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LOG
06-27-2010, 01:12 AM
Is there a set temperature for how hot something would have to be to cause liquids (water specifically) to evaporate before it can touch the source of the heat?

efkelley
06-27-2010, 02:26 AM
Well when a substance reaches a certain temperature it changes state. Period. For water at sea level this is 100C (212F). The heat source needs to radiate enough energy into the surrounding medium (assuming normal air here) to boil whatever volume of water you're dropping on it before the water impacts.

As for the math though, I have no idea. Likely the amount of water is going to be the biggest factor. Water has a crazy high specific heat, and gasses are inefficient conductors of thermal energy.

veinglory
06-27-2010, 02:37 AM
There are far too many variables in that situation for a single simple law to cover.

RainyDayNinja
06-27-2010, 11:43 AM
I believe the proper technical term is "really freaking hot."

Xelebes
06-27-2010, 11:49 AM
It's not a matter of how hot an element is but rather how much energy is radiated from the element.

geardrops
06-28-2010, 09:44 PM
Is there a set temperature for how hot something would have to be to cause liquids (water specifically) to evaporate before it can touch the source of the heat?


It's not a matter of how hot an element is but rather how much energy is radiated from the element.

Water, or whatever other material, would have to come in contact with heat. Whether the water physically touches the solid matter which is the source of the heat or not is another thing.

But there are instances on earth, if my grade-school science readings have stayed with me correctly, where the terrain has been so hot that rain has evaporated before it hits the ground. Rare, and only in the absolutely balls-hottest parts of the earth.

At that point you're going to have to find out how much heat a material will radiate (some materials radiate heat better than others) and go from there.

pdknz
06-28-2010, 10:17 PM
You are right that water can boil so fast that it refuses to touch a hot surface. My observation is that a hot woodstove in the range of about 400 to 600 degrees F is about the range where that phenomenon occurs. It is frequently an issue for heat treating steel by quenching in water. The temperature of the heated surface is only one factor, however--if the water is hot, it is more likely to dance on the hot steel than cold water. The depth (actually the pressure) of the water is also an issue. Salt water is sometimes used for quenching steel because it has a higher boiling point, and therefore cools the steel faster.

Round number for most purposes, though, 400 F or more.

RainyDayNinja
06-29-2010, 04:43 AM
I just did some rough calculations (keep in mind that I never took a class in heat transfer), and assuming a drop of 0.1 mL water, a radiating surface area of 1 mm2 per drop, and a time of .1 seconds to evaporate, you'd need a temperature of about 14 000 degrees C.

This is to vaporize the entire water droplet before it reaches the surface, via blackbody radiation, not to merely float on a cushion of water vapor. Again, I never learned how to calculate this properly, made a lot of assumptions, and looked up equations on Wikipedia, so take this with a grain of salt.

Lhun
06-29-2010, 05:12 AM
I suspect that conductive heat transfer is actually a lot more important here than radiative, since the air around the hot object will have pretty much the same temperature as the hot object. Without air, a.k.a. in a vacuum, the liquid would evaporate anyway.

RainyDayNinja
06-29-2010, 05:17 AM
I don't know about more important, but now that you mention it, I'm sure it's a factor. But I have no idea how to calculate it. I'd guesstimate that it's still up in the several-thousand-degrees range, though.

Lhun
06-29-2010, 05:24 AM
Rule of thumb, conduction transfers more heat than radiation. That's why vacuum is a better insulator than air.

RemusShepherd
06-29-2010, 07:47 PM
It might help you to look up the Leidenfrost Effect. This is a phenomenon where water refuses to touch a hot surface -- it creates a pillow of steam to keep the water and the hot element separate. This doesn't evaporate the water right away, but it insures that the hot element never gets wet. Maybe that's of use to you. Maybe it's a problem: If the hot object is hot enough to cause a Leidenfrost Effect, it will evaporate water *more slowly* than a cooler object would.

pdknz
06-30-2010, 01:05 AM
It might help you to look up the Leidenfrost Effect. This is a phenomenon where water refuses to touch a hot surface -- it creates a pillow of steam to keep the water and the hot element separate. This doesn't evaporate the water right away, but it insures that the hot element never gets wet. Maybe that's of use to you. Maybe it's a problem: If the hot object is hot enough to cause a Leidenfrost Effect, it will evaporate water *more slowly* than a cooler object would.

