*01 Forces&Energy (8-45).qxd
12/11/08
2:10 PM
Page 26
26 FORCES AND ENERGY
Temperature scales
Key words absolute zero Celsius Fahrenheit Kelvin scale
1 Fahrenheit
2 Celsius
3 Kelvin
°F
°C
K
212 °F
100 °C
373.16 K
Boiling point
1 Fahrenheit Fahrenheit was devised at the start of the eighteen century. On this scale the ice point is 32˚ and the steam point 212˚. These figures arose because Fahrenheit did not use the ice point and steam fixed points but chose an unspecified ice/salt mixture for the lower fixed point, to which he gave the value 0˚, and human body temperature which was given the value 96˚. ● The Fahrenheit scale is still sometimes used in the context of weather but is only of historical interest in science. ● The
Freezing point of water 32 °F
0 °C
273.16 K
–459.67 °F
–273 °C
0
2 Celsius ● In
1742 the Swedish astronomer Anders Celsius proposed a temperature scale in which ice melted at 0˚ and water boiled at 100˚. The Celsius scale is widely used, sometimes being referred to as the centigrade scale since there are 100 degrees between the fixed points. Temperatures on this scale are given in ‘degrees Celsius’, ˚C. ● One disadvantage of the Celsius scale is that temperatures below the freezing point of ice are negative.
3 Kelvin
© Diagram Visual Information Ltd.
● In
1848 the physicist William Thomson (later Lord Kelvin) suggested a temperature scale which started at the lowest theoretically possible temperature, absolute zero. This is known as the absolute or Kelvin temperature scale. Degrees on this scale are called kelvins and are denoted by K (not ˚K). A kelvin is exactly the same size as a Celsius degree i.e. 1 K = 1 ˚C.
Fahrenheit to Celsius and vice versa 5 °C = (°F –32) × — 9
9 + 32 °F = (°C × —) 5
Kelvin to Celsius 1 K = 1 °C
°C = K –273
Absolute zero