We often associate temperature with how hot or cold something is. If we see a piece of metal glowing red, that would be hot to the touch. A ground covered with snow would be cold to touch. This is only part of the story.

In physics, temperature is the measure of how internal particles move.
 

The image above shows an example of this. If this image represents a gas, the particles that constitute the gas would move depending on temperature. In a hot gas, the molecules (represented by the dots) are free to move - the faster they move, the hotter the gas. Alternately, a cold gas would equal slow or very minimal movement of molecules.

Absolute zero is defined as molecules in a medium that are not moving at all. In the Celsius temperature scale, absolute zero is -273.2 degrees C.

In an effort to standardize the temperature scale, Kelvin's are preferably used as 0 (zero) on the Kelvin scale is absolute zero. As such:

Celsius has by no means been abandoned. Much of our standards are based on the Celsius scale:

• 0 degree Celsius       =   Freezing point of water

• 100 degrees Celsius  =   Boiling point of water

The Fahrenheit scale is used prominently in the United States, and mostly for weather and cooking measurements. This scale is not at all used in science.

• 32 degrees Fahrenheit    =   Freezing point of water

• 212 degrees Fahrenheit  =   Boiling point of water

The temperatures above do not follow a nice neat pattern like the Celsius scale, but of special note:

-40 degrees Fahrenheit = -40 degrees Celsius

To convert Fahrenheit to Celsius:

Other important notes:

The symbol for degrees (^o) or the word degree is NOT used on the Kelvin scale:

Absolute zero = 0 K

The symbol for degrees or the word degree IS to be used for Celsius or Fahrenheit temperature scales.

In science, DO convert Fahrenheit to Celsius and avoid using Fahrenheit altogether.

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