Transformation of energy

What is the temperature definition? Scales and units

What is the temperature definition? Scales and units

Temperature is the thermodynamic magnitude that shows the thermal energy of one body to that of another. This thermodynamic property only describes a macroscopic state.

The definition of temperature in terms of molecular kinetic energy is the measure of the average of the molecules’ kinetic energy that makes it up. That is the movement of the particles inside it.

On the other hand, according to statistical mechanics, we can define it as the derivative of energy concerning entropy at constant volume.

Being a macroscopic magnitude, it has an exclusively statistical character. It means that it does not make sense to speak of an isolated molecule or atom’s temperature but as a whole.

The Boltzmann constant relates the average kinetic energy for each degree of freedom of an item in equilibrium to its temperature. For example, the Boltzmann constant relates the average kinetic energy of particles in an ideal gas with the gas’s temperature.

What are the scales of temperature?

The three most common scales for measuring temperature are:

  • The Celsius scale in which his unit is the degrees Celsius.

  • The Kelvin scale in which his unit is the kelvin.

  • The Fahrenheit scale that uses degrees Fahrenheit.

  • The Rankine scale refers to the temperature scale. It is defined by measuring in degrees Fahrenheit above absolute zero, so it has no negative values. 

Celsius scale and Kelvin scale

The Celsius scale is the scale most used to express temperature. Almost everyone uses the Celsius scale (degree Celsius) for the measurement of most measurements. The variation between one degree and the next on a Celsius scale is the same as on a Kelvin scale.

The difference between the Celsius and Kelvin scales is in fixing their null point:

  • On the Celsius scale, 0ºC corresponds to the freezing point of water.

  • On the Kelvin scale, 0 degrees corresponds to the minimum level that a body could theoretically reach.

Kelvin scale intervals are measured in kelvin but were previously called Kelvin degrees.

Fahrenheit scale and Rankine Scale

However, most notably in the United States, a few countries are still using the Fahrenheit scale daily. It is a historical scale. In this scale, the freezing point of water is 32 ° F, and the boiling point of water is 212 ° F.

The Rankine scale refers to the temperature scale defined by measuring in degrees Fahrenheit above absolute zero. 

The Scottish physicist and engineer William Rankine proposed Rankine Scale in 1859.

Examples of temperature



Celsius degrees

Fahrenheit degrees

Rankine degrees 

Absolute zero





Melting water temperature





Boiling water temperature





Human body temperature





Ambient temperature is the temperature of the outside air. The maximum recorded has been 56.7 degrees Celsius. Registered at Furnace Creek Station, located in the well-known "Death Valley" of the USA. The minimum recorded is -93 degrees Celsius recorded in Antarctica.

Unity in the international system of measures

The unit of measurement in the International System of Units (SI) is the kelvin. Kelvin is, therefore, the unit used by scientists. It is frequent to see it referenced as a Kelvin degree.

The triple point of water

The International System of Units (SI) defines a scale and a unit for thermodynamic temperature based on the triple point of water.

Why do we feel cold and hot?

The sensation of cold or heat that a person feels according to a combination of meteorological parameters is called thermal sensation.

The physiological sensation is generated when there is an exchange of thermal energy between body temperature and other bodies.

If we touch an object at a lower temperature, the heat flows from our body to the item, and then, we feel cold. Otherwise, if we touch an object at higher temperatures, the heat flows in the opposite direction, and we feel hot.

Relationship between temperature and thermodynamics

It is one of the main properties studied in a thermodynamic system. In thermodynamics, differences in thermals between different regions of matter are incredibly significant. These differences allow the movement of heat from one part to another.

What is the temperature definition? Scales and unitsHeat flows only from hot regions to cold regions. The second law of thermodynamics establishes this in Clausius's statement. Two objects at the same temperature do not transfer heat.

Is the temperature the same as thermal energy?

The molecules of all material substances are always in continuous motion. They can be in vibration or agitation due to the multiple interactions they undergo within the body.

As a consequence of this random agitation, the atoms and molecules of matter have specific internal energy since they have kinetic energy in the form of movement. They also possess potential energy due to the forces exerted between the particles.

Internal energy is also known as the thermal energy of bodies.

On the other hand, the temperature is the magnitude that allows the average value of a body's thermal energy to be recorded.

The specific heat is the amount of heat necessary to change 1.00 kg of mass by 1.00 ºC.

How is the temperature measured?

Different measurement systems depend on the application or whether very high or shallow values ​​are to be measured. However, the best known and used tool is the thermometer.

To determine the temperature of a system, it must be in thermodynamic equilibrium. Variations in a body’s thermal state cause changes in physical properties such as dilatation, the evolution of electrical resistance, the creation of electromotive forces, pressure changes, volume changes in gas, etc.

Consequently, variations in these properties allow them to be used for the construction of instruments that detect temperature variations.

Previously, the mercury thermometer was used to measure body temperature. It is currently in disuse due to its dangerousness.

What does it mean in thermal equilibrium?

When two systems are in thermal equilibrium, they have the same temperature. The extension of this principle fundamentally justifies using the thermometer and establishes the principles of its construction for its measurement.

The empirical definition is derived from the thermodynamics’ zeroth principle’s thermal equilibrium conditions in a more fundamental approach.

Publication Date: November 9, 2016
Last Revision: October 29, 2020