Temperature

In thermodynamics, temperature is a measure of the tendency of an object or system to spontaneously give up energy. Temperature is a physical property of a system that underlies the common notions of "hot" and "cold", in which something that is hotter has the greater temperature. Temperature arises from the random microscopic motions of matter, where temperature is related to the average energy of these microscopic motions. The concept of temperature, defined as a tension associated with entropy, follows from the zeroth law of thermodynamics.

Temperature is a measure of the average energy contained in the microscopic degrees of freedom of a system. For example, in an ideal gas, the relevant degrees of freedom are translational, rotational, and vibrational motion of the individual molecules. In this case, temperature is proportional to the mean kinetic energy of the constituent atoms. But in more complicated systems, magnetic, electronic, photonic, or other exotic degrees of freedom can play a significant role in determining temperature.

Thermal motion is the reason gasses have pressure, since the particles in the gas collide with the walls of the container and exert an outward force. Although very specialized laboratory equipment is required to directly detect thermal motions, thermal collisions by atoms or molecules with small particles suspended in a fluid produces Brownian motion that can be seen with an ordinary microscope. The thermal motions of atoms are very fast and temperatures close to absolute zero are required to directly observe them. For instance, when scientists at the NIST achieved a record-setting cold temperature of 700 nK (billionths of a kelvin) in 1994, they used optical lattice laser equipment to adiabatically cool caesium atoms. They then turned off the entrapment lasers and directly measured atom velocities of 7 mm per second in order to calculate their temperature.

Molecules, such as O2, have more degrees of freedom than single atoms: they can have rotational and vibrational motions as well as translational motion. An increase in temperature will cause the average translational energy to increase. It will also cause the energy associated with vibrational and rotational modes to increase. Thus a diatomic gas, with extra degrees of freedom like rotation and vibration, will require a higher energy input to change the temperature by a certain amount, i.e. it will have a higher heat capacity than a monatomic gas.

The process of cooling involves removing energy from a system. When there is no more energy able to be removed, the system is said to be at absolute zero, which is the point on the thermodynamic (absolute) temperature scale where all kinetic motion in the particles comprising matter ceases and they are at complete rest in the "classic" (non-quantum mechanical) sense. By definition, absolute zero is a temperature of precisely 0 kelvins (-273.15°C or -459.67°F).