Naked Science Forum

General Science => General Science => Topic started by: Edge03zn on 22/06/2009 10:45:25

Title: Firewalking
Post by: Edge03zn on 22/06/2009 10:45:25
How are people able to firewalk and come out physically unharmed [?]
Title: Firewalking
Post by: Chemistry4me on 22/06/2009 10:58:49
-Water has a very high specific heat capacity (4.184 kJ/K kg), whereas coals (and lava) have a very low one. Therefore the foot's temperature tends to change less than the coal's.
-Water also has a high thermal conductivity, and on top of that, the blood in the foot will carry away the heat and spread it. On the other hand, coal has a poor thermal conductivity, so the hotter body consists only of the parts of the coal which is close to the foot.
-When the coal cools down, its temperature sinks below the flash point, so it stops burning, and no new heat is generated.
-Firewalkers do not spend very much time on the coals, and they keep moving.
-Calluses on the feet may offer an additional level of protection, even if only from pain, however most people do not have calluses that would make any significant difference.
Title: Firewalking
Post by: on 22/06/2009 11:12:46

Firewalking and Science

Why is it possible for bare feet to touch red-hot coals without getting burned? The coals start out as pieces of wood. The wood consists of lots of carbon, some "volatile organic molecules," and water.

A volatile organic molecule is a carbon-based molecule that evaporates when you heat it. Gasoline is a volatile organic chemical. We see these volatile organic molecules from wood as smoke rising from the fire. The heat of the fire evaporates all the volatile organics, as well as all the water. Because they've been burning for a while before the stunt, the coals have burned down to nearly pure carbon.

If you were to pick up one of these pure-carbon coals, you would notice that it is extremely light. Carbon is a lightweight element -- that's why carbon-fiber bike frames and tennis rackets don't weigh very much. A coal is mostly lightweight carbon atoms and air spaces (it does contain a few other elements, like potassium and calcium -- that's what left behind in the ash).

This lightweight carbon structure is a poor conductor of heat. It takes a relatively long time for heat to transfer from the glowing coal to your skin. If the coal were made of red-hot metal, heat transfer through conduction would be almost instantaneous -- you would get a severe burn.

Now, add to that the fact that ash is a very good insulator. People used to use ash to insulate ice boxes and such. The red-hot coals covered with ash transfer their heat even more slowly because the ash acts as a layer of insulation.

Then there is the short time span. Heat transfer from a red-hot coal is slow, but it still happens. If you were to stand still on the coals for several seconds, you would definitely get a burn. By walking briskly, you limit your contact with individual coals to a very short time span. You also get across the bed of coals very quickly, and that limits your total amount of coal time. So, your foot never gets hot enough to burn

Or watch Penn And Tellers "Bullshit", season one episode entitled "self-helpless"
Title: Firewalking
Post by: Bored chemist on 22/06/2009 21:38:50
There's also the fact that feet tend to be sweaty. The moisture evaporates and provides further cooling.

What I'd like to know is who was it first tried this given that, at that time, latent heat of vaporisation, heat caacity thermal conductivety, and the circulation of the blood were unknown concepts?
Title: Firewalking
Post by: DrChemistry on 14/08/2009 11:32:48
Might be because of the thickening in the skin that often appears on our feet. The thickened skin is much less sensitive in the otherwise painful situation.