|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 water).
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)
capacity of hand) x (mass of hand) x (temp change
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
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.
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 pretty weakly.
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.
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.
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