Yeah. That's what I was talking about. Let me add a little digression--if you are ever, say, working in a blacksmith shop and you pick up a piece of steel and it feels oddly slippery--drop it. You probably just got a third degree burn, but you also felt the Leidenfrost effect.

Don't Ask Me How I kNow This (DAMHINT)

Pthom
06-30-2010, 05:17 AM
This is slightly obtuse from the OP question, but certain substances "subliminate" (pass from solid to gasseous state without becoming liquid). Carbon dioxide does this at normal temperatures and pressures. Under normal conditions water will not subliminate. But it can be made to do so, such as when it's in a vacuum (ref Luhn above). The Leidenfrost effect, however, manifests itself at very short distances. And the liquid (water) must contact the heated surface. The temperatures required to evaporate water via radiation would cause all kinds of other havoc before the water evaporated. Unless, of course, you squirt water into a vacuum. But that's different, too, isn't it?

RainyDayNinja
06-30-2010, 07:50 PM
It's "sublimate" or "sublime", not "subliminate".

Pthom
07-01-2010, 05:10 AM
Many thousands of begging your pardon for making the spelling mistake. You are of course, correct.

Some substances sublimate, which is to say that they sublime (change from solid to gas without passing through the liquid state, and just as often return to solid without becoming first liquid).

The harder thing is to decide when to use the spell checker (or to trust the results of using it). Sometimes I avoid making that decision. Maybe I should give up typing responses in a hurry and .... nah. No one else in here does. :D

RemusShepherd
07-01-2010, 05:08 PM
The harder thing is to decide when to use the spell checker (or to trust the results of using it). Sometimes I avoid making that decision. Maybe I should give up typing responses in a hurry and .... nah. No one else in here does. :D

My advice is to never use a spell checker. The brief embarrassment you get when you spell a word wrong will serve as reinforcement, so you *will* get that word right next time. Strike me down, spelling nazis, and you will only make me that much more powerful! :)

RainyDayNinja
07-01-2010, 07:37 PM
But you can't really trust the spell checker when you're writing about technical scientific topics. Half the words I use in my lab reports are marked as misspelled.

benbradley
07-01-2010, 08:12 PM
Well when a substance reaches a certain temperature AND YOU ADD HEAT it changes state. Period.
My edit - this is an important point:
The heat of vaporization of water is the amount of heat to convert (a certain standard amount of) water at 100C into vapor at 100C. It actually takes a LOT of heat:

...the molecules (http://en.wikipedia.org/wiki/Molecule) in liquid water (http://en.wikipedia.org/wiki/Water_%28molecule%29) are held together by relatively strong hydrogen bonds (http://en.wikipedia.org/wiki/Hydrogen_bond), and its enthalpy of vaporization, 40.65 kJ/mol, is more than five times the energy required to heat the same quantity of water from 0 C to 100 C
For a demonstration with a pot on the stove, you can heat a chilled pot of water to boiling in maybe five or ten minutes, but it will take another half hour or so to boil away all the water.

For water at sea level this is 100C (212F). The heat source needs to radiate enough energy into the surrounding medium (assuming normal air here) to boil whatever volume of water you're dropping on it before the water impacts.

As for the math though, I have no idea. Likely the amount of water is going to be the biggest factor. Water has a crazy high specific heat, and gasses are inefficient conductors of thermal energy.
THERE you go, you DO know what you're talking about!


Back to the OP, there is no "instant" vaporization any more than there is "instant" acceleration from 0 to 100MPH. How fast water vaporizes is dependent on how fast you can put energy into it. If you really want to vaporize a lot of water quickly, I might suggest neutrons - a big burst of lots of high-speed neutrons. These neutrons will hit the water molecules, transferring energy to them.

There is such a device which will generate a large burst of high-energy neutrons - I'll leave it to the OP to research such devices. :)

This is slightly obtuse from the OP question, but certain substances "subliminate" (pass from solid to gasseous state without becoming liquid). Carbon dioxide does this at normal temperatures and pressures. Under normal conditions water will not subliminate.

