Thermal physics

Temperature and its scales

Temperature indicates thermal state; Celsius, kelvin, and Fahrenheit scales are related through fixed points.

Temperature indicates how hot or cold something is and sets the direction of spontaneous heat flow: from higher to lower temperature.

Reference points and conversion

At 1 atm, pure water freezes at 0 °C = 273.15 K = 32 °F and boils at 100 °C = 373.15 K = 212 °F. A negative Celsius temperature can still correspond to a positive kelvin temperature.

Quick check

A temperature scale assigns numbers to reproducible thermal states. To convert Celsius to kelvins, add 273.15; a temperature interval, however, has the same numerical value on both scales. Fahrenheit has a different interval: a change of 5 °C corresponds to 9 °F. When reading a thermometer, allow it to reach thermal equilibrium with the object and distinguish temperature from heat: temperature is a state variable, whereas heat is energy transferred because of a temperature difference. Water's freezing and boiling points are reference values only near standard atmospheric pressure; at high altitude, lower pressure makes water boil at a lower temperature. Physics equations use kelvins because absolute zero provides a natural zero for thermodynamic temperature, not because matter literally becomes perfectly motionless there.

A good measurement records both the number and the scale, such as 20 °C or 293.15 K; numbers cannot be compared meaningfully when the scale is omitted. In Celsius, 20 °C does not mean twice the temperature of 10 °C; only ratios of absolute temperatures are meaningful in gas laws. Outdoors, keep a thermometer out of direct sunlight when measuring air temperature, since absorbed radiation can make the instrument warmer than the surrounding air.