By some eldritch means I am able to heat matter (raising the kinetic energy of the matter's particles).

I want to weaponize this ability, and need some guidance for its implications.

The obvious use-cases would be to have an instant source of ignition, right?
Is there something like an explosive effect I can achieve? What would for example happen if I suddenly heated the core of an iron sphere to 8000°K while the matter outside stays cool?

How can the ability to heat portions of matter to high temperatures be used as a weapon, if I cannot affect the opponent's body matter directly and I need to minimize the number of heating instances and volume? Which secondary effect (i.e. explosions or something) would I be able to generate?

Limitations:

• I can only affect a small volume of matter, lets say 1m³.


• At the moment I am thinking about capping the maximum temperature at 8000°K. What would change if that was not the case?

Personal Amendment: Wow. I underestimated the implications severely. Thanks guys!

Is there something like an explosive effect I can achieve?

Oh yes.

Given the handy metre-cube limit, lets think about the energy required to raise the temperature of that volume of liquid water at ambient temperature (say, 20 degrees C) to 8000K. Lets say it is a nice round tonne of water. Specific heat capacity of water is 4181J/kg, so you need about 335 MJ to bring the water up to the boil. Next you need 2.2564MJ/kg to convert that water to steam, which gives you another 2-and-a-bit GJ. The SHC of steam is more like 1996J/kg, so the next 3000-odd degrees will need another 6GJ.

edit: as cmaster rightly pointed out, once you get to about 3000 degrees, water disassociates into hydrogen and oxygen. The average energy of an H-O bond in water is 461kJ/mol or 25.6MJ/kg, so that'll need another 25.6GJ* to fully disassociate the entire tonne (also note that all those excited and solo hydrogen and oxygen atoms are gonna want to hook back up again, and that's gonna produce an exciting effect too!).

Now, the SHC of monatomic gasses is nice and simple and related only to their atomic mass (because there are no complicated atomic bonds to wiggle about). Hydrogen gets a high 12kJ/K/kg, and oxygen gets a slightly more modest 750J/K/kg. For the remaining 4700-odd kelvin, the hydrogen will need 6.2GJ and the oxygen will need 392MJ. This is about half the energy needed if the water molecules were still intact. The total energy is therefore about 35GJ.

The "tonne of TNT equivalent" energy measure used for nuclear bombs and the like is defined as about 4.184GJ, so the amount of energy you've just poured into the system is the equivalent of a 8.3 tonne bomb. This is is in the same ballpark as the the 11-tonne yield of the GBU 43/B MOAB (which looks like this in action [youtube link]) or its predecessor the BLU-82 Daisycutter which has a slightly more subtle yield of about 5-7 tonnes, shown in action here:

Now, I don't think this magical instantaneous superheated-monatomic-gas transition is quite the same as explosive detonating (I'm carefully ignoring temperature/pressure relationships, because I'm lazy), but I would absolutely not want to be anywhere near it. That incredibly hot gas products will be under formidable pressure when it forms, and quite a lot of that energy you've poured into it will be released in the form of vigourous and rapid expansion. Boom.

This could perhaps be even worse if you picked a material which took even more energy per unit volume to heat this much. There are a bunch of factors at play here, especially given the energy sucked up by molecular disassociation, so it isn't clear what would be the most destructive. Some form of stone is probably the winner here, but working out by how much it wins is quite tricky and left as an exercise to the reader.

At the moment I am thinking about capping the maximum temperature at 8000°K. What would change if that was not the case?

At a "mere" 8000K, you're probably below the temperature at which a significant amount of the hydrogen and oxygen in the aforementioned water explosion would be ionised. Push the temperature to over 10000K though and you'll need another 1300kJ per mole of hydrogen or oxygen to knock off some electrons and make a nice singly ionised plasma. It might quadruple the total energy released in the blast compared to 8000K (but I'm too lazy to work the actual output).

But what if we went even higher? Arbitrarily high temperatures and a magical effect that propagates at lightspeed. What could possibly go wrong?

If you had access to the right kind of fuel, this sounds like a recipe for Inertial Confinement Fusion.

Now I want to weaponize this ability, and need some guidance for the implications.

My recommendation is to make it much less hot, and affect much smaller volumes of material. I mean, seriously, just bringing stuff up to a few hundred degrees C is enough to start fires, deform or melt soft metal objects, render other objects unusable or downright dangerous (would you want to be wearing a helmet that has suddenly become 500K?). Your enemies would have to approach you nude (but for some asbestos sandals, perhaps) and emptyhanded. That seems reasonable enough, compared to town-flattening explosions.

_{* I'm not 100% sure on this; the bonds might just fall apart because they've already had enough energy pumped into them by heating, and there might be no additional threshold to overcome. If this were the case, the yield drops by about 2/3rds, but still leaves you with a respectable couple of tonnes. Also, the disassociated H and O will vigorously recombine to release that much energy afterwards, contributing to the fireball if not the blast.}

One, instantly zapping a cubic meter of just about anything to 8000 K is going to be a major explosion. You can amplify the destructive effect to a specific area by applying more fun physics. For example, in a fantasy setting, a glacier fortress was built around a nearly impervious stone geode suspended in the glacier. Superheating the geode (through magical means) instantly converted the surrounding ice into gas, not only exploding but turning the glacier into a shape-charge that violently expelled the geode and disintegrating fortress into the sea amidst a rain of ice and stone.