FWIW, Firefox underlines subliminate as I type it, but not sublimate or sublime, indicating subliminate isn't an actual (correctly spelled) word, but I suspected it just MIGHT BE an actual word and so wouldn't have been flagged for being misspelled. There's that problem with spiel chequers

While I'm being pedantic, wouldn't that be ICE that "will not sublime?" And I thought ice in a freezer was a classic example of sublimation - it doesn't happen fast, but if you leave ice in the freezer for days or weeks it gets rounded edges, supposedly (I read it in a book decades ago, so it MUST be true) from the ice that has sublimated. Or sublimed. Firefox says they're both good words.

Pthom
07-02-2010, 06:17 AM
And yet, Google returns some 16000 entries for "subliminate." Correct spelling or no, the word exists.

Be afraid.

Be vary afraid.
Those in the United Kingdom (and elsewhere) call the most useful and common element "aluminium." Those of us in the United States (and other elsewheres) call the exact same stuff "aluminum."

And while you're at it, be very afraid.

FOTSGreg
07-03-2010, 01:27 AM
Oh (waves hand)!

I have a related question.

How much energy would it take to completely evaporate a human body along the same lines as a phaser in Star Trek in the 2-3 seconds it seems to require in that show?

I'm thinking it's probably going to be a lot more than you'd like to be standing nearby or holding in your hand at the time.

In addition, let's say you can completely disintegrate and evaporate a human body in 2-3 seconds times. What is the resultant effect on the environment around that body, heat-wise, in the immediate, say 10 seconds, afterward?

RainyDayNinja
07-03-2010, 01:52 AM
If you assume a human being made entirely of water (this is sci-fi we're talking about, you know), it takes about 1 kilowatt per gram to vaporize him. A human on the large side is 100 kg, so we're looking at 100 000 kW output from that phaser. That comes out to about 3 days' worth of the average energy consumption of an American home, all pumped out in seconds. In reality, it would be higher because of all of the organic material and metals, etc.

FOTSGreg
07-03-2010, 02:33 AM
Um, check my math - 100 thousand kW is around 1 GigaWatt, right? Or is that 100 MegaWatts?

100 MW or 1 GW applied to a roughly 1 cubic meter solid in 2-3 seconds time resulting in total vaporization of the affected solid would have what side effects on the local neighborhood?

RainyDayNinja
07-03-2010, 03:25 AM
It's 100 MegaWatts

Considering water expands by something like 14000 times when it evaporates, you're looking at a serious explosion. Anyone nearby would probably suffer severe steam burns, and you might have some broken windows etc. on your hands.

benbradley
07-03-2010, 05:52 AM
Or 0.1 gigawatts, though it's more conventional to pick the prefix that makes the number something between 1 and 999, thus 100 megawatts.

I'd prefer to watch from uphill and upwind, maybe 500 feet away from the target/victim.

I agree with Ninja, one whole human's worth of flesh turned to vapor would probably cause bad burns on anyone within 20 to 50 feet. Kirk and Spock should stand further away when they fire with a setting of annihilate.

Lhun
07-03-2010, 12:21 PM
If you assume a human being made entirely of water (this is sci-fi we're talking about, you know), it takes about 1 kilowatt per gram to vaporize him. A human on the large side is 100 kg, so we're looking at 100 000 kW output from that phaser. That comes out to about 3 days' worth of the average energy consumption of an American home, all pumped out in seconds. In reality, it would be higher because of all of the organic material and metals, etc.Oh boy where to start. Well, in general your thoughts run in the right direction with some small mistakes.
Water has extremely high heat capacity, so a human would need (a little) less energy to vaporize than 100kg of water. There's not much metal in humans, and most of the organic stuff will take much less energy than water. Also, most biological organic substances are impossible to vaporize, they burn or degrade first (this will also reduce energy needed).
Watt is a unit of power, not energy. You do not need a certain amout of watt to vaporize something, you need a certain amount of joule (or wattseconds)
Assuming a 100kg body of water, we need about 200 227 MJ for vaporization. (For comparison: that's about twenty times as much energy as a tank cannon projectile) That means to vaporize in 2 seconds, we need an output of 113,5 MW. The Phasers on StarTrek usually do that in a second or less, so we'd need even more.

