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Q & A: is aluminium foil an insulator or a conductor?

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Most recent answer: 01/27/2016
Q:
is aluminium foil an insulator?? i read on internet that aluminium foil is conductor but how?? if we pack a hot food in aluminium foil, it dosent allow the food heat to escape, i.e., its an insulator.... but it is used in electric circuits also.... so is the aluminium foil an insulator or a conductor??????
- tinkle (age 13)
india
A:

If you have a cheap Digital Voltmeter (DVM) you can do what I just did- measure the resistance between contacts to different parts of a piece of aluminum foil. It conducts very well. If there's any insulating polymer or oxide on it, the contacts I used easily broke through that to the conducting aluminum. 

As you say, since Al conducts electricity well it will also conduct heat well. So why doesn't a samosa wrapped in aluminum foil cool down quickly? Think of where the foil goes. It just wraps around the samosa, it doesn't reach away from it. So it doesn't have a way to conduct heat far away.  The foil doesn't much help to insulate the food from being cooled by the air, but it does help in another way. One main way that typical hot food cools is by evaporating water. The foil keeps the water vapor from escaping quickly, slowing down the evaporation and thus slowing down the cooling.

Mike W.


(published on 01/27/2016)

Follow-Up #1: electrical and heat conduction

Q:
I think the questioner is mixing up the two meanings of the word conductor, i.e., electric conductor vs. heat conductor. Al is an electric conductor, in fact, some wires are made of Al. Its non-conductivity of heat, i.e., its insulation is due to two reasons: One is, as Mike W. said, the tight wrapping of Al around the food stops convective heat transfer, i.e., it stops the flow of hot air from inside the foil to outside. The other reason is that Al's reflective (mirror-like) surface reflects the radiation (heat) back to inside. In fact, Al blankets work this way. See Space blanket - Wikipedia. Heat also may transfer by conduction. In this regard, Al is a good conductor, i.e., a bad insulator of heat -- touch an aluminum window frame in winter and you will feel cold. As to why a good conductor of electricity is also a good conductor of heat, see: https://van.physics.illinois.edu/qa/listing.php?id=1854 ((((OR)))) https://van.physics.illinois.edu/qa/listing.php?id=1818 ((((OR)))) http://van.physics.illinois.edu/QA/listing.php?id=1305
- Mehran (age 65)
Arlington Heights, IL
A:

Hi Mehran- Tinkle was right to assume that good electrical conductors are also good heat conductors, as described by the Wiedemann-Franz law. You raise some other good points, though.

That same electrical conductivity makes the aluminum highly reflective. As you say, that reduces the loss of heat by electromagnetic radiation. I'd forgotten to include that. 

As far as the convection goes, since the foil gets almost as hot as the food, and air can convect past the foil, I don't think the foil reduces that heat loss much. 

Still, in between reducing evaporation and reducing radiation, the foil should help noticeably.

Mike W.


(published on 01/27/2016)

Follow-up on this answer.