Cooling trends

In a previous piece about density and the insulating power of whipped cream, I mentioned that trapped "nothingness" was even better at insulating things than the trapped air in the whipped cream.

A hot object can transfer heat to its surroundings by conduction and convection, in other words by having molecules (or atoms) in the surroundings collide with the surface of the object, pick up some of its energy and move off. Imagine a little bucket brigade, stepping up to the object, grabbing a piece of heat (energy, really) and heading off to dump it elsewhere. Air doesn't conduct heat very well, and trapping it reduces convection (the bulk movement of the air - air currents), hence trapped air is an effective insulator.

If you halt the bucket brigade - prevent any molecules from walking off with a chunk of energy, all the energy stays in the object and voila my cocoa stays hot. So if I could envelop my cocoa in a bubble of nothingsness — a vacuum — I could keep it from cooling via conduction and convection. (Of course, I'd have to put a vacuum tight lid on the thing, lest it instantly boil off — but that's another post…) A convenient way to do this is to use a vacuum flask, where a 'layer' of vacuum is enclosed between two walls. Originally glass walls were used. If you're my age, you might remember dropping your lunch box, and then opening your thermos at lunch to find shard of glass inside. These days stainless steel or plastic walls make lugging your milk to school a less risky proposition.

Chemists still use the glass walled version of a thermos. We call it a Dewar (for James Dewar, who invented the contraption in 1892).

But an object untouched by other molecules can still lose heat by radiation, by emitting infrared photons - light at wavelengths longer than visible light. Reflective wrappings, like metalized Mylar, keep the light - and the warmth - inside.

A friend recently wondered why clear weather was cold weather. The earth radiates some of its heat back out into the universe as infrared radiation. Clouds act as insulating wraps for the earth, the water molecules grabbing the heat before it radiates out into space and re-emitting some of it back toward the planet's surface. (This is the greenhouse effect — it's not up for debate.) On a clear night, no clouds, so less heat is retained. Suddenly the temperatures are well below freezing.

(Other molecules besides water help trap infrared radiation within the atmosphere, including carbon dioxide and methane.)

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Photo is from Wikimedia.