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Why does aluminum foil absorb heat?

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2009-09-05 13:26:19

== Aluminum foil absorbs heat because the foil, like anything

else, will absorb energy from a heat source according to

thermodynamic principles. What that translates into is this: heat

goes from where it is to where it isn't, or heat goes from where

it's hot to where it's not. There are three ways for that to

happen. One is convection, one is radiation and the third is

conduction. If foil is exposed to flame, the hot combustion gases

directly heat the foil when they come in contact with it. If the

foil is sitting on the top of the range when the oven is on, the

oven heats the air around it and convection currents are set. These

convection currents take heat away from the oven and transfer it to

things which the hot air passes over. Anyone who has had a hair

dryer pointed at them knows what this is like. The skin reacts to

the hot air. We feel it. Convection. In a situation where the foil

is exposed to radiant energy, infrared radiation, it will react to

the incoming radiation by absorbing some of it. Radiation happens

because something that is hot is generating heat radiation

(electromagnetic waves that are below the optical or visible

spectrum). And the foil catches some of them without reflecting

them. It absorbs the energy and gets warmer. The best "life

experience" example of this kind of radiation may be something

familiar. If we suffer a sunburn on our face, the skin is

sensitized to heat by the trauma. If we wander into the kitchen

where things are cooking on the stove top and look at the stove

from fairly close, we might notice a bit of pain where we were

burned. That's direct heat (infrared) radiation striking the skin

of the face. Another example might be looking into a fireplace

through a glass screen. If the fire is hot and we are close enough,

we can feel the heat on our face because a bunch of infrared

radiation makes a direct route from the fire through the glass and

onto our skin. Conduction is the transfer of heat in matter that is

touching; for example, from one end of a metal bar to the other

end, or from a metal bar that is touching another metal bar.


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