Surprisingly, this topic hasn’t been extensively discussed by nerds on Reddit, so I guess I’ll start the trend. I don’t know about you, but my favorite type of fantasy is the kind that attempts to explain magic through physics and chemistry, I suppose that’s why I love the Dresden Files series, Brandon Sanderson, and the The Kingkiller Chronicle (soon to be) trilogy so much. Not that I don’t appreciate the weird, random explanations for magic as well (GoT, Harry Potter), but the more scientific the explanation, the more ‘real’ magic is, I suppose.
Let’s begin by breaking down the main ideas in thermodynamics (literally translated to ‘energy movement’). The system and the surroundings are two terms which describe the two components that participate in thermodynamic reactions, the system is the ‘portion of the universe we are considering’ and the surroundings is ‘everything in the universe besides the system’. These definitions are super confusing, so let me put it this way: the system is the wielder of magic (the warlock/witch/wizard) and the surroundings are the rest of the universe (the dragon they’re fighting, the air they’re breathing, the dwarves accompanying them on their journey, etc.).
The other essential piece to these reactions is energy. Energy is defined as the capacity to do work and/or transfer heat. Energy can take many different forms: kinetic (movement), heat, light, sound, electricity, etc. This may sound familiar as ‘magic’ is often represented as some of these forms. For example: Harry Dresden releases energy in the form of fire when he uses the spell ‘fuego’. Kvothe uses kinetic energy when employing sympathetic magic on objects to make them move. Vin can use potential energy by storing her magic in a variety of metals.
The last thing someone needs to learn in a crash course of thermodynamics is the first law (which you’ve probably heard before). In any process energy can be changed from one form to another; but it can never be created nor destroyed. In those ‘weird’ explanations of magic I mentioned earlier (mostly Harry Potter) individuals are able to create magic (energy) seemingly out of nowhere (or at least the transfer of this energy is not described), thus breaking this first rule.
When talking about the movement of energy there are two different flavors of it: enthalpy and entropy (a kickass band name if you ask me). Enthalpy, put simply, is the change in energy (usually heat) from before the reaction to after the reaction has occurred. Thus, enthalpy within the system of Harry Dresden may be higher (more energy stored) before he casts a spell and lower after the spell has been cast. Furthermore, if the energy of doing the spell (the universe) is more than the energy within Harry (the system) before casting the spell it may cause faintness or even death (which has been discussed in the series). Furthermore, if Harry stores a bunch of energy within himself (the system) without releasing it through a spell (to the universe) it could also cause sickness.
Entropy is a more confusing term, its definition is the measurement of chaos within a system. Put simply, a system is more energetically favorable if the energy is more randomly dispersed. For example, in a pure crystalline substance the molecules are packed tightly together, is more ordered (and thus less disordered), the entropy is 0 and potential energy is lower. Entropy increases and potential energy increases as well as the system changes from a solid, to a liquid, to a gas because the molecules begin to move more randomly. The principle of entropy in magic has been explored in many universes and is often referred to as ‘chaos magic’, those who use chaos magic usually are extremely powerful and destructive, but the spells are often random and not as precise.
The Scarlet Witch’s powers are often defined as the ‘power to alter probabilities’, she can change the odds of something happening and thus her power is the epitome of the principle of entropy. In the Dresden File book ‘Blood Rites’ Harry is hired to investigate many strange deaths (for example, being hit by a car while waterskiing) which he eventually figures out is an ‘entropy curse’. Ruin in the Mistborn series is the God of destruction who seeks to unmake the world through destruction. Of course Ethan Rayne of Buffy the Vampire Slayer is also a great example of entropic magic, he worships the God of Chaos and uses his powers to create chaos in the natural world.
Now that we understand the main principles of magic it’s easy to categorize systems of magic and magicians by their entropy and enthalpy. If entropy and enthalpy is high then the magic is powerful, but less controlled. If entropy and enthalpy is low, then the magic is less explosive, but more controlled. If entropy is low, but enthalpy is high then the magician is powerful and more refined. If entropy is high, but enthalpy is low then the user gets short spurts of uncontrolled energy release. Remember, entropy=amount of chaos and enthalpy=amount of energy, maybe this infographic will help.