Sunday, 28 December 2008

Very Cold December

It’s been the coldest start to December since 1976. Very cold Decembers are not uncommon but a little unusual as our coldest months in the UK are usually January and February. I was talking about the cold weather to some friends over Christmas and we got on to talking about why we can feel hot or cold. I don’t mean that the heating isn’t on or there’s a freezing wind outside, I mean where the sensation of heat actually comes from.

Well, something feels hot to us because the atoms inside it are moving fast, and when it feels cold they are moving slowly. We interpret this movement through the feeling of something being hot or cold, and a thermometer interprets as a certain temperature, in for example degrees Centigrade of Fahrenheit.

Scientists often use Kelvin as a measure of temperature rather than deg C or deg F. The bottom of the Kelvin scale is 0 K or Absolute Zero. What got me thinking about this recently was my God Son who asked me what is Absolute Zero. I was able to give the temperature 0 K or -273.15°C, but not the definition. It’s one of those things that I am sure I did know and could make a reasonable guess of what I thought it was, but to be precise I looked it up for myself. And it is as you might guess a point at which no more heat exists.

What that means specifically in classical kinetic theory is that there should be no movement of individual molecules at Absolute Zero. In practice we don’t of course know this as we have never been able to observe anything at 0 K, but we have come close in the laboratory. The experimental evidence shows us that as we reduce temperatures close to 0 K molecules do slow right down, but there is still movement.

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