Transformation of energy

Thermal energy and combustion.
Effects of thermodynamics


What Is The Temperature?

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.

What is the temperature?

This property is related to the physiological sensation of cold and heat. This sensation is generated when there is an exchange of thermal energy between the human body and other bodies or, simply, the environment that surrounds it. However, it must be considered that within the human perception there is the concept of thermal sensation that alters this perception due to other external factors.

From the physical point of view, the temperature of a substance can be defined, according to molecular theory, as the measure of the average kinetic energy of the molecules that make it up. On the other hand, temperature 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 a molecule or an isolated atom, but of a set.

What are the scales for measuring temperature?

The three most common scales are:


    Theoretically, the temperature could be measured using units of energy because, since entropy has no dimensions, according to the definition of statistical mechanics. Traditionally, however, temperature scales have been created in parallel with energy units. The Boltzmann constant is the factor that allows to go from a system of energy units to temperature.

    Celsius scale and Kelvin scale

      The Celsius scale is the scale most used to express temperature. Almost everyone uses the Celsius (° C) scale for the measurement of most measurements. The temperature variation from one degree to 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 temperature that a body could theoretically reach. Colder than 0 Kelvin is impossible.

      To convert a Celsius scale magnitude to a kelvin scale, add 273.15 kelvins. That is, the freezing point of water on the Kelvin scale is 273.15 k.

      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. This is a historical scale in which the freezing point of water is 32 ° F and the boiling temperature of water is 212 ° F.

      What are the units of temperature in the international system of nations?

      The unit of measurement for temperature 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.

      Within the fields of science, the International System of Units (SI) defines a scale and a unit for thermodynamic temperature based on the triple point of water. The triple point is one in which the solid state, the liquid state, and the gaseous state of a substance coexist in equilibrium. It is defined with a temperature and a vapor pressure. The triple point of water is an easily reproducible second reference point.

      For historical reasons, the triple point of water has been set at 273.16 units of the measurement interval. This interval is called a kelvin (lowercase) represented by the symbol K (uppercase) in honor of the Scottish physicist William Thomson (Lord Kelvin) who first defined the scale. Previously it was called Kelvin degree.

      What is the relationship between temperature and thermodynamics?

      What is the temperature?One of the main properties studied in the field of thermodynamics is temperature. In thermodynamics, temperature differences between different regions of matter are especially important. These differences are what allow the movement of heat from one region to another. Heat is that it is the transfer of thermal energy.

      Spontaneously, heat flows only from the regions with the highest temperature in the regions with a lower value. As stated in the second law of thermodynamics in Clausius's statement. So if heat is not transferred between two objects it is because both objects have the same temperature.

      According to the classical thermodynamic approach, the temperature of an object varies proportionally to the speed of the particles it contains. It does not depend on the number of particles (of the mass) but on their average speed: the higher the temperature, the greater the average speed. Therefore, the temperature is directly linked to the average kinetic energy of the particles that move in relation to the object's center of mass.

      Temperature is an intensive variable, since it is independent of the amount of particles contained inside an object, be they atoms, molecules or electrons. It is a property that does not depend on the amount of substance or the type of material.

      Is the temperature the same as thermal energy?

      The molecules of all material substances (solids, liquids and gases) are always in a continuous state of 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 and also 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 registering the average value of a body's thermal energy.

      How is the temperature measured?

      Thermometers are the tool to quantitatively measure temperature, which can be calibrated with respect to different measurement scales (Celsius scale, Kelvin scale or Fahrenheit scale).

      In order to determine the temperature of a system, it must be in thermodynamic equilibrium. The temperature can be considered to vary with position only if for each point there is a small area around it that can be treated as a balanced thermodynamic system. In statistical thermodynamics, instead of particles we speak of degrees of freedom.

      Within the field of thermodynamics, a system is said to be in a state of thermodynamic equilibrium, if it is incapable of spontaneously undergoing any change of state or thermodynamic process when it is subjected to certain boundary conditions.

      Currently, there are several ways to measure temperature. Different systems usually depend on the application or on whether very high or very low temperatures must be measured. However, the best known and used tool is the thermometer.

      Variations in the thermal state of a body cause changes in some macroscopic properties (dilation, evolution of electrical resistance, creation of electromotive forces, changes in pressure or volume in a gas, etc.). Consequently, the variations of these properties allow them to be used for the construction of instruments that detect temperature variations.

      What does it mean in thermal equilibrium?

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

      When two systems are in thermal equilibrium they have the same temperature. The extension of this principle as an equivalence relationship between various systems fundamentally justifies the use of the thermometer and establishes the principles of its construction for its measurement.

      Although the zero law of thermodynamics would allow the empirical definition of many temperature scales, the second law of thermodynamics selects a single definition as the preferred one: absolute temperature, known as thermodynamic temperature.

      This function corresponds to the variation of the internal energy with respect to the changes in the entropy of a system. Its natural, intrinsic or null point origin is absolute zero, where the entropy of any system is minimal. Although this is the absolute minimum temperature described by the model, the third law of thermodynamics postulates that absolute zero cannot be reached by any physical system.

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      Last review: March 17, 2020