Thermodynamics: Temperature

Definition and Units of Temperature

Temperature is a physical quantity that expresses hot and cold. It is measured in Degrees Celsius (°C), Kelvin (K), or Fahrenheit (°F).
On the Kelvin scale, 0K, referred to as absolute zero, is the lowest possible temperature at which particles have minimal kinetic energy. A change in 1 Kelvin represents the same change in energy as a change in 1 Degree Celsius.
Thermal equilibrium is when two systems in contact with each other cease to exchange energy. In such a state, they are said to be of the same temperature.

The concept of ‘hot’ or ‘cold’ is related to the direction of heat flow. When two bodies at different temperatures come into thermal contact, heat will flow from the hot body to the cold body until they reach thermal equilibrium.
The sensation of hot or cold is subjective and depends on the temperature of the human body. Therefore, it cannot be used in scientific measurements.

Physicists have discovered a temperature at which all molecular motion would cease. This temperature, called absolute zero, is equivalent to -273.15°C or 0K on the Kelvin scale.
Absolute zero can never be reached, but temperatures very close to it can be achieved in labs for certain low-temperature physics experiments.

The physical, chemical, and electrical properties of matter strongly depend on temperature. For example, a metal’s electrical resistance will typically increase with temperature.

An increase in a body’s temperature is due to an increase in the internal kinetic energy of the body’s molecules. When a body cools, it loses internal energy.
In a system, average kinetic energy is directly proportional to the temperature. This is basis of gas laws, with the proportionality constant being Boltzmann’s constant.

Temperature is typically measured using a thermometer, which comprises a temperature-sensitive material (like mercury or coloured alcohol) that expands or contracts when heated or cooled.
Thermocouples are important tools used in industries for the measurement of temperature. They consist of two different metals that generate a voltage proportional to a temperature difference.
Newer forms of temperature measurements involve the use of infrared sensors or lasers, allowing for contactless measurements.