scrape_r_rational/data/cache2/3x3fky.md

Energy of D&D spells in kJ [D]

• Author: u/notmy2ndopinion Concent of Saunt Edhar*

• URL: https://www.reddit.com/r/rational/comments/3x3fky/energy_of_dd_spells_in_kj_d/

• Score: 10

• Created: 2015-12-16T16:30:53

Post:

In preparation for next week's Writing challenge, I had a question come to my mind.

What is the approximate amount of energy in classic low level D&D spells?

I'm primarily interested in quantifiable cantrips and first level spells.

For example: Mage Hand can lift 5lb. Prestidigitation can light a fire, create a temporary hand sized object for 6-10 seconds. Color Spray can blind people.

Spells like Magic Missile, while classic, have much more variability in their interpretation and are more difficult to translate in physics (although everyone pretty much pictures it as a heat-seeking missile/bullet.)

I'm sure that this has been done before by someone in the Internet. Any thoughts?

Comments:

u/alexanderwales [+11] Time flies like an arrow (29 minutes later)

[Wikipedia's table on orders of magnitude is quite helpful.](https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)) There's also this article on relative energy levels though I don't think it gives you quite what you want.

If we're just looking at 0-level wizards spells then Light is a decent baseline. If we assume that it's roughly equivalent to an incandescent light bulb, then that's 60W over 10 minutes at level 1, which is 36kJ (multiply that by level if above level 1).

Prestidigitation's most powerful effect is the creation of an object, which presumably is a real object and has actual mass. The energy required per gram of matter is 9×10^13 J, or 90 billion kJ. It's not clear to me whether the object sticks around; the spell only specifies alterations to an object revert back and I don't know whether creation is the same as alteration. There are other spells with similar effects though. Also, there's probably something hinky going on with pulling in matter from an alternate dimension, which is how the Decanter of Endless Water and some similar items function.

It can chill, warm, or flavor 1 pound of nonliving material.

This really depends on what we mean by "warm" in terms of heat, and what the specific heat of the nonliving material is, and whether our ability to warm that object is dependent on its properties. It's 3.3×10^2 J (0.3 kJ) to melt one gram of ice, but we'd need specific quantities and some experiments to know more.

Generally speaking, D&D makes light and movement expensive, heat cheap, and matter creation really cheap, relative to each other in spell levels.

Edit: Mage Hand can propel a 5lb object 15 ft. in "any direction", with the worst (most energy-intense) being upward. The kinetic energy produced by 100 grams falling 1 meter against Earth's gravity is 1 joule, which means that the potential energy generated by lifting 100 grams 1 meter is the same. 15 feet is 4.572 meters, 5 pounds rounds to 2268 grams, so that's 126 J of potential energy generated.

Edit 2: Ghost Sound can produce as much volume as four normal humans per level. It includes shouting, so we can assume four humans screaming at the top of their lungs. Wikipedia gives a person a maximum of 135 decibels, which converts to 158 dB(SWL) assuming 1 inch from the screaming person, which converts to 6.3 kJ. Multiply that by four, because we have four people, gives total energy of 25.2 kJ at level one.

u/Gurkenglas [+8] (an hour later)

I'd say that since Prestidigitation's Conjuration effects can be ended by a dispel and the results are very brittle, the magic doesn't create an object but a semblance of one, which would explain why the duration of the object is limited, and why the objects can only withstand a small force: The Newtonian reaction that withstands external forces is supplied directly by the magic. Imagine Prestidigitation's telekinesis effect turned around, affecting not an object but everything that would interact with the space the "object" is simulated into. (Including some crude interaction with photons that only serves to produce an artificial look.)

To find the energy-matter-conversion factor, you'd have to look at spells like True Creation.

u/alexanderwales [+4] Time flies like an arrow (2 hours later)

We'd need to have access to a D&D laboratory to know for sure.

While I'd accept your judgement if you were my DM, Prestidigitation isn't actually in the Conjuration school (it's Universal), so we're not necessarily following those guidelines. Notably, a wizard who has chosen Conjuration as a prohibited school can still create an object via Prestidigitation. After all, the spell allows you to soil something and that ends up being permanent. Can you soil something while standing in clean room? If so, you have to be creating matter, and if Prestidigitation can do that, then I don't see why it couldn't create a crude, fragile object (which is clearly something that magic in general can do).

