|MadSci Network: Physics|
Greetings, Spencer: I think I have to start this Answer by taking issue with your statement about "wrapping up a hot potato". It is my understanding that the aluminum foil is wrapped around the potato before it is cooked, not afterward. The reason is that the foil, like the nail you described, helps the potato to cook faster. And now that I've probably confused you even more, I need to review the three different ways that heat energy is transported from Point A to Point B. First is conduction, about which you seem to be comfortably knowledgeable. Each hot molecule near A bumps into a cooler molecule located slightly closer to B, transfering some kinetic/heat energy. For as long as heat pours out of A (the heating element in an oven, for example), the molecules near A can continue to transfer heat toward B (the potato, of course), by simply bumping other molecules. Second is convection, which requires the molecules near A to physically move toward B. They generally acquire heat at A by conduction, and deposit heat at B by conduction, but there is very little wastage of time and energy, heating up lots of other molecules. A good example of natural convection in action can be found on a cold day, when a house is warm and the windows are single panes of glass. In this case the whole face of the window conducts heat from inside the house to outside -- but if that was all, the heating costs for that house would be quite less than is actually the case. See, as air molecules inside the house next to the window are cooled, they tend to sink; as air molecules outside the house next to the window are warmed, they tend to rise. Well, if those molecules move to become no-longer-adjacent to the window, what takes their place? Inside the house, warm air flows toward the top of the window; outside the house, cold air flows toward the bottom of the window. Thus, inside the house, we can imagine a loop made of moving air: At the top of the window the warm air begins conducting heat into the glass, and being cooled by that, so it sinks -- and encounters more cool glass, so conducts some more heat into the glass, and sinks some more.... At the bottom of the window the quite cool air now flows across the floor of the house, where the heating system may warm it up again, causing it to rise, and eventually flow back toward the window. This is the essence of the convective process. Meanwhile, outside the house, cold air at the bottom of the window has some heat conducted into it, so it rises a little, encounterings glass that is a bit warmer, so even more heat can be conducted into it, so it rises some more.... At the top of the window it just keeps on rising, encountering a sky full of cold air. And there is plenty more cold air to move toward the bottom of the window. The total amount of heat that can be lost from any house in this fashion is expensive! Anyone interested in cutting down on heating costs should be willing to do what they can to stop the convective flows. Inside the house curtains can be placed snugly against the wall surrounding the window, keeping air from FLOWING toward and away from the glass. Outside the house shutters might be installed, for much the same reason. One could also invest in double-paned windows -- the gap between the two panes is too small to allow a convective loop to exist, and so heat must waste time merely conducting through the air (a relatively poor conductor it is) separating the panes. If you have ever seen a "convection oven", you might have noticed that it is generally smaller than the average kitchen stove. It features a fan to push hot air from the heating element toward the potato, and cooks it faster because very hot air begins arriving at the food rather soon after the oven is turned on. Forced convection is an extremely efficient way to move heat from Point A to Point B. Most restaurants use convection ovens these days, because shorter cooking time is almost always a benefit to the customers, and energy costs are lower, too. The third and final heat-transference mechanism is radiation. Every type of substance is able to absorb and emit photons of pure energy. Some substances are more efficient at emitting; some are more efficient absorbing. The glow you see coming from any heating element is pure energy that, simply because it went straight from the heating element to your eyeballs, isn't helping to cook any food. But no matter how much glow you actually see, you should be aware that the spectrum of radiant energy includes a great deal that you cannot see at all -- in fact that heating element is radiating far more invisible infrared energy than it is radiating visible energy. Meanwhile, most ovens are made of metal these days, and metal is a good reflector of radiant heat energy. This means that photons can bounce around inside the oven until they get absorbed by something -- usually the food. And now to tackle the question of how a wrapping of aluminum foil helps a potato to cook faster. As you probably know, aluminum is a pretty good reflector of radiant energy, and so it certainly does not help to cook the potato by reflecting photons emitted by the heating element. However, those photons eventually all get absorbed somewhere inside the oven, usually by the very air. Hotter air means that it can bounce all the better off the aluminum foil, transferring heat by conduction. And as you already noted, the aluminum will do a good job of conducting the heat toward the potato. So the potato heats up. What happens then? It starts to radiate photons! Which cannot escape, because of the layer of aluminum foil! Instead the photons bounce back and get re-absorbed by the potato, even as more heat is conducted to it from the air in the oven by that same layer of foil. Thus the potato cooks faster, because it cannot leak very much of the heat that is getting conducted into it. Finally, when the fully-cooked potato is removed from the oven, the layer of foil continues to prevent the escape of heat-energy photons. It is to be acknowledged that the foil can conduct heat from the skin of the potato to the exterior air, but inside the potato, heat must transfer to the skin before the aluminum can help. This cannot happen via convection, because the potato is a reasonably solid object, and convection only works in gases and liquids. It certainly happens via conduction, but the body of a potato is a poor conductor. Finally, heat can radiate from the interior of the potato. Much of that will probably be absorbed by the body of the potato before it reaches the skin, but some will make it that far -- only to be bounced back into the body of the potato by the aluminum foil. Therefore, because conduction operates poorly INSIDE the potato, convection can only exist outside the potato, and radiation is foiled, that is why the potato stays hot.
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