Fire-walking

Fire-walking is the act of walking barefoot over a bed of hot coals. This feat sounds like defying the laws of nature - one would expect to burn one's feet - but according to physicists it is just an application of those laws.

Physical explanation

When two bodies of different temperatures meet, the hotter body will cool off, and the cooler body will heat up, until they are separated or until they meet at a temperature in between. What that temperature is, and how quickly it is reached, depends on the thermodynamical properties of the two bodies. The important properties are:

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• temperature,
• mass,
• specific heat capacity, and
• thermal conductivity.

The product of mass and specific heat capacity is called heat capacity and tells us how much heat energy the body needs to heat it up by one degree. Since the heat taken in by the cooler body must be the same as the heat given by the hotter one, the end temperature will lie closer to the temperature of the body with the greater heat capacity.

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The bodies in question here are:

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• a human foot, which mainly consists of water,
• burning coals.

Several factors act together to prevent the foot from burning:

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• Water has a very high heat specific capacity (4.18 kJ/K kg), whereas coals have a very low one. Therefore the foot's temperature will change considerably 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. So effectively the mass of the cooler body is increased. 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.
• The coals are often covered with ash, which is a poor heat conductor.

This does not mean that it is impossible to burn your feet. Fire-walking is still dangerous.

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• People have burned their feet when they remained in the fire for too long (enabling the thermal conductivity of the coals to catch up).
• Also, there should be no foreign objects in the coals. Metal is especially dangerous since it has a high thermal conductivity.
• The coals should have burned for a while. Coals contain water, which increases their heat capacity as well as their thermal conductivity. The water must be evaporated already when the firewalk starts.
• The feet should be dry. Wet feet can cause coals to cling to them, increasing the exposure time.

It has been claimed that the Leidenfrost effect, which is based on a layer of water vapor between the hot and cold body, is involved in firewalking. But this would lead to extremely reduced friction - you wouldn't be able to walk on the coals, you'd slip on them.

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