Why can tinfoil be touched immediately after coming out of a super hot (hundreds of degrees) oven?
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Isn’t the eli5 answer just: it’s thin, so it can’t hold a lot of heat?
It isn’t the temperature of the object you touch that burns you, it’s the temperature it’s able to bring your skin to via conduction that determines how bad your burn is. Tinfoil can be blazing hot, but it’s very thin. So, when you touch it, the heat gets conducted into your finger, but since your finger has so much more thermal mass, the heat spreads out, and your skin never actually gets that hot. Also, it doesn’t hurt that tinfoil has so much surface area relative to its mass that any heat in it can get carried away by convection within seconds of coming out of the oven.
Its been mentioned but I like to think about the amount of “heat energy” something has like glass of water. If something can’t hold lots of heat energy what little energy it has will dissipate VERY quickly once it touches you. So yeah, the aluminum may be hot but it contains so little “hot” that once you touch you skin is easily able to absorb the little amount of energy without getting burned.
What kind do you use?
Mine burns me for several minutes after coming out.
You can look at this in a more thermodynamics-centric way, but I’ll try to stay brief:
– the amount of heat that needs to be extracted away from an object to lower its temperature by 1°C is (among other things) tied to its mass. A typical sheet of aluminum foil is extremely thin and thus extremely light. As a result you don’t need to extract that much energy to lower the temperature of a sheet of foil.
– The speed at which heat radiates away from an object is strongly dependent on its surface area. On top of the previously discussed low, a sheet of aluminum foil has a LOT of surface area.
– Additionally, aluminum itself is a good thermal conductor, meaning heat has no trouble reaching the outer edge of a sheet of foil.
A solid cube of aluminum would cool much slower, for the sake of example.
In order to get burned, it’s your fingers that need to get hot, and there are three things that are needed for that to happen:
A high temperature. (heat flows from a high temperature to a lower temperature)
Thermal conductivity (the ability to transfer heat quickly from one object to another)
Thermal mass. (how much energy can be transferred before the hot object reaches the same temperature as your fingers.)
Aluminum foil from your oven has the first two, but has very little thermal mass, so it cools off before it can heat your fingers up enough to burn them. If it was thicker, like a baking sheet, it would burn you.
Something like space shuttle insulation tiles have very low thermal conductivity, and can be picked up while still hot enough to glow, because they can’t transfer that heat quickly.
There’s just not much actual object to be hot, so not much heat goes into your fingers, it’s very thin so it cools down before your finger stars to burn.
Aluminum is a high thermal rate conductor.
In a foil form the heat is transferred nearly as soon as heat source is applied or removed.
Thickness is critical. If you take aluminum foil and heat a tightly wound roll, the heat will transfer much slower.
However the instant air is permitted between the layers heat retention drops.
If you have ever seen a car radiator, you have seen this technique for temperature exchange.
Air conditioners work the same way.
An additional benefit of the rapidly cooling foil, is the nice headwear that may be made so soon after the foil has been in an oven.
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