|Why does aluminum foil not get hot when it is in
a hot oven?
|Question Date: 2004-10-22|
Actually, it does get hot. In fact, if
you wrap a potato in aluminum foil and put it in a
hot over, the foil will get hot first. This is
because metals like aluminum are very good
conductors of heat, so they absorb heat very
quickly. When you pull out the potato, the same
property of being a good heat conductor makes sure
that the foil cools down very fast.
It does get hot, but since it is so
thin, and an excellent thermal conductor
(being metal), it radiates/conducts away all of
its heat so rapidly that it cools off much faster
than anything else.
Actually, aluminum foil does get hot
when it is in the oven. For example, if you wrap
up a baked potato in foil and cook it in the oven,
the foil will be too hot too touch. What you
might be thinking about is, for example, laying a
piece of foil in the oven (like when people make a
"tent" over their turkey). If you reached into
the oven, you could probably touch the foil
without burning yourself. The aluminum foil has
a very small mass and holds a relatively small
amount of thermal energy. When you touch the
foil, this thermal energy is quickly dissipated
into your hand, which has much more mass. Plus,
your hand is mostly made up of water, and water
has a very high heat capacity (meaning it
takes a lot of heat to raise the temperature of
If you want to think about it more
quantitatively, the thermal energy being
transferred goes as (heat capacity) x (mass)
x (temperature change).
If you touch the foil, let's assume the case
where all the energy transferred from the foil is
transferred to your hand, so that:
(heat capacity of foil) x (mass
of foil) x (temp change of foil)
(heat capacity of hand) x (mass of hand) x
(temp change of hand).
Now, since as we said, the heat capacity and
the mass of foil are both small, this
means that the foil will change temperature by a
lot. On the other side, the heat capacity and
mass of your hand are both large, so the
temperature change of your hand will be smaller.
So, while the foil is initially hot, it will
cool very quickly and your hand will not "warm up"
by that much.
Since aluminum foil has a lot of surface area
and is very thin and heat travels within
aluminum very well, it is going to cool off
very quickly in air when you remove it from the
oven. In addition, since thin foil doesn't weigh
much and thus can't hold much "heat," when you
touch it not much heat can transfer to your
fingers and thus it doesn't feel particularly
warm. Here is a question for you:
Let's say that you are about to jump into a
swimming pool. Both the air temperature and water
temperature is 60 degrees F. When do you think
you will feel cooler - out in the air or after you
jump into the water? Why? Which do you think is
better at transferring heat - air or water?
Hmmmm. I don't know what kind of fancy
aluminum foil you've been using, Nicole, but
aluminum foil gets really hot in my oven. To
understand why this happens, it's good to know
what "temperature" and "heat" really mean.
Everything is made of molecules and atoms, and
all atoms are made of a very heavy nucleus with
a bunch of small electrons orbiting around it like
moons. The temperature of an object tells you
how fast the molecules in that object are moving.
When something gets really hot, its
molecules get moving around really fast. When
you heat up a liquid (like water), the liquid
molecules start moving so fast that they stop
sticking together, and they change from a liquid
into a gas (this is what happens when water
boils). The same thing happens when you heat up
a solid (like ice) -- the molecules stop sticking
together, and the solid melts into a liquid. Most
of the things you use every day will melt inside
an oven -- think of ice, or glass, or plastic
things (please don't try to experiment with
melting things in your oven!). That's because the
molecules making up those things are held together
But metals, like aluminum foil, are different.
Instead of being made of lots of individual
molecules, atoms are one big block of nuclei that
all share electrons with each other. You can
think of it like this: most materials (like
plastic) are like a bunch of grapes -- each
nucleus is weakly attached to the others by a tiny
little stem. It's easy to cut the stem or
knock off a few grapes if you bump into the bunch,
and if you shake the bunch, all the grapes move
around in different ways. But the nuclei in
metal are more like the grapes in a Jell-O fruit
cocktail. They're all linked together by a goop
of electrons (the Jell-O), it's tough to get just
one grape out without pushing a bunch of the jello
around and making a big mess, and if you shake the
jello, all the grapes bounce in the same way.
So when you heat up a metal, the goop of
electrons (physicists call it the 'sea' of
electrons) can start getting hot and moving
really fast, but all the nuclei stay in the same
place and just pass the electrons back and forth
amongst themselves. So a metal can get really,
really hot before it gets so hot that the nuclei
stop sticking together. That's why you
can put your aluminum foil in the oven and it will
get hot, but you won't be able to turn up the heat
enough to melt it.
Click Here to return to the search form.
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.