This week, I'd like to focus on how it applies to a simple law of physics. To be precise, I refer to the law of conservation of energy, which basically states that the total energy in the universe is conserved (though note that not all energy is useful, and entropy means a tendency towards energy that isn't useful).
So what does this mean for magic? Well, the energy to make that fireball had to come somewhere. As with all applications of the rule, this comes with some flexibility. Often, the simple solution that people revert to, without even considering it, is that there's some sort of limitless (or damn near limitless) supply of magical energy that magic users can access.
While that's certainly an option, I've got problems with it. Anyone who stops to think about it for a second (and depending on what your target audience is, that may or may not be an issue, and if it isn't you can ignore the rest of this post) will call bullshit. It makes magic sort of reality breaking by the universe's own logic. Why are there problems if there's limitless magical energy to fix them?
A reader who thinks about will expect that, at some point, something will show some strain from using a lot of magic. What it is that shows the strain varies depending on your energy source(s), of course, but the usual concept is that the magic user himself or herself will begin to weaken as the magic draws the energy out of them. Most often you can leave it there, but the more you know about how something works in your universe, the more consistent and believable it is, no?
So let's establish where the energy actually comes from. Starting with those sources which cause strain for the magic user, we have:
- The magic user's own physical energy (this can be difficult to work with, expect mages to eat A LOT because otherwise they can't do much)
- Some sort of inborn energy source in the magic user, which can only be used for magic (in which case you should establish how they refill it, if they even can)
- A combination of the above (with a combination of consequences)
- Drawing it from magic floating around in the environment (Visible or invisible can be relevant, and energy from anything in the environment is possible as long as you explain it. The strain here is mental exhaustion from gathering the energy.)
- A combination of the above (see previous)
However, there are other sources to bring up. For instance, what if magic only comes from magical sources in the environment? You would expect this to cause some strain on those sources, or even the general environment itself (which in and of itself can lead to interesting issues when it's overused in an area). Or what if the energy is extradimensional somehow? Expect to deal with consequences for that as well, whether it be the destruction of reality itself, or people coming over from the other dimension demanding their energy back, or anything else you can think of.
The sources are endless, and so are the consequences. In general, your best bet is to be creative and logical. Hell, the characters could be under the impression that the energy comes from one place when it really comes from another. It doesn't matter, as long as you figure it out and keep it consistent. That, after all, is the major key to believability with magic. My effort here is only to get you thinking about it, so that when you use magic in your story it is believable.
So until next time, have fun writing.