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Thermodynamics is the description of how heat in a system is distributed when properties of the system are changing over time.

edit: or prior to reaching a stable state in a system with fixed properties.

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.

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?

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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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.

Thermo - heat

dynam - change

ics - of or relating to, the study of

Thermodynamics. Of or relating to how heat changes. Or slightly rephrased, the study of how energy flows from place to place.

Energy flow is what allows anything to do anything. Keyword being flow -- all the energy in the universe won't help you do anything if it has nowhere with relatively low energy to flow to. A waterwheel set up on a small rushing stream will turn, but the same waterwheel on a glassy smooth sea will not. Studying thermodynamics helps you discover where all the proverbial streams are and how to tell them apart from useless seas.

Thermodynamics' sister, physics, will tell you how things move and bounce around after being kicked off. Thermodynamics tells you what kicked them off in the first place.

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>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.

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.

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

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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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Three laws of thermodynamics:

1st Law of Thermodynamics - Energy cannot be created or destroyed. 2nd Law of Thermodynamics - For a spontaneous process, the entropy of the universe increases. 3rd Law of Thermodynamics - A perfect crystal at zero Kelvin has zero entropy.

You grow on these laws to understand how energy exists and changes in forms of heat, motion, chemical potential, and pressure. Understanding these three laws allows you to explore subjects of like physics, heat transfer, physical chemistry, fluid mechanics and so much more. Thermo can be used to create new processes and debunk inachievable ideas.

As a chemical engineer it’s the most useful class I took. It made me see the world with a different perspective!