later issues in dealing with the relationships between entropy and energy. The zeroth law just states that heat always flows from hot to cold, which is true, but which is a consequence of the three laws of thermodynamics working together.

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SheeshKebabi OP t1_j1z2dj2 wrote

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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Jew-fro-Jon t1_j2006ty wrote

This is a great explanation. I have a masters in physics, so let me know if you have a deep question.

In class, we start by learning that “temperature” is a macroscopic quantity, and it is trying to describe the microscopic motion of particles. Heat transfer is really momentum transfer (of small things).

In grad school the class is labeled as statistical mechanics, because the coursework focuses on the statistics of it all.

Some cool concepts are things like heat capacity, which is the amount of energy it takes to change the temperature of a material. Water takes a lot of energy to change the temperature. We learn that this is because of the degrees of freedom that the molecule has. So a single atom can move in 3D, a pair of atoms like H2 can also spin along its axis, and H2O can spin along multiple axis. Each extra degree of freedom means it can store energy in more ways before a it “speed” increase (which is measured by temperature).

One useful application in life is diamonds: real diamonds need more energy to change temp. So if you touch it, and it’s cold, then it’s real. If it’s warm, then it’s fake.

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Retrrad t1_j203xoq wrote

Can’t win, can’t break even, can’t quit.

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[deleted] t1_j1zz134 wrote

[removed]

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r3dl3g t1_j1zz9xn wrote

>I've no idea where you got this idea from. The zeroth law is not tautological, can't be derived from the other laws, and (like the third law) is often ignored in pedagogical contexts because of its relative subtletly and obscurity.

The zeroth law is absolutely tautological; it's not remotely needed because the phenomena described by the zeroth law are necessary consequences of the second law.

>Well, the entropy of what? If I put a glass of room-temperature water in the fridge, its entropy will certainly decrease.

I mean, I'm simplifying for the purposes of ELI5.

The correct description only applies to closed systems.

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