A Question for the Scientifically Versed.

[emmline]

I took the aluminum pizza pan, lined with heavy duty aluminum foil on which had cooked 4 veggie corn dogs, out of the 350º oven.

"Tin foil," says the kid, calling it by the incorrect former name, "doesn't conduct heat very well."

I felt the pan. Hot.
I felt the foil. Not hot. At all.
The corn dogs, otoh, were hot.

"Probably just cooled off really fast because it's thin," I say.

"No," says the kid. "It's hot. It just doesn't feel hot because it doesn't conduct heat. Like the special metal alloy NASA uses on the
outside of rockets. It can be burning up in your hand, but you won't feel it because it doesn't conduct heat."

"That makes no sense," I say. "Maybe that kind of metal just burns at a really low temperature."

"No," insists the kid. "It feels cool because it doesn't conduct heat very well."

I remain skeptical. "Let me ask some smart people," I say.

Over to you, smart people...

[dubhlinn]

In my humble opinion, the foil stays cool.

It's the container and the food that ye gotta be wary of.

I speak from experience as opposed to any knowledge of matters scientific.

That's what Oven Gloves were invented for.

Slan,
D.

[jsluder]

Index Key: PHY062
Author: Alex Botvinnik
Subject: Transfer of heat
Text: How come when I put a pizza in the oven, the aluminum foil does not get hot?

Response #: 1 of 1
Author: mooney
Text: It (aluminum foil) DOES get hot. In fact it gets hot much more
quickly than the nearby pizza. It also cools off much more quickly, which is
why we tend to think of it as not getting hot. Here is the deal: At any
given temperature, aluminum holds much less energy than an equivalent mass of
pizza (or an equivalent mass of human flesh--and now we are getting to the
point). When you touch the aluminum foil, your hand and the foil share the
thermal energy in the foil. Your hand says "big deal! It takes MUCH more
energy to raise my temperature because I am made mainly of water" (and water
is phenomenal in its ability to hold energy without increasing temperature
much). However, it is also true that the aluminum foil is much less massive
than your hand, and so part of the answer is that there is just not much foil
there. And its also true that the air cools the foil more quickly than the
pizza since the foil has more surface area in addition to holding less energy
per degree of temperature.

[emmline]

Aha. So I am right, but I still don't know about that NASA stuff.

[herbivore12]

[D'oh! jsluder answered faster and better than I did below. But I'm glad the answer he found validates what I thought the case...]

I don't think the youngling has it quite right; aluminum conducts heat relatively well (which is why it's used in cookware, after all). But that thin layer of foil can't hold very much energy at all, and will cool really fast when brought out of the oven and exposed to air. It will use up the meager amount of energy it can store in heating the air and surroundings, and so will cool. The thicker aluminum sheet, with much more mass, will store much more energy, and so will cool more slowly. Your hand, being made of water and meat, is also hard to heat up, and the energy stored in the thin layer of aluminum foil won't be able to affect it much.

If you wadded that same aluminum foil sheet into a ball and heated it up to 400 degrees and then picked it up, you might well notice it (don't do this!).

[emmline]

Good. I will attempt to explain to him.
But what's the deal with the NASA alloy? Would it actually not feel hot while burning?

[izzarina]

I don't think it truly does conduct heat as well...if I use an aluminum pan and a stainless steel one, the aluminum one tends to cool much more quickly than the stainless one. But with foil, I would imagine the fact that it's so thin does contribute greatly to it's cooling down so quickly. That just seems to make logical sense to me.

[jsluder]

But what's the deal with the NASA alloy? Would it actually not feel hot while burning?

Without knowing what "NASA alloy" he's talking about, it's rather difficult to be specific. However, any material will feel hot while it's burning. It is, after all, burning. You can't feel it without also burning.

For lots of info about heat protection for the space shuttles, see NASA Facts Online: Orbiter Thermal Protection System.

[herbivore12]

Good. I will attempt to explain to him.
But what's the deal with the NASA alloy? Would it actually not feel hot while burning?

I don't think so. Your kid may be confused about insulating/conducting properties. If a spaceship is swathed in metal, it's not because it burns cool, it's because it protects the rest of the ship from heat. For an impressively poor conductor/good insulator, check out aerogels. You could hold a bit of certain aerogels -- which are made almost entirely of air --in your hand and then direct a blowtorch at it, and not feel a thing.

The only way a burning metal could feel cool in your hand, I think, would be if it were a metal that burned at a temperature at or below your body temp. Otherwise, burning -- which is, after all, a release of energy -- would definitely be something you'd notice. Like, a lot.

But my physics is pretty old and now is mostly limited to popular and lay books on the subject, so I could be wrong. Maybe there's some new wonder-metal I've not heard about.

[djm]

If you grabbed hold of the aluminum foil soon enough when it was coming out of the oven, it would still be hot enough to burn your hand, but it loses heat so quickly once removed from the heat source that it may well be too cool to harm you even in less than a minute after being removed from the oven.

djm

[FJohnSharp]

As an experiment, take a piece of foil, fold it in half as many times as you can, then put it in the hot oven for a few minutes. Now put it in your hand. No wait, let the kid do it.

[gonzo914]

And no matter what your kid or NASA or djm says -- DO NOT USE ALUMINUM FOIL AS A POTHOLDER.

[SteveShaw]

Aluminium (note correct spelling, Merkins ) is a very good conductor of heat. A sheet of aluminium foil has a very large surface area compared to its small volume, therefore it will lose heat extremely rapidly from its large surface due to radiation or conduction, once in contact with a cooler atmosphere, and at the same time lack the capacity to hold much heat energy by dint of its small volume. The same principle explains why African elephants have big, thin ears - they use 'em to lose heat as rapidly as possible, just like the sheet of foil. Don't forget that, in the oven, the foil is losing heat at the same rate as it is gaining it, and the large surface area-to-volume ratio ensures that the loss and gain are both very rapid. Take away the heat source (e.g. by taking it out of the oven) and the loss is very rapid. A thick chunk of aluminium has a much smaller SA/Vol ratio and will therefore be far less speedy at radiating heat away - it just doesn't have enough surface. So it stays hotter for longer.

[chas]

We use aluminum foil as an aid for baking vacuum systems. Because the 0.001" thick aluminum foil conducts heat better than the 0.120" stainless steel -- that means that it conducts heat hundreds of times better than stainless. In fact it's one of the best thermal conductors there is.* Also, it doesn't radiate very much (it's shiny), so it only loses heat to the atmosphere conductively. That means that you can put your hand within an inch or so of the baking vacuum system, which might be as hot as 250 C or so, and not feel much heat. Every newbie who comes through does this, thinks the bake must not be on, and burns himself when he touches it "to make sure".

*Any good electrical conductor will be a good thermal conductor (Wiedeman-Franz law). Aluminum is an extremely good electrical conductor, thus is also an extremely good thermal conductor, one of the best after silver and copper.

[Dale]

Many years ago, I saw a NASA guy do a demonstration at a high school. One part of the demo I'll never forget. He had a cube, maybe 2" on a side, made out of the material used to make the tiles on the undersurface of the Space Shuttles. He put bunsen burners on it until it was glowing. Then he turned off the heat, paused for MAYBE 2 seconds, and then picked it up with his bare hands. Cool. Literally and figuratively.