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Thermodynamics.
Transformation of energy

Thermal energy and combustion.
Effects of thermodynamics

Entropy

Heat

Heat

In physics, particularly thermodynamics, heat is defined as:

The contribution of energy transformed 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 contained energy, as internal energy.

Heat and work are forms of energy that cannot 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, flow.

The work identifies the moment when the force makes a change. In other words, workflows are done the minute they happen; just as heat is identified only at the time of transmission.

Units to express heat

The heat is measured in the International System in joules.

In practice, however, it is often still used as the unit of measurement for calories.

A calorie is the amount of heat required to raise the temperature of one gram of distilled water by one degree Celsius. This definition is valid under the pressure conditions of 1 atm.

Sometimes purely technical units are also used: like kWh or BTU.

What is specific heat?

The specific heat (or heat of specific mass) of a substance is defined as: the amount of heat required to increase or decrease the temperature of a unit of mass by one kelvin.

Remember that the difference between a degree Celsius and a kelvin is the same.

A similar amount is specific molar heat, also called molar heat, defined as: the amount of heat required to increase or decrease the temperature of one mole of substance by one degree.

In the international system the unit of measurement of specific heat is J / (K · kg), even if the kcal / (kg × ° C) is used a lot, while that of molar heat is J / (K · mol).

What are the 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 in any form of energy (such as internal energy, kinetic energy, or potential energy).
Q -> represents heat.
W -> indicates the job (by volume change or isochoric).

The consequences of heat transfer can be mainly of two types:

  1. energy variation
  2. job exchange.

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

What are latent heat and sensible heat?

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

The classic formula for sensible heat is:

Q  = c·m·ΔT

while that of latent heat is:

Q = λ·m

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

  1. a contribution related to sensible heat
  2. a contribution related to latent heat.

Example

For example, the increase in water temperature from 20 ° C to 50 ° C under standard conditions (i.e. at a pressure of 1 atm) is determined by the fact that sensible heat is provided. If the water has already reached boiling temperature, it 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.

Reaction heat is also referred to when heat is consumed or generated by a chemical reaction.

What relationship do heat, temperature and internal energy have?

Heat is not a property associated with a thermodynamic equilibrium configuration. In the presence of a temperature gradient, heat flows from points at higher temperatures to those at lower temperatures, until thermal equilibrium is reached.

The amount of heat exchanged depends on the particular path followed by transformation to get from the initial state to the final state.

In other words, heat is not a state function.

Internal energy

Internal energy, instead, is a function of the state associable 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".

Energy in transit

On the other hand, heat is not a thermodynamic property. Phrases like "the body has heat, gives up 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, heat flows due to a temperature difference between the system under study. The environment that interacts with it. So, heat only manifests when it passes between the system and the environment due to a temperature difference.

It is not recognized in any way within the system and the environment as an intrinsic property of it.

How does heat spread?

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

  • Driving.
  • Convection.
  • Irradiation.

Heat spread 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 conduction energy it is transferred through matter, but without macroscopic movement of the latter.

Convection heat propagation

In a moving fluid, the fluid parts can heat or cool on contact with the outer surfaces. Then, in the course of its movement (in the often turbulent character), the transfer (always to run), the energy acquired to other surfaces, which results in an advection heat transfer.

Heat spread by irradiation

Between two systems, heat transmission can take place at a distance (also in a vacuum).

The transfer is carried out by the emission, propagation and absorption of electromagnetic waves: the lower body temperature heats up, the higher temperature cools down.

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

Although it is possible to compare the relative temperatures of two bodies with touch (with some caution), it is impossible to give an absolute evaluation.

Calorimeters are used to calculate the heat transfer.

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 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 rise, then there is a sharper sensation of heat, but there are directly measurable variations in some physical properties.

Examples

For example. We submerge one hand in cold water for a few seconds and the other in hot water. Then we submerge both in warm water. The first will have the sensation that the water is hot, the second that it is cold, because the perceived temperature is relative to that of the hand that is doing the measurement.

A relative evaluation is also often impossible. For example, by touching a piece of wood and a piece of metal. We assume that both materials have been in the same environment long enough to achieve thermal equilibrium with the environment. Touching them gives the feeling that the metal is much colder, due to the different thermal conductivity of the two materials.

At the same time we place a thermometer. First in contact with wood and then with metal. We observe that the temperature in both materials is the same. The same as room temperature.

Historical background of heat

During the first half of the 18th century, scholars used the elemental substance called phlogiston to explain the heating of some materials and combustion.

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

Despite Boyle's 17th-century studies of the relationship between particle motion and heat, it was only around the middle of the 19th century that the foundations of thermodynamics were laid. These foundations were laid thanks to the studies Mayer (1842) and Joule (1843), regarding the amount of heat and the work necessary to achieve it.

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Published: August 24, 2018
Last review: April 21, 2020