Temperatuuri

Temperatuuri is a physical quantity that expresses the degree to which a system's thermal energy is manifested as hot or cold. It is a scalar property that helps characterize the state of matter and its energy distribution, and it is independent of the amount of substance in most practical senses. In physics, temperature is related to the average kinetic energy of particles in a substance and to the direction of heat flow: heat transfers from higher to lower temperature bodies until equilibrium is achieved. Temperature is measured with thermometers.

Common scales include Kelvin, Celsius, and Fahrenheit. The Kelvin scale is absolute, beginning at zero, known as absolute zero. The Celsius and Fahrenheit scales are defined by fixed reference points, such as the freezing and boiling points of water for the Celsius scale. Conversions between scales are linear: T_C = T_K − 273.15 and T_F = 9/5 T_C + 32. Instruments include liquid-in-glass thermometers, RTDs, thermocouples, and infrared sensors. Calibration and traceability ensure comparability across devices.

Applications of temperatuuri span meteorology, engineering, medicine, food safety, and industrial processes. Temperature governs chemical reaction rates, phase transitions, and material properties. Thermal comfort, climate control, and cryogenics rely on precise measurement. The concept also intersects with heat, energy, and enthalpy, distinguishing between the state variable temperature and the energy exchange that accompanies heat transfer.

Historically, thermometric devices evolved from thermoscopes to mercury and alcohol thermometers, followed by the Celsius and Fahrenheit scales and eventually the Kelvin scale in the 19th and 20th centuries. Modern temperature measurement includes non-contact methods such as infrared thermography and optical thermometry.