The lowest level creation spell that demonstrably makes matter is Create Water, but that's not available to wizards and I guess you could argue that it's really coming from the Elemental Plane of Water, even though the spell description doesn't say it does.

u/derefr [+3] (2 hours later)

Can you soil something while standing in a clean room? If so, you have to never wrong matter

Or just using mass from your own body. The name "prestidigitation" has always suggested to me that this is the D&D "spell" that represents the non-supernatural "magic" we have in the real world: card tricks and the like. Like our "magic", it's accomplished through deception.

u/DCarrier [+5] (4 hours later)

It's just a magic trick. That only wizards and sorcerers can learn. Which takes up a spell slot. And doesn't work in an antimagic field. And can be dispelled. Seems legit.

u/derefr [+8] (6 hours later)

It's a magic trick accomplished with the assistance of "magic" (i.e. laws of physics we don't have), but not by using any particular formed spell. In Mother of Learning terms, it's using "basic shaping" to do something interesting. D&D wizardry is all about arcana—knowledge others don't have about the nature of the world. Whereas Prestidigitation could be learned by a Rogue who happens to have some natural magical ability, but no arcane knowledge whatsoever.

Imagine a card trick where you're moving the cards around while shuffling them with weak telekinesis, or tagging certain cards with a bit of light only you can see. The trick is still, fundamentally, a trick; it just relies on physics that only MUs have access to.

In this case, with the soiling thing, I imagine the "trick" is basically to create a force-field jarate jar, fill it up (no magic required), and then splash it on the thing.

u/alexanderwales [+3] Time flies like an arrow (2 hours later)

It's one of those things you would have to consult with your DM about, or if you were the DM, make up your own mind on. As is the case with lots of D&D physics questions.

u/notmy2ndopinion [+1] Concent of Saunt Edhar (4 days later)

HPJEV makes that same point about Aguamenti (but it's a spell beyond 1st years, so the experimentation doesn't take place, regrettably!)

u/eaglejarl [+6] (2 hours later)

5 pounds rounds to 2268 grams,

That's not very round. Elliptical at best, but really closer to trapezoidal.

u/notmy2ndopinion [+1] Concent of Saunt Edhar (4 days later)

Wonderful -- the combination of the Wikipedia table of Orders of Magnitude and the back of the napkin calculations are just what I need to make my story!

I agree about the D&D 'cheapness' of certain effects -- it is so bizarre to me that illusions and light are so expensive, but matter creation and time-reversal/Cure Wounds spells are so cheap.

u/Charlie___ [+5] (4 hours later)

Matter-creation spells are, of course, totally bonkers in terms of energy, if they're actually creating matter. Common ways around this are to use nearby matter as feedstock (e.g. making a hat by stealing molecules from the walls, or condensing water out of the air to summon water), summoning it from somewhere else in the world (summon water from a random lake, or summon a randomly selected hat in the world - this explains why sometimes you lose your hat even when you put it down just a moment ago), or summoning it from an otherworldly plane (in this case, the plane of hats).

u/GaBeRockKing [+5] Horizon Breach: http://archiveofourown.org/works/6785857 (27 minutes later)

It's hard to quantify, because acceleration typically isn't mentioned in spell descriptions. Accelerating 5lb to three miles per hour, then decelerating it in time to stop on the limits of mage hand at is orders of magnitude less expensive than accelerating it to mach 10 and then subsequently decelerating it. And for prestidigitation, while combusting oxygen is fairly easy, there's technically nothing that tells you that you can't use something absolutely ridiculous for an oxydizer, like, I don't know, osmium.

u/Nighzmarquls [+3] (2 days later)

This kind of thing with matter creation spells is why I decided that in my to be started fantasy setting magic actually has infinite energy. It's just most people who use it are not directly controlling it and those that are have to be super precise and take a long time 'spooling' out the structure their making if they want anything more complicated then raw grand unified force in an incoherent burst.

u/notmy2ndopinion [+2] Concent of Saunt Edhar (4 days later)

stay tuned to my submission... i have an entirely different take on it.

u/lucidobservor [+3] (an hour later)

Here's my analysis of 5e cantrips. Hope it helps!

