Exploring Heat Transfer: The Leidenfrost Effect and Walking on Hot Coals

[tacloban87]

I've always wondered how people are able to walk on hot coal without suffering from burns.

Moreover, earlier, a teacher demonstrated to my class how if one spills liquid nitrogen on one's hand in open air that one does not suffer from frostbite, but only feels the cold temperature.

I got to thinking that the explanation for these two cases are in some way related to one another...is it because they are both open systems? So with the hot coal, heat is leaving the system, but with the liquid nitrogen, heat is entering the system?

 

[Panda]

Thats pretty much it - What is detecting the temperature and what is between that and the heat source/sink?

I would say through experimentations/showing off as a stupid student both do actually burn but only very superficially. How would that actually help you?

P.S. don't be like me, whilst it earnt me shed loads of beer and loads of admiring sighs from the gals, I had a nasty burn where a coal kicked up and landed on the top of my foot whilst walking through a beach BBQ.

 

[HallsofIvy]

How long is the foot in contact with the coals or the hand in contact with the liquid nitrogen? Certainly the greater the difference in temperatures, the faster damage takes place- but it still requires a specific amount of time.

 

[cesiumfrog]

Nitrogen gas is a good insulator. http://en.wikipedia.org/wiki/Leidenfrost_effect" about the way a small droplet of liquid nitrogen will tend to quickly skitter off of your hand, without taking the time to absorb noticeable heat from your skin. However, if the liquid nitrogen isn't allowed to run off, your skin will keep cooling down.. the process will obviously rocket once the outer layer of dead skin cools too much, then it's just a question of whether the amount of heat stolen from your hand was enough to do serious damage (and if you stick the frostbite straight under a cold water tap, you might be able to curb the damage spread).

Similarly, though coals are hot they don't have the highest heat capacity and they especially don't conduct heat very well. I presume it only takes an instant to equalise the temperature of the hard sole surface of your foot with the top surface of the coal, but you get several seconds before too much heat is conducted into your foot from the other parts of the coal.