Kelvin is the SI base unit of temperature. It is given the symbol . Since temperature measures the energy of atoms and molecules, a true temperature scale must be a positive scale since atoms cannot have a negative energy. On the Kelvin scale, 0 Kelvin is the temperature of absolute zero. Absolute zero is the temperature in a system where all particles stop moving (ignoring some important quantum mechanics). A degree Kelvin and a degree Celsius are the same unit, as they have the same "spacing" between each degree, but the scales have different zero points (see converter below).
Kelvin is usually avoided for everyday use, instead it is always used for temperatures in physics (and also often in chemistry), in equations such as the ideal gas law. If a gas had a temperature of -52.15°C (220 K) the negative number would make no sense in the ideal gas law.
Temperature is a measure of the kinetic energy of molecules in a system, and using Kelvin units are essential to explaining this concept clearly. Temperature can be related to energy through a value known as the Boltzmann constant. This constant is derived from the ideal gas law, given by the equation where is the Boltzmann constant, J/K.