Posted by: Kash Farooq | December 5, 2012

## Charles’ Law and Absolute Zero

Blogging while studying S207 – The Physical World.

In the 1780s, by experimentation and measurement, Jacques Charles discovered the following:

If a fixed mass of gas at constant pressure is heated, the volume increases and this volume increase is proportional to the increase in temperature.

i.e. the gas expands as the temperature rises and there is a linear relationship: Charles’ Law shown for three different pressures: at a fixed pressure, there is a linear relationship between volume of a gas and temperature.

So the above graph shows that if you increase the temperature of the gas, it’s volume rises. But what happens if you lower the temperature? The graph would suggest that the volume decreases. Let’s extend the graph: Charles’ Law: on extrapolating the lines into negative temperatures, we find they all converge at -273.15 Celsius.

Amazingly the lines of the graph all converged on a single point. As the volume of the gas cannot be negative, we can’t go beyond this point. The graph suggests that Charles’ Law does not work at temperatures lower than this. We now know the reason for this: the lines converged on absolute zero and there are no temperatures below this temperature!

That’s worth repeating: there are no temperatures below -273.15°C. There is something fundamental in the laws of physics that means this temperature is a lower limit.

The closest we have gotten to this temperature, i.e. the current world record, was set in 1999. Researchers of the Low Temperature Laboratory of Helsinki University managed to get to 100 picokelvins (0.000 000 000 1 of a kelvin).

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