Afaik, 8000K is significantly higher than the temperatures reached by any chemical reaction. As such, instantaneously heating a ton of whatever matter to 8000K definitely gives a larger explosion than a 1 ton TNT bomb. The military will be very interested in your skills.

If we remove that limit, the explosion gets larger. This is proven by all the nuclear bombs out there: All they do is to heat their core to the multi-million degree range, and let thermodynamics turn that energy into destruction. The military will be extremely interested in your skills.

So, basically, you can be as destructive as you allow yourself to be (by capping the achievable temperature and heatable volume). Take care not to be too close to the explosions you create, though.

How does your superheating work?

First, you need to decide on what your superheating skill is like.

• Transmission speed: You said it transmits at the speed of light.


• Transmission distance: Line of sight? Mental image of area? Anywhere on the planet?


• Superheating time: How long does it take to get from 20°C to your 8000°C? Is it like 'bam' instantaneous, or does it take e.g. 1 second per 1000°C?


• Radius of effect: You mentioned that it can be 1 cubic meter at the most. What is the smallest possible radius of effect?


• Temperature control: Can your person vary how hot they make stuff? Can they decide if they want boiling temperature only (100°C), or are they always the full 8000K?


• Radius control: Can your person vary how much stuff they affect? Can they decide 'nah, 1 cubic meter is far too big - let's go for 1 cubic centimeter'?


• Reload time: how quickly can your person do a second superheating feat? Couple seconds? Hours? Days?

Only if you can answer those questions, it becomes apparent how much damage a person with your ability can really do.

Damage Dealer

Living Beings

If you can do rapid shots of small targets, your superheating ability is going to be very effective against people (and animals).

If you have enough control and are able to do precise targeting, heating even as little as 1cm3 to only 100°C will be fatal if you do it in the right spot (brain stem, heart).
If you aren't as precise and heat one cubic decimeter to 100°C, that's going to be crippling in the short term no matter where in the body you hit with that.
Heating 1cm3 to 1000°C will be fatal anywhere on the torso - the resulting steam (bodies are 80% water) will explode the person quite messily (and possibly steam-broil the surrounding people alive).

The problem comes when dealing with multiple people - are you quick enough to target them before they get to you? If you are a one-hit wonder, you might refrain from targeting living beings when there are more than 1.

For multiple targets, you might consider superheating 1 cubic meter of air. It will expand rapidly and create a blast wave that will can damage the body, hearing, and the surroundings (think of an explosion without fire). It will also result in a flash of very bright light (if you manage to heat it up to plasma levels - like a lightning strike) and potentially blind your enemies.

Transportation

How effective you are against transportation vehicles is decided by how precise you can be, and whether you know where the engine is in the vehicle. Superheat any part of the engine - it will not work anymore and prevent your enemy from getting away.

With something as large as ships, you might consider superheating some of the water right beneath the ship - the resulting explosion will damage the hull severely and hopefully sink it.

Structures

Here, it comes down to how well you can target without line of sight, how far away and how quickly you can superheat matter.

To get an order of magnitude of how powerful your ability is, I have crunched some numbers. One kg of water takes about 4.18 kilo Joules to heat 1K. One kg of TNT has an explosive power of about 4.6 MJ. So, 1000kg of water (= 1 cubic meter of water) needs approximately the energy of 1kg TNT to heat for 1°C. This changes once the water turns into steam to where the energy of 1kg TNT is sufficient for heating 1 cubic meter of steam for about 2°C. Of course, that also changes with the pressure (which rises the hotter the stuff is), but this is just a calculation to get a ballpark figure...

Result: heating 1 cubic meter of water to 1000°C takes approximately the same power as 1 ton of TNT. If you get the complete explosive power out of the water probably depends on how quickly you can heat it. A tomahawk cruise missile has about half a ton of TNT in power...

See Wikipedia https://en.wikipedia.org/wiki/TNT_equivalent#Examples for more examples of what you can do with your power and how it compares to atomic bombs e.g.

Technology

If you have a lot of technology around and manage to superheat air to plasma levels, you might be able to create an EMP effect. Plasma is an electric conductor, no matter whether the original material was or not. It might result in some interesting electromagnetic impulses when it explodes outwards and temporarily disable electric devices in the area. Maybe it is even strong enough to fry delicate circuitry.

1) How can the ability to heat portions of matter to high temparatures be used as a weapon, if I cannot affect the opponent's body matter directly and I need to minimize the number of heating instances and volume?

As you have been aswered by @AlexP, you can case explosions up to tons of TNT equivalent. It is very dangerous to youself as you cann't know for sure that say, this iron sphere has tangsten core or is hollow: result would greatly vary.

It would be safer to just create a denial zones with heating earth and disable any mechanisms (cars, helicopters, firearms) with overheating or melting critical parts.

2) Which secondary effect (i.e. explosions or something) would I be able to generate?

The major effect you can case beside explosion is heat radiation. If you just slowly (i.e. not explosion-like) rise the temperature of iron (steel) sphere to melting point (about 1000 C), or, better, make lava from rock - the heat radiation would cause burns and ignite flamable materials (including cloth, hairs and flesh) in vicinity.

Other effect would be a small firestorms and fire tornados.

Also you can lit your way in the dark by just heating up some small object to about 600-700 C and holding it with a stick.

In general it is quite a destructive ability.

upd:
I would suggest not to breake first law of thermodynamics much and make any heating to cause smth in vicinity cooling (down to absolute zero). This would make this "Maxwell Devil" even more destructive! You would be able to rise down any construction by heating one part and cooling another.