OCR A-Level Physics/The Newtonian World/Thermal physics

Units of temperature

Temperature can be measured in using many different units; using degrees Kelvin (K) or degrees Celsius (°C).

Degrees Celsius

The Celsius scale is derived from the temperature of pure melting ice (defined as 0 °C) and the temperature of steam coming from boiling water under atmospheric pressure (defined as 100 °C). One degree Celsius is therefore 1/100 of the difference in temperature between pure melting ice and steam from boiling water.

Degrees Kelvin

The Kelvin scale, also known as the thermodynamic temperature scale, is derived from absolute zero (0 K) and the triple point of water (273.16 °K). A change in temperature of one degree Kelvin is equal to a change in temperature of one degree Celsius.

Absolute zero is the lowest possible temperature a substance can have, where none of the molecules are moving. The kinetic energy of the molecules is therefore equal to zero.

The triple point of water is the temperature and pressure at which the three states of matter of water (ice, water, water vapour) can coexist in thermal equilibrium. This has a temperature of 273.16 K and a pressure of 611.66 Pa.

Converting units of temperature

The temperature of the triple point of water is roughly equal to the temperature of pure melting ice. Therefore 0 °C is roughly equal to 273 K. The equations below can be used to convert between the Celsius and Kelvin scales;

${\textstyle T(K)\approx \theta (C)+273}$

${\textstyle \theta (C)\approx T(K)-273}$

Thermal equilibrium

When a hot object is placed next to a cool object, there is a net transfer of thermal energy from the hotter to the cooler. The temperature of the hotter object will decrease and the temperature of the cooler object will increase until both objects have the same temperature. At this point there is no longer a transfer of heat energy.

This is thermal equilibrium; when there is no net transfer of heat energy between two or more objects.