Before we start discussing, please no one ever think of attempting this. Okay, check this out.
http://www.woodswarriors.com/Liquid%20Metal.html
Now, let’s discuss this scientifically, with no intent of trying/testing it. Is this possible to do without getting burned? (even if the answer to the question is yes, do not go try it). If so, how?
Is it possible or is the video edited. If it is a fake happening by means of video alteration, how can you tell, and how was it done?
No way that’s real.
I have no idea about the specific numbers, but I’m sure even bone would be instantly incinerated at 2800*
Not to mention the fact that nobody who worked in a foundry would be so freakin stupid as to stick their hand in that.
I’m going to say that’s probably real…
You’re not going to get a scientific reason from me, just one based on my own stupidity. I often like to feel how hot things are. I’ve dipped my finger in boiling water before. I’ve put my hand through a fire. The key is how fast you do it. In the video he does it so fast that even when the footage is slowed down his hand still looks like it never touches the molten metal. In reality it does but it happens so fast that his hand is in the heat for only microseconds and then cools.
I think that video is probably real. I also don’t have any “scientific reason” as to why you would be able to do such a thing, but maybe he covered his hand in some kinda of heat retardant gel or something like that.
I agree that it is probably real. I cannot prove this (no one can prove anything on this vid.), but I have a friend that likes to wave his hand through candle flames to see what happens and how slow he has to go before it starts to hurt. Apparently, in his experience if you take more than like a hundredth of a second to get out, then it starts to hurt a lot.
So this is probably done at very high speeds by a very stupid person.
i dont really want to admit it, but i would probably try that at least once
I think if you did, one try is all you’d get, unless you wanted to lose the other hand too. So please don’t.
However I’m hoping that someone experienced in thermodynamics wil stumble upon this thread and prove why this either can or cannot be done without injury.
Actually, it is possible to do stuff like this without getting hurt too badly (no personal experience, but my Physics book had a little bit on this). And sanddrag, don’t tell me you have never heard of firewalking. They use the same principle. The theory is that if you wet your finger (foot, hand, whatever) before you do something like this, the water will take most of the heat until it is gone, so if you are quick, you can get away with it. (Although firewalkers use coals, so they step quickly and don’t get as much energy to their feet as if they used metal.) I think this has some basis, but I’m too lazy to go find my book and check it out 
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Yes, but remember, a candle flame can’t be hotter than 400 degrees farenheit and this is 2800 degrees.
I wonder if that guy used something like Aerogel to insulate the part of his hand that hits the molten metal. Aerogel looks almost clear (it is 99.8% air), yet it can withstand amazing temperatures.
http://upload.wikimedia.org/wikipedia/commons/thumb/b/b4/Aerogel_matches.jpg/441px-Aerogel_matches.jpg](http://en.wikipedia.org/wiki/Aerogel)
Its not temperature that burns you, its heat transfer
if you put a pizza in a 400° oven it doesnt not instantly reach 400°, it takes about 20 minutes to heat-soak (cook)
likewise, if your skin makes contact with molten metal, it would take time for the heat to transfer, and cook your skin
It does not say in the video what metal that is - the lower the density of the metal, the less heat it would contain, so maybe its aluminum?
but the bottom line is, how long is his skin in contact with the metal? 1mS? 10µS? If his hands are calloused they would be poor heat conductors, and it would take longer for his hand to be burned.
BTW, the reason you can walk on burining coals is, they are mostly carbon by the time they are coals, and they conduct heat very poorly - again, its not a question of temperature, its a matter of heat transfer.
Does the temperate effect how fast the heat transfers?
the temperature difference (between the metal and your hand)
and the heat conductivity between the two.
thats why sticking your hand into a 400° oven is not a problem, the air is a poor heat conductor, but if you touch the metal rack for a second or two you will cook your finger.
Some of us have 1/4"+ callouses on our feet. Asphalt in the summer ain’t no thang. I can’t imagine coals being that much of a problem either.
Wetzel
Here comes the science:
They state that the metal is 2800 degrees. If we assume this is in F, then we can reasonably assume that the metal is iron because 2800F is the melting point of iron. Aside from that, iron is a very common metal for casting. So, we’re going to assume that we’re working with iron.
