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Effects of thermodynamics


What Is Temperature? Definition, Scales And Units

What is temperature? Definition, scales and units

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

According to molecular theory, temperature is defined as the measure of the average kinetic energy of the molecules that make it up. That is, the movements of the particles inside it.

On the other hand, it can be defined according to statistical mechanics, as the derivative of energy with respect to entropy at constant volume.


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

What are the scales of temperature?

The three most common scales for measuring temperature are:

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 variation 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

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

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 USA. The minimum recorded is -93 degrees Celsius recorded in Antarctica.

Unity in the international system of measures

The unit of measurement of the 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.

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?

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

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

If we touch an object at a lower temperature the heat flows from our body to the object. We feel cold. If the object is a good conductor of heat, heat flows faster. Then we will have the thermal sensation that the object is even colder.

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 especially important. These differences allow the movement of heat from one region to another.

What is temperature? Definition, scales and unitsHeat flows only from hot regions to cold regions. This is established by the second law of thermodynamics 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 a certain 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, temperature is the magnitude that allows the average value of a body's thermal energy to be recorded.

How is the temperature measured?

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

In order to determine the temperature of a system, it must be in thermodynamic equilibrium. Variations in the thermal state of a body cause changes in some macroscopic properties:

  • Dilatation.
  • Evolution of electrical resistance.
  • Creation of electromotive forces.
  • Pressure changes.
  • Volume changes in a 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 the use of the thermometer and establishes the principles of its construction for its measurement.

In a more fundamental approach, the empirical definition is derived from the thermal equilibrium conditions, which are expressed at the zero principle of thermodynamics.


Published: November 9, 2016
Last review: May 18, 2020