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I had a look at this question, and though about magic and the human body's energy output.

I remember calculating the energy require to walk up a 3.4m staircase quickly in a school physics lesson once. I looked up the results in my old book, and I got the power outputs of everyone in the old class. The greatest output was around 840W in 4 seconds, and the least was 130W in 14 seconds.

I remember being surprised by this, especially considering that a really good lightbulb is 17W, and an LED lamp is about 60W. It doesn't take too much energy to start a fire, either, though admittedly it probably takes much more to sustain one.

If 'magic' in humans were possible by using energy produced by the human body, what limits would this magic have? How would this magic differ from the Stereotypical, Tolkein or Harry-Potter style magic we're used to? What effect would this have on people using magic?

And, more importantly, what would magic be able to do that we can't do better with electricity?

In response to PipperChip: I don't see why it should be constrained to any type of energy, so molecular energy and such is viable in my eyes.

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    $\begingroup$ You should read Eragon. The Inheritance Cycle style of magic seems to be almost exactly what you're describing. In those books (Spoiler alert), casting magic takes a direct toll on the casters body. The first time the MC tried to cast a spell, he fainted... $\endgroup$ – Aify May 4 '15 at 16:03
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    $\begingroup$ "The Kingslayer Chronicles" also has magic that draws energy from sources, including one's own body heat. (Take too much body heat from yourself and you could very well die) $\endgroup$ – DoubleDouble May 4 '15 at 16:07
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    $\begingroup$ Related: Energy in a human body, How much can a magician lift..., and Explaining where energy comes from.... $\endgroup$ – Frostfyre May 4 '15 at 16:24
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    $\begingroup$ Can you further define what energy types are allowed? Can we use atomic energy from the atoms in people's bodies, the heat given off by a person's body, the energy stored in chemical bonds (this is where our bodies store energy for later already), or the mechanical energy we can produce? $\endgroup$ – PipperChip May 4 '15 at 16:33
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    $\begingroup$ LED lamps are less than a few watts (with brightness equivalent to a 60W traditional bulb). $\endgroup$ – Memming May 4 '15 at 17:17
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There is, I think, a problem with your question. The problem is that specifying total energy is not enough. Additionally, specifying power is not enough. Adding force is not enough. Adding speed is not enough. Let me explain.

Let's say energy is the only constraint. Then you can levitate rocks to high altitude and let them sit there, waiting for the command to drop. Making a modest assumption of 100 J/sec, you can lift a 1 pound rock about 20 meters in one second, do this for 50 seconds for a final height of 1 km, and it takes no energy to leave it there. Doing this for an 8-hour day will give you about 500 man-killers. Doing that for a month will give you a cloud of 15,000 rocks, enough to obliterate any village or small army.

Is this reasonable? Many don't think so, since it seems obvious that nobody can hold up 15,000 pounds. So applied force needs to be considered. And once you start considering force, how does magic deal with mechanical advantage? And how fast can a magician apply force? Consider throwing a rock. A major-league pitcher can throw a 5-ounce baseball 100 mph. That's 140 grams at 44 m/sec, or about 140 J. A 9 mm pistol slug has about 500 J, but is far more lethal than a baseball. Can a magician accelerate a small, dense body faster than a pitcher accelerate a baseball? Let's take a 1/3 oz flechette, a small dart about an inch long. At 140 J, it will have a speed of 140 m/sec, and will pierce a person's body all the way through. Remember those floating rocks? Replace each one with a 1 oz metal dart 3 or 4 inches long. Dropped from 1000 feet, one of these will punch all the way through a person - lengthwise. And a cloud of 320 of them will only weigh 20 pounds, and almost anybody can hold that weight indefinitely.

And then there's power density. Sunlight, with a power density of 1 kW/sq meter, can be focused by a 6-inch diameter magnifying glass to a spot size of less than 1 mm (as lots of kids have discovered, to the detriment of the local ant population). That's less than 20 watts to produce a nasty little burn. So it's clear that, if only power or energy is limited, 100 watts can do very bad things to one's enemies.

Finally, it gets worse if rather specialized knowledge is invoked. Creating light is a staple of magic. Knowing it's possible, what's to prevent the creation of coherent light? A laser beam of less than 100 mW is more than capable of temporarily blinding someone. Is there any good reason that a magician can't do it, rendering an attacker helpless at almost no energy cost? Focussed acoustic energy of a few watts is more than enough to destroy a person's eardrums. Is this allowed?

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There are a few aspects I'd raise: type of energy, looking at power vs. energy, and the most interesting one for me: lack of physical requirements / infrastructure.

Type of energy (and entropy)

Electricity is a low-entropy form of energy, so it's very organised, whereas heat is very chaotic. Converting from electricity to heat can be simple and efficient (just use coils of wire!), but the other way around is more complicated (we can get ~40% efficiency with some setups, and even then it takes a bit of engineering).

So: is the energy you're using high-entropy or low-entropy? Does it translate into some forms of energy more easily than others? (e.g. motion is easier than light?). What if you can only output one form of energy, and to produce light you need a "light generator" to convert it?

Power vs. energy

Something like a bullet obtains all its energy at once. However, if your magical system allows action at a distance, then you shouldn't just compare instantaneous force/energy, but total over a period of time.

Say one magician can lift a cannonball (comparable to human muscle) - that means they can apply as much force as gravity can. If a second magician applies the same force sideways, the cannonball can "freefall" in any direction you like.

In fact - I'm capable of giving a heavy friend a piggyback. That means I can exert an extra force of ~1000 kilo-Newtons (on top of my normal standing around) for at least ten seconds. That means I can accelerate a 1kg weight to three times the speed of sound in only a second.

Physical requirements

You asked "what would magic be able to do that we can't do better with electricity?". For that, I would respond with the biggest advantage that magic has in most representations: it requires no infrastructure.

So with a long enough lever I can lift pretty much anything - but I need space for the lever. With electricity, I can produce a bright light on demand - but I need to have placed a bulb and electricity source there first.

If magic can work at any kind of distance, then this requirement is freed up. I really can unlock the box with the crowbar that's inside it - more relevantly, I can easily walk through any door that can be opened from the inside. I can punch someone in their internal organs - or perform surgery without having to open them up. In the previous examples, I can accelerate an item without having to attach the energy source and conversion apparatus (like in a rocket) to the item itself.

This, I think, is often the most effective part of a magic system - even if you have to waste magical energy to get it in the form or place you want, you don't have to set up the physical infrastructure to deliver it, in the way you do when dealing with physical energy sources.

Mitigation

When viewed from a point of view of energy alone, it's easy for magicians to become insanely overpowered. If you want to avoid this, you could hobble the above properties - e.g.:

  • an awkward form of energy that's hard to translate (and then a better generator/convertor could be a plot point / force for change)
  • limiting physical distance to avoid extreme kinetic buildups
  • requiring magical infrastructure (so spell-casting to launch something is the equivalent of building a catapult in "magical space")
  • requiring physical infrastructure
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