Monday, December 28, 2009

Tongue in Cheek

What a classic holiday image - a child in distress. Part of growing up is learning lessons about the world around you, and this early physics lesson is one that most kids don't forget. It's the principle of thermal conductivity.

The sensation of having your tongue stick to a metal pole sends chills up my spine. Not for the cold or the taste, but the removal. Unless you've got a cup of hot water, it's rather painful to pull your tongue off. It's that or the risk of remaining tethered to a pole. In 8-year-old lore, it's metal poles and mailboxes that you have to watch out for. Frost on wood or rubber doesn't present such a high risk. Why doesn't your tongue freeze to your glove when you eat a bit of snow off of that? Why not?

Quite simply, when you lick cold metal, the water in your tongue freezes to ice and binds you to your temporary prison. Here's why: Heat is sort of a socialist. It wants everyone to be even. It's always trying to get the heat in one place to move over to colder areas and won't rest until they're the same temperature. So eventually the pie, hot when it comes out of the oven, cools off, and the air around it is a bit warmer. If two surfaces touch, the heat wants to flow from the hotter to the cooler object. What prohibits this from making our whole world the same uniform temperature is thermal conductivity (and other stuff if you want to talk global, but we're just talking about tongues and frost here...). The heat from your hands wants to flow out to the cold air - but is stopped by your gloves. Those gloves are good insulators - they don't allow the heat to pass very quickly. Metal, on the other hand, transfers heat very well.

So when you stick your tongue to that metal pole, assuming it's cold enough that the pole is well below freezing, the heat from your tongue is sapped by the metal before your body can replenish it. The water in your tongue freezes, and there you are, stuck like fool. Warm water will unfreeze you, as will saliva if you can get enough of it running before it, too, turns to ice. You can also manage to get a moist hand stuck to a metal pole - but you'll notice it would be almost impossible to get it stuck to a piece of wood or rubber. Your body would heat you up much faster than those materials could suck the heat away. Ice has a thermal conductivity higher than wood but lower than metal - which is why your tongue might stick to a Popsicle, even if it's a warm day.

To test this in a less painful way, try leaving a piece of metal and a piece of wood lying near each other for 24 hours. Put them outside, inside, under your mattress, just a few places with distinctly different temperatures. Once they've sat for a day, long enough, we assume, to become the same temperature as their surroundings, touch them. Which one is colder?

Normally, your immediate sensation will tell you that the metal feels colder. If you have a means of taking the temperature of the wood and the metal before you touch them, do so. Even though the metal feels colder, you'll find that it is usually the same temperature as the wood. But the metal sucks up your hand's heat very fast, making your hand feel cool.

Oh, childhood lessons.

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