Why isn’t aluminium foil hot when I take it out of the oven?
It does get hot, just not as hot as you might think it would. It’s because there isn’t much there to hold the heat. The aluminum reflects a lot of the heat.
Thanks for the info, but I’m looking for a more technically based answer. I know aluminium is not a particularly good conductor for electricity, but I don’t know how that translates to heat. For example, I take a 400 degree baked potato out of the oven after 2 hours, and the aluminium foil is cool enough to touch. Is there a reflective property at work? Is there a radiative effect at play, or does it not absorb the heat in the first place?
As already stated, it is as hot as the air in the oven. The deal is is that it is very thin and when you grab it, the heat drains into your fingers very quickly but instead of your fingers getting hot, the foil cools down.
Fold it over a few times to make it thicker and then put it in the oven. It will both be, and feel, very hot when it comes out.
Reeder did reply to your question. Aluminum is a very reflective material. Heat is emitted from the elements in your oven (or from any other source) and strikes the aluminum, which reflects it back rather than absorbs it. There is no radiation from the aluminum itself, as that would suggest that the foil on it’s own could produce heat, which it doesn’t. Any “radiation” there is coming from that piece of foil is merely relected heat energy. The reason why it’s cool enough to touch after being in the oven with your baked potato is because it’s THAT GOOD of a relective material - it absorbs so little that there is no heat in it for you to detect.
Actually, aluminum is a reasonably good conductor of electricity, it’s sort of falling back out of fashion but houses used to be wired with aluminum wire. Not as good as copper of course.
Aluminum has fanatstic reflective properties in the IR. So much so that even a non-polished aluminum panel will give reasonably good reflections in a IR camera. This reflective property however, does not prevent the air in the oven from heating it up. It radiates in the IR but the air in the oven heats it directly.
The foil comes out of the oven as hot as the air in the oven. It does radiate heat off as you thought but the real trick is how thin it is.
Aluminum is highly conductive, and therefore looses its thermal energy fairly quickly when exposed to cooler air.
Aluminum foil is not massive, compared to a human hand, or almost anything for that matter. But it has more to do with the aluminum not being much of a storage medium for thermal energy in the first place. That is, it takes very little thermal energy to raise the temperature of a thin layer of aluminum compared to the much larger amount of energy it takes to raise the temperature of your (comparitively much more massive) fingertips. Add to that the fact that your skin is partly made of water, which is known for keeping a steady temperature even when exposed to short-term heat energy.
To rephrase, the thin layer of aluminum foil heats up quickly to 400º (the temperature of the air in the oven) because it is highly (thermally) conductive and so little energy is required to bring the foil up to this temperature. Likewise, the foil looses what little bit of energy it can retain very quickly, and this tiny bit of thermal energy is insufficient to raise the temperature of your skin enough to cause discomfort.
Nt true, aluminum is a very good conductor of electricity. It is also a good conductor of heat.
In the oven, the aluminum is as hot as the air and the potato. As soon as you take it out, it loses a lot of heat to the air and some to radiation. Aluminum foil doesn’t have much thermal mass, because of it’s low specific heat and the low mass of aluminum that you’d use to cover a potato, so it can lose the heat easily, whether it’s into the environment or into your skin.
Some math: the specific heat of water is 4.184 Joules per gram-degree C (or K) and aluminum is .91 Joules per gram-degree C. That means that a gram of foil takes one fourth as much energy as a gram of water to heat up (or one fourth as much must be removed for it to cool off one degree).
So, in some magical land where hot foil wouldn’t radiate IR, and where there’s no atmosphere for the foil to conduct to, your friend takes a ball of foil out of the oven and tosses it to you, and you catch it with your bare hand. If the ball is made of 2 square feet of foil, it will have a mass of about 8 grams. 330 deg F diff (180 deg K) by 8 is 1466. 1466 divided by (4.184/.91) gives you 385.
That 385 is the number of gram-degrees that would be absorbed by your hand. Assuming the surface of your hand is mostly water, and you put 50 grams of hand water in contact with the ball, it would raise that 50 grams of water ~8 degrees C, which is 14 degrees F.
So, even if you come into contact with all 2 square feet of foil at once, and it’s still at 400 degrees F, the surface of your hand would only be raised 14 degrees F, a long way from a burn.
Once you take into account radiation, conduction by the air, and the miniscule area of the foil that you actually touch, it’s not odd that it doesn’t feel warm.
Ben
Yeah, you don’t get burned mostly because there is so little aluminum and it has such a large surface area over which to cool fast. You could take a little peice of aluminum foil and hold it in the flame of a lighter for 5 minutes, take the flame away, and it would be room temperature again in a few seconds. Now take the entire roll of aluminum foil still tightly rolled up, throw that in the oven for an hour, and then grab that an hour later and you definately will get burned.
This is correct. Ovens work mostly by convection, with hot air heating the contents directly instead of by radiation. A polished aluminum block will heat up almost as fast as a black anodized aluminum block, and will reach the same temperature. If aluminum didn’t heat, nor would the food wrapped inside the aluminum foil.
As already stated, aluminum foil is extremely thin and has excellent heat conductivity, which means it loses its heat almost instantly to the air once removed from the oven. It will be cool to the touch by the time you touch it.
Aluminum house wiring was outlawed about 20 years ago in most parts of the civilized world that used it. No one makes in anymore. The problem was electrolysis between the aluminum and the copper and brass used in other wiring and electrical components such as switches and outlets. The electrolysis would cause corrosion which in turn would cause the joints to fail electrically. This would happen at poorly tightened connections. In some instances, the extra resistance would cause thing to heat up and a fire would result. My brother just remodeled a house built in 1966 and he had to replace the aluminum wiring with copper. My house was built in 1969 and before I could buy it, the wiring was inspected to make sure there was no aluminum wiring. Aluminum is a very good disipater of heat though. Many cars built in the past 15 years have aluminum radiators.