According to P.92 of Thermodynamics: An Engineering Approach, 5th edition, we have the following formula for heat conduction:
Qdot = kA(delta T / delta x)
We have two volumes separated by a thermally conductive material. Qdot is Q with a dot over it - as in rate of heat transfer (J/s, W). k is the thermal conductivity. A is the area of the contact surface. delta T is the difference in temperatures of the volumes. delta x is the thickness of the separating material.
According to Table 2-3, human skin has a thermal conductivity of 0.37 W/m*K. Human body temperature is 98.6F (310.15K). The metal is at 2800F (1810.9K). According to Wikipedia, human skin is 1-2mm thick. I think finger skin is on the thin side, so we’ll go with 1mm (0.001m). I also think that 20cm^2 (0.02m^2) sounds like a good contact area.
Qdot = 0.37(0.02)(1810.9-310.15)/0.001] = 11000W = 11J/ms
That means that our hand will be able to absorb 11J of heat per millisecond. So, what do we consider to be too hot for our fingers? I’ll go with 120F (322.0K). That means we need an increase of 11.85K.
My book doesn’t have the specific heat of human, so I went with “beef round”. From Table A-3, beef round has a specific heat of 3.08kJ/kgK (3.08J/gK). I also couldn’t find the density of either substance, so I’m just going with 1g/cm^3 (density of water). So, if we assume 1cm thick flesh over our 20cm^2 contact area, we have 20cm^3 of flesh. This gives us 20g.
To find how much heat we need to raise the temperature:
delta Q = m * c * delta T
delta Q = 20 * 3.08 * 11.85 = 729J = ~730J
So, we need 730J to burn our hand. With a transfer rate of 11J/ms, we can keep our hand in the metal for 66ms.
Addendum:
I forgot to mention why working with iron was important. Actually, it probably isn’t as important as the fact that it has a higher thermal conductivity than skin. The thermal conductivity of iron is 80.2 W/m*K. As long as the hot substance has a thermal conductivity higher than that of skin, this analysis will hold. If the thermal conductivity is lower, however, it will be less willing to give its heat up for your hand and you’ll have to use the conductivity of the substance. Such substances, listed in Table 2-3, are wood, rubber, and air.
As I have said on several occassions before…Learn to recognize people like this because when they go they take others with them.
sciguy,
your analysis is excellent (esp for an electrical engineer :^)
the only point that doesnt sit right with me is you used the mass of your whole finger (1cm thickness) and if I understand the equation you used then which part of the finger is rasied to 120ºF? the surface of the side in contact with the metal, or the entire mass?
While the finger is in contact with the metal, there will be a temperature gradiant from the contact side to the back side, with the mass of the finger drawing heat away from the contact area.
The reason I ask is your answer seems pretty long - 66mS is only 1/15th of a second. My intuitive basis for this comes from experience, like licking your finger and tapping an clothes iron to see if its hot.
The other question is, what temp can we endure without being burned? 120ºF seems low. Tap water is usually 160 - 180ºF.
I’m assuming that the whole finger is rasied to 120F. There was a couple issues that I couldn’t easily resolve. First off, I wasn’t sure how to model the heat transfer within the finger. I suspect that if I worked on this long enough, I could probably come up with some differential equation that’ll do it. We’d need an equation that describes the rate of temperature rise in the finger at some distance from the surface. I doubt that it would be a very pretty one though. But hey, computers can solve anything.
The other issue was blood. Because it’s flowing, it’s essentially acting as a coolant. Although, because of the short time, I’m not sure how much it actually does.
Well, I was going off of cooking temperatures. For beef (which I’m assuming is the closest animal to human), medium-rare is 130F. And through the wonder that is Google, here is a handy dandy chart.
the answers to these questions (heat transfer) are learned in college Thermodynamics classes
Its normally taken by mechanical engineering students, after taking 4 semesters of calc and one semester of differential equations, and in many university’s its one of the ‘weed-out’ courses (where D is for Diploma :^)
D isn’t for diploma anymore.
It’s now C for continuation.
Wetzel
Yup, I’m taking ME113 (Thermodynamics). I don’t need it for EE, but I needed some extra units and it looked interesting. It covers topics more along the lines of power generation from heat engines. However, it looks like heat transfer is covered in the other two thermo classes (“heat transfer in electronics” and “thermal engineering”). They both cover heat transfer, but the one I have to take has an emphasis on heat transfer applications in electronics rather than engines and whatnot.
As for math, thermo’s prereq is the last semester of calc. The heat transfer classes need differential equations though.
And, according to the course catalog, ME majors need a C- or better in their thermo classes.
