SI Units Explained with Worked Examples





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Unlike some of the other SI units temperature is one of those things that we instinctively have an understanding for. Temperature sensitive nerve endings in our skin send signals to the brain to tell us when we are too hot or too cold. But how cold is "cold" and is it possible to lower temperatures indefinitely?

We are all affected by a property of the Universe called temperature

The two most widely used measurements of temperature are Fahrenheit and Celsius. The first was proposed in 1724 and is still the official scale in the United States. On this scale the freezing point of water is 32 degrees Fahrenheit (F) and the boiling point 212 F. The other is called the Celsius scale which is also defined by the freezing and boiling points of water. In this scale water freezes at close to 0 degrees Celsius (C) and boils at close to 100 C. The reason I say "close to" is because the modern definition of Celsius is quite complex, but we don't need to worry about it here. For our purposes we can say that water freezes at 0 C and boils at 100 C.

These two scales work very well on a everyday basis. We know that it's cold when the temperature is 5 C (41 F) and that it's hot when the temperature is 30 C (86 F). However, temperature is unusual in that it has an absolute starting point.

If we consider the SI units for length the starting and end points are not fixed. A two metre ruler can be placed anywhere and it will still measure two metres. Temperature is different and has a fixed starting point called absolute zero, which is defined as zero kelvin (0 K) and is the equivalent of -273.15 C (-459.76 F).

Nothing, no matter how much it's cooled, can be as cold or colder than this temperature. This is because temperature is a consequence of the movement of particles - the faster a particle moves the hotter it is, and the slower a particle moves the colder it is, and it's not possible for a particle to be absolutely still. Particles, all particles, such as atoms, electrons and so on, would cease to move at all at absolute zero and so it's not possible to lower their temperature any further.

The kelvin scale has the same graduation scale as Celsius. That is, a temperature difference of 10 degrees on the kelvin scale is the same as a temperature difference of 10 degrees on the Celsius scale. To convert from one to the other we simply add or subtract plus or minus 273.15. For example, 30 degrees C is:

30 + 273.15 = 303.15 kelvin

Note that it's incorrect to say "degrees kelvin" and the unit is just K, not K.

Here are some examples of temperatures and their equivalents in other scales (note that beyond a certain point the percentage difference is so small that it makes sense to simply consider kelvin and C as being equivalent):

Example kelvin C / F
Absolute zero 0 K -273.15 C / -459.76 F
Boiling point of water 373.1339 K 99.9839 C / 212 F
Incandescent lamp 2500 K 2,000 C / 3632 F
The Sun's surface 5,778 K 5,505 C / 9941 F
A lightning bolt 28,000 K 28,000 C / 50,000 F
The Sun's core 16 million K 16 million C
An H bomb's core 350 million K 350 million C
Particle collisions in experimental nuclear colliders 1 - 10 trillion K 1 - 10 trillion C
The Universe 5.391 x 10-44 seconds after the Big Bang 1.417 x 1032 K 1.417 x 1032 C

Finally, temperature sensitive nerve endings were mentioned at the start of the page. Here's a fun little experiment to try. Gently touch different spots on the back of your hand using the tip of a pencil or a fork's prong. Most of the time you will only feel a little pressure but every so often you will touch a temperature sensing nerve-end which will feel noticeably cold.
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