Temperature Scales

Celsius Scale (symbol: °C)

The degree Celsius is a unit of temperature on the Celsius scale.

This temperature scale was originally known as the centigrade scale as it corresponded to dividing the temperature difference between the freezing and boiling points of water by 100 and calling each 'one degree centigrade' - it comes from the Latin 'centum', which means 100, and 'gradus', which means steps. In 1948 it was renamed after the Swedish astronomer Anders Celsius (1701–1744).

The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty.

The Celsius scale is based on 0 °C at the freezing triple point of pure water and 100 °C for the boiling point of water at 1 atm pressure.

Fahrenheit Scale (symbol: °F)

The Fahrenheit scale is based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736) - he invented the mercury-in-glass thermometer - the first widely used, accurate, practical thermometer.

Fahrenheit was the first standardized temperature scale to be widely used and until the UK joined the Common Market in the 1970s it was used in the UK. Its worldwide use is now limited - the Celisius scale being used in science but it is still the official temperature scale in the United States.

It uses the 'degree Fahrenheit' (symbol: °F) as the unit.

There are several accounts of how he originally defined his scale, but the original paper suggests the lower defining point, 0 °F, was established as the freezing temperature of a solution of brine made from a mixture of water, ice, and ammonium chloride (a salt).

The other limit established was his best estimate of the average human body temperature (set at 96 °F; about 2.6 °F less than the modern value due to a later redefinition of the scale). However, he noted a middle point of 32 °F, to be set to the temperature of ice water.

The Fahrenheit scale is now usually defined by two fixed points: the temperature at which pure water freezes into ice is defined as 32 °F and the boiling point of water is defined to be 212 °F, both at sea level and under standard atmospheric pressure (a 180 °F separation).

To convert Fahrenheit to Celsius

100 ^oC = (212 - 32) ^oF

Δ100 ^oC = Δ180 ^oF

Δ10 ^oC = Δ18 ^oF

Δ5 ^oC = Δ9 ^oF

Δ ^oC = Δ⁹/₅^oF and

Δ ^oF = Δ⁵/₉ ^oC

212^oF = 100^oC and

32^oF = 0^oC

T_C^oC = 5/9 (T_F- 32) ^oF

T_F^oF = ( 9/5T_C+ 32) ^oC

The Kelvin Scale

The Kelvin temperature scale is an 'absolute temperature scale'.

It is defined using the third law of thermodynamics. Because it is an absolute scale, temperatures recorded in Kelvin do not have 'degrees' - the symbol is K NOT ^oK...

The zero point of the Kelvin scale is Absolute Zero, which is the temperature at which particles have the absolute minimum kinetic energy and therefore cannot get any 'colder'.

Each unit (a degree, in other scales) is 1 part in 273.16 parts of the difference between absolute zero and the triple point of water. This is the same size unit as the Celsius degree.

To convert Kelvin to Celsius you subtract 273.16 (Although in examination questions you simply subtract 273 most of the time )