<snip>I'm thinking it's probably going to be a lot more than you'd like to be standing nearby or holding in your hand at the time.Well, given how many shots a phaser on StarTrek can fire, i'd prefer to be really really sure that the nuke in my never explodes on a misfire.

In addition, let's say you can completely disintegrate and evaporate a human body in 2-3 seconds times. What is the resultant effect on the environment around that body, heat-wise, in the immediate, say 10 seconds, afterward?Heat-wise? Doesn't really matter.
If you vaporize 100kg of water, you just created 100kg of steam, which will quickly expand to about ten thousand times its current volume. The technical term for this is "big-ass explosion". For comparison: Instantly evaporating a human body is about the same as replacing it with TNT and detonating it. I hope those phasers have a "not for indoor use" sticker somewhere, or there's a mighty lawsuit coming as soon as someone shoots a burglar with it and levels the building.

FOTSGreg
07-04-2010, 12:40 AM
Lhun wrote, For comparison: Instantly evaporating a human body is about the same as replacing it with TNT and detonating it.

I thought as much and your answer left me laughing so thanks very, very much.

I have another (somewhat related) question and it's something that Star Trek gets wrong, apparently, even in their "Bible" technical handbooks.

In one of the technical "Bibles" I've seen they give figures for the power output of a ship's phasers in gigawatts and the defensive capability of a shield in gigajoules or vice versa and they tend to switch back and forth here and there.

Looking at the online information it says that the Enterprise's shields can fend off energy bombardment in the terawatt range and direct torpedo hits in the kiloton or low megaton range (source: http://www.stardestroyer.net/Empire/Tech/Shields/Shield1.html) and phaser output is in the 30 thousand terawatt range against shields and 1-10 terawatts against dense armor.

I'm confused. What is the direct energy relationship between power output (say in joules) and absorptive capacity?

Lhun
07-04-2010, 01:35 AM
In one of the technical "Bibles" I've seen they give figures for the power output of a ship's phasers in gigawatts and the defensive capability of a shield in gigajoules or vice versa and they tend to switch back and forth here and there.Well, you can use both units to describe both weapons and shields, as long you keep in mind the differences.
Joule (or wattsecond, more commonly used in the kilowatthour transformation i.e. 3,6 million wattseconds) is a measure of energy, watt is a measure of power. Power is energy per second, so if you describe a weapon or shield in watt, that would give you the figure of how much energy can be emitted/absorbed per second. If you describe a weapon or shield in joule, you'd have the total energy output of single (usually) shot, or the maximum absorbed energy without recharging shields or something.

Looking at the online information it says that the Enterprise's shields can fend off energy bombardment in the terawatt range and direct torpedo hits in the kiloton or low megaton range (source: http://www.stardestroyer.net/Empire/Tech/Shields/Shield1.html) and phaser output is in the 30 thousand terawatt range against shields and 1-10 terawatts against dense armour.

I'm confused. What is the direct energy relationship between power output (say in joules) and absorptive capacity?The relationship is purely fictional, since there is no equivalent to energy shields in reality, and measuring the defensive capability of armour in joule doesn't make a whole lot of sense. In reality, armour and weapons behave differently in different matchups. Tank armour for example might survive impact from a HEAT projectile barely scratched, but will get penetrated by high velocity kinetic projectile, yet, the HEAT projectile has a lot more total energy.
Generally speaking though, if giving such figures for energy shields which always work the same no matter the type of energy (which even star trek shields don't btw) you would want to give the capacity of the shields in joule, i.e. what is the biggest amount of impacting weapons energy at once the shield can fend off. Then you might also state the recharge rate of the shield, in watt. I.e. if the shield can survive a 100joule impact and has a power rating of 100watt, we'd calculate that it can fend off a 100joule impact every second, or two 50joule impacts per second, or it will let some energy pass through when hit by a 120joule impact. (Or possibly all of it depending on how the shield's supposed to work)
Similarly, you'd rate a weapon in watt to describe its continuous power output, and give the energy of every single shot in joule.

This however
phaser output is in the 30 thousand terawatt range against shields and 1-10 terawatts against dense armour just makes no sense. Terawatt is not some word that describes damage done to a target, the power output of a weapon is completely independent from what it's pointed at. Maybe it does 30k times as much damage to shields, but that doesn't mean the energy output is any less when shooting at armour.