Temperature

Heat refers to the total amount of heat energy in a substance - two liters of boiling water have more heat energy than one liter. Temperature, on the other hand, is a relative term, and refers to the average kinetic energy of the atoms and molecules in a substance. One fundamental property of temperature is that

There are three main temperature scales used in the world - Celsius, Fahrenheit, and Kelvin. These are compared in the following table.

 

Table 8.1: Temperature scale comparisons

	oC	K	oF
Water boils	100	373	212
Water freezes	0	273	32
Absolute zero	-273	0	-459

One can see from this that the size of 1 ^oC and 1 K is the same, while both differ from the size of 1 ^oF; in fact, the difference between the Celsius scale and the Kelvin scale is simply a shift by 273 ^o. The Kelvin scale is more convenient to measure substances with very low temperatures. Moreover 0 Kelvin has a very deep physical significance: absolute zero, as the name suggests is the lowest temperature that can, even in principle, be achieved in Nature. It is the temperature associated with empty space that is completely devoid of all all motion and/or energy. In practice it is impossible to obtain, although one can get arbitrarily close. As we will see in the chapter on cosmology, not even the empty space between distance galaxies is at absolute zero: it contains energy and an associated temperature of about 2.7 Kelvin.