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4.12 What is the lowest temperature possible? The Kelvin absolute temperature scale

We know the temperatures of the cores of stars can be millions of degrees, so there doesn't seem to be any upper limit to temperature!

BUT, is there limit at the lower end of the temperature scale?  The answer is YES!

The first experimental evidence for a lower temperature limit came from graphs of volume versus temperature and pressure versus temperature for a fixed mass of gas.

The plots for these were linear until the gas liquefied or the liquid solidified.

BUT, if you extrapolated the gaseous data back, you find every line ended up at a theoretical pressure of zero and temperature of -273^oC.

Explanation ....

As you cool a material, the particles have less and less kinetic energy of movement around (gases or liquids) or vibration in fixed positions (solid).

The kinetic energy of particles is a function of temperature.

You can also say, that what we measure as temperature, is a measure of the average kinetic energy particles have.

BUT, eventually virtually all movement ceases at a temperature of -273^oC, particles have ~zero kinetic energy.

Therefore that's it as regards particle KE, and the temperature as we know it, cannot fall any further - there is no more internal energy to remove!

So, the lowest possible temperature that is -273^oC. (-273 on the Celsius scale, unit ^oC).

Theoretically, at this temperature, the particles have no kinetic energy of movement or vibration, the coldest they can be - nothing left in the kinetic energy store of the particles.

In fact by then, every substance will have solidified, but at -273^oC there is zero vibration of the particles.

In 1848 a Scottish-Irish scientist called William Thompson (later became Lord Kelvin) proposed a new temperature scale starting at zero (called absolute zero), which became known as the 'Kelvin' scale of temperature - unit K.

Therefore the difference between the Celsius and Kelvin scales is 273.

To convert from one to the other, the following simple formulae apply.

K = 273 + ^oC     or     ^oC = K - 273

(absolute zero 0 K is the same temperature as - 273^oC)

Examples of temperature Celsius (^oC) and Kelvin (K) scale conversions:

Freezing point of water = 0^oC, therefore 0 + 273 = 273 K.

Boiling point of water = 100^oC, therefore 100 + 273 = 373 K.

Melting point of pure iron = 1811 K, therefore 1811 - 273 = 1538 ^oC

Note: Do NOT write degrees Kelvin, do NOT write ^oK.

and don't write just C for Celsius, you need the degree symbol ^o too.

INDEX of notes on Particle model theory state changes and latent heat

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Keywords, phrases and learning objectives for particle models and thermal energy

Appreciate from graphical data the evidence for a lower limit to the scale of temperature and this led to the formulation of the Kelvin scale temperature.

Know the lowest possible temperature possible -273^oC (0K on the Kelvin scale).

Know how to convert between celsius and kelvin temperature scales in calculations.

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