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Temperature is not Heat

First things first. PhysicsStop is back on-line after an enjoyable two-week break in warm and sunny southern England.

Second things second. What advice can anyone give to the parents of a fourteen-month-old with jetlag who insists that 4 am is time to get up, have breakfast, and feed the chickens (or the "Choo Chuk" as he calls them)? In the end we got up, had breakfast and let the chickens and the neighbours get some sleep. Consequently I'm rather tired and feel like it's ten past two, not ten past eleven. 

So, here's a bit of quick physics from our travels. If you're fortunate enough to fly with an airline that still gives out hot towels to its economy class passengers, you'll know that the towels can be extremely hot - almost untouchable at first. But, once you've unrolled them and used them for whatever purpose you can think of, they cool down very rapidly. They don't stay hot for long, and you hand them back cold.

In physics terms, the towel, as it is presented to you, has a high temperature, but it doesn't have a great deal of heat. While we can use these words loosely and almost synonomously in everyday conversation about the weather, in physics they are very distinct quantities. Heat is a measure of the thermal energy in something. It's measured in joules, just like any other form of energy. The energy resides in the thermal vibrations of the water molecules. Heat is an extrinsic quantity. If one doubled the amount of the material (had a towel twice as big), one would have twice the amount of heat. 

Temperature is a lot harder to define in simple terms. (Try making sense of the Wikipedia entry on it). There's a nice physical definition (rate of increase of energy with respect to entropy) but that's not terribly intuitive. It's easier to think of temperature as an 'average' thing - broadly speaking temperature is proportional to the average kinetic energy per atom in the material. Each atom in something that's hot will have more kinetic (movement) energy than something that's cold. Any physicists reading this will realise I've given a horribly simplistic definition, but it is roughly correct. A key thing is that temperature is an intrinsic quantity - if you double the amount you have (a towel twice the size), the temperature (average energy per atom) stays the same.

Our towel starts at a high temperature. However, because it is thin, there isn't actually a lot of heat in it. That means that it quickly loses what heat it had once unrolled, and the temperature, which is what you perceive on your skin, drops.  Contrast this to a lump of rock that's been sitting in your oven at 70 degrees for five hours. Pick that out and see how long you can hold it for (actually, don't try it). It contains far more heat than a hot towel, so it takes much, much longer for it to be lost.

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