• Acid splash, or for that matter any other conjuration spell, packs a frankly insane amount of energy if you assume it involves conjuring matter from nothing. If that's not what's actually happening, such spells are rather hard to analyze.
• Mage hand can exert 10 lb of force in this edition.
• Prestidigitation - "You chill, warm, or flavor up to 1 cubic foot of nonliving material for 1 hour." You can probably exploit this relentlessly by selecting very dense / high specific heat materials, but to a first approximation, warming up 1 cubic foot of water from room temperature to the temperature of hot tea is about 6 megajoules.
• Shocking grasp delivers enough lightning damage to kill your average commoner (the average commoner has 4 (1d8) hit points). A defibrillator in hostile hands seems like it could have roughly the same effect, and the internet tells me the average defibrillator delivers around 500 J at full charge (citation).
• Thunderclap creates a sound loud enough to seriously cripple or possibly kill the average commoner standing 5 feet away from the caster. I don't know enough to try to convert that to energy units.
• Gust can move a 5 lb object 10 ft, with wind.
• Frostbite - back of the envelope calculation says that slightly lowering the body temperature of a human is on the order of 300 kJ.
• Mold Earth - moving a 5 ft cube of earth 5 feet takes roughly 85 kJ. This is slightly more energy than shape water accomplishing the same effect with water.

u/nicholaslaux [+3] (2 hours later)

My read of "warming" for prestidigitation is the equivalent of putting something in the microwave for 30 seconds, though that's obviously very subjective from dm approval.

u/DCarrier [+2] (an hour later)

Mage hand has an energy proportional to the distance moving while using it. If you use it on a wall, that's no energy. If you use it on a photon for one round, that's about 80 GJ.

u/mhd-hbd [+3] Writes 'The World is Your Oyster, The Universe is Your Namesake' (17 hours later)

Could you care to clarify how you got 80 GJ?

u/DCarrier [+1] (a day later)

Speed of light times 10 pound force times six seconds.

u/BoilingLeadBath [+1] (2 days later)

Anybody knows how far an average atom will tunnel at 80 GJ?

Mage Hand has a range of 25 ft = 8 meters. If an object leaves that range, the effect ends, so your maximum energy is of the order of 16m x 10lbf x4.5N/lbf = 700J, comparable to modern handguns.

(However, the wizard needs to point, so probably the lightest you can go with any accuracy is about 1lb, which reaches 100 miles-per-hour in a bit under a second. Wizards shooting arrow canister round (5/per reaches 230 ft/s: shortbow territory) are scary good at ambushes, but only a dumb beast would let you line up that shot in a pitched battle...)

u/ConvertsToMetric [+1] (2 days later)

^(Mouseover or click to view the metric conversion for this comment)

u/DCarrier [+1] (2 days later)

You could try doing it between a pair of mirrors or something. Although even 700J is far more than enough for a photon to pass through any mirror. You could use it on all the photons in that area, but at best it will just get to the point of melting the mirrors. You could also use it while moving really fast, which has the added bonus of letting you use it longer (assuming relativity is a thing, which is not a given). You could probably find another spell or magic item to get this to work, but then it's not like you're just using a cantrip to get tons of energy.

u/DCarrier [+2] (an hour later)

You can permanently soil an object with prestidigitation. It's not clear how much you can soil it with, but let's say it's one gram. That's a mass-energy equivalence of 90 TJ.

u/Transfuturist [+2] Carthago delenda est. (4 hours later)

"soil?"

u/DCarrier [+3] (4 hours later)

You can get things dirty.

u/Transfuturist [+2] Carthago delenda est. (a day later)

...with ...what?

What is that even used for?

u/alexanderwales [+2] Time flies like an arrow (a day later)

It's one of those spells that requires a lot of interpretation on the part of the players and the DM. I've seen it used to soil a lawyer's shirt to make him appear like a slob. I've also seen it used to soil goods in order to haggle for a lower price from a vendor ("This fruit has some shit on it, can I get it for half price?").

Mostly it's a spell that exists for the same reason that people find threads like "You have the ability to summon a nickel into your hand" fun. It's not that it's there for a reason, it's just a minor effect that you can find your own uses for (especially when you're out of spells).

u/DCarrier [+1] (a day later)

You can make it look like someone peed their pants. I can't think of any other uses.

u/diraniola [+2] (an hour later)

Magic missile deals 1d4+1 per hit, which is the same as a moderately strong person punching.

u/fljared [+2] United Federation of Planets (14 hours later)

There's no upper limit. The 2nd level spell enlarge increases the height and weight of the object by 10% per caster level. For a hurled object, velocity, not momentum is conserved- A rock that is doubled in size by a level 10 wizard hits twice as hard. The only limit is that the object cannot be more than 10 ft^3 in volume.

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