| MadSci Network: Physics |
Hello Michael It depends on the temperature. Transparent plastic-wrap (clingfilm, saranwrap) allows light through but is opaque to heat. Aluminum is opaque to everything below ultraviolet because it REFLECTS it. OK so we have a thing wrapped. It is at temperature t and is in a room whose temperature is T. Heat is lost if t is greater than T. It is gained if t is less than T. The rate of heating or cooling is proportional to T-t. See Newton's law of Cooling in Google and Wikipedia. Newton's law is true enough even for radiation (heat and light). For things that are red hot (t or T) you need Stephan's Law (see Google). The point about aluminum is it is an excellent CONDUCTOR of heat, so the outside of the aluminum film IS t, the same as the outside temperature of the wrapped-up thing. So it is a very POOR blanket (insulator). But it is an EXCELLENT reflector. So heat cannot radiate THRU it - not either way! And excellent reflectors are very BAD at emiting heat waves. That is something Einstein used and is part of the story of the invention of the laser. Plastic wrap is the opposite - a nice warm insulating blanket but a GOOD radiator AND absorber of heat. In other words what happens IN THE END is the same, whatever your wrapping. The temperatures t and T become the same. The wrapping only fixes HOW LONG this takes. So to keep things cold the very best of all is a multilayer arrangement of aluminum first (to keep the heat out) and then styrofoam (blanket) and then as a final layer aluminum to keep the heat out. If you do that you have invented "the vacuum flask" or "Thermos Flask", known in US as a "Dewar flask" (see Google). You will find previous answers to questions like yours which you will find under "Aluminum plastic colder" using our search engine. John
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