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Heat. What is it?

Heat. What is it?

In physics, in particular in thermodynamics, heat is defined as the contribution of transformed energy as a result of a chemical or nuclear reaction and transferred between two systems or between two parts of the same system. This energy is not attributable to a job or a conversion between two different types of energy. Heat is, therefore, a form of transferred energy and not a form of energy contained as internal energy.

As the energy is exchanged, the heat is measured in the International System in joules. In practice, however, it is often still used as the unit of measurement of calories, which is defined as the amount of heat necessary to raise the temperature of one gram of distilled water, is subjected to the pressure of 1 atm passed from 14.5 at 15 ° C, 5 ° C. Sometimes purely technical units such as kWh or BTU are also used.

Heat and work are forms of energy that can not be associated with the state of the system, that is, with its equilibrium configuration; in particular, both forms of energy are recognized the moment they transit, they flow. The work identifies the moment when the force is made a change. In other words, workflows are performed at the moment they are produced; just as the heat is identified only at the moment of its transmission.

Effects of heat

The effects of heat transfer are described by the first law of thermodynamics in its most general form:

ΔE = Q - W

where ΔE indicates a change of any form of energy (such as internal energy, kinetic energy, or potential energy), Q represents heat and W indicates work (by volume change or isocorous). The consequences of heat transfer can be mainly of two types: energy variation or work exchange.

A particular form of energy that can be modified after the passage of heat is the internal energy; the variation of internal energy can have different consequences, including a change in temperature or a change in the state of aggregation.

If the heat transfer results in a change in the state of aggregation, this heat takes the name of latent heat, whereas if the heat transfer results in a decrease in the temperature difference (because the two systems or two parts of the same system tend to reach thermal equilibrium) we speak of sensible heat.

The classic formula of sensible heat is:

Q = c · m · ΔT

while the latent heat is:

Q = λ · m

Finally, in the case that heat transfer involves both a decrease in temperature difference and a phase change, this heat can be considered as the sum of two contributions: a contribution related to sensible heat and a contribution related to the latent heat.

For example, the increase in water temperature from 20 ° C to 50 ° C under standard conditions (ie, at a pressure of 1 atm) is determined by the fact that sensible heat is provided, whereas, if the Water has already reached the boiling temperature, stores energy (in the form of latent heat), keeping its temperature unchanged, until the phase change from liquid to vapor occurs. For this reason, a jet of water vapor at 100 ° C, which has energy stored during the passage of the state, can cause more severe burns than water in the liquid state at the same temperature.

Heat of reaction is also spoken of when heat is consumed or generated by a chemical reaction.

Heat, temperature and internal energy

Heat is not a property associated with a thermodynamic equilibrium configuration. In the presence of a temperature gradient, heat flows from the points at higher temperatures to those at lower temperatures, until thermal equilibrium is reached. The amount of heat exchanged depends on the particular trajectory followed by the transformation to arrive from the initial state to the final state. In other words, heat is not a state function.

The internal energy, instead, is a function of the associable state with an equilibrium (or thermodynamic state) configuration of the system, depending on the state variables.

For internal temperature and energy they have logical expressions (that is, they are scientifically correct) of the type: "the body has a certain temperature, it has a certain internal energy, it acquires energy, it gives energy".

On the other hand, heat is not a thermodynamic property, so phrases like "the body has heat, yields heat, acquires heat" have no scientific value. In fact, heat can be defined as "energy in transit", not as "energy possessed by a body"; heat is exchanged between two bodies (or two parts of the same body) and not possessed by a single body (as is the case with internal energy). In particular, the heat flows due to a temperature difference between the system under study and the environment that interacts with it, then the heat only manifests when it passes between the system and the environment due to a temperature difference and is not recognized. no way within the system and the environment as an intrinsic property of it.

Heat propagation

The transfer (or exchange or propagation) of heat between systems can be done in three ways:

  • Propagation of heat by conduction: in a single body or between bodies in contact there is a transmission, by impacts, of kinetic energy between the molecules that belong to the neighboring areas of the material. In the driving energy is transferred through matter, but without macroscopic movement of the latter;
  • Propagation of heat by convection: in a fluid in motion, the fluid parts can heat or cool to run in contact with the outer surfaces and then in the course of their movement (in the turbulent character often) , the transfer (always to execute), the energy acquired to other surfaces, which gives rise to a transfer of heat by advection. In a gravitational field such as terrestrial (associated with the force of weight), this method of heat transfer is due to the natural occurrence of advection currents, heat and cool, due to temperature diversity and, for therefore, of the density of the fluid regions involved in the phenomenon, with respect to those of the surrounding fluid;
  • Propagation of heat by irradiation: between two systems, the heat transmission can take place at a distance (also in a vacuum), for the emission, propagation and absorption of electromagnetic waves: also in this case the lower body temperature it is heated, and that at a higher temperature it cools. The irradiation mechanism does not require physical contact between the bodies involved in the process. An example is the heat that propagates from the Sun to the earth through solar radiation.

Temperature detection

The feeling of "heat" or "cold" that you feel when touching a body is determined by its temperature and the thermal conductivity of the material it is made of, in addition to other factors.

Although it is possible to compare with the touch (with some caution) the relative temperatures of two bodies, it is impossible to give an absolute evaluation. For example, when submerging one hand in cold water for a few seconds and the other in heat water, and then submerging both in warm water, the first will have the feeling that the water is heat, the second that it is cold, because the temperature perceived is relative to that of the hand that is doing the measurement.

A relative evaluation is also often impossible. For example, when you touch a piece of wood and a piece of metal that have been in the same environment long enough to reach thermal equilibrium with the environment, you have the feeling that the metal is much colder, due to the different thermal conductivity of the two materials. A thermometer placed first in contact with wood, then with metal, instead would measure the same temperature, which coincides with that of air in the environment that is approximated as a source of heat for everything that is contained in it.

Temperature is an index of the average kinetic energy of the body particles under examination, heat is the energy that a body at a higher temperature transfers to a body at a lower temperature (until both bodies have the same temperature ). The sensation of cold and heat is due both to the temperature difference between the hand and the object and to the speed with which the object can transfer (absorb or release) heat to the hand (or other object at different temperatures).

However, by providing heat to a body, not only does the temperature increase, then there is a more acute sense of heat, but there are directly measurable variations in some physical properties.

Historical background of heat

During the first half of the eighteenth century, scholars used the elemental substance called phlogiston to explain the heating of some materials and combustion.

In the following years, the thermal phenomena went back to the theory that heat was an invisible fluid, that when entering the matter of a body it could increase its temperature.

Despite Boyle's seventeenth-century studies of the relationship between particle movement and heat, it was not until the middle of the nineteenth century that the foundations of thermodynamics were laid. These bases were laid down thanks to the studies Mayer (1842) and Joule (1843), relating to the amount of heat and the work needed to achieve it.

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Last review: August 24, 2018

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