All substances have internal energy. This value is characterized by a number of physical and chemical properties, among which special attention should be paid to heat. This value is an abstract mathematical value that describes the forces of interaction between the molecules of a substance. Understanding the mechanism of heat exchange can help answer the question of how much heat was released during cooling and heating of substances, as well as their combustion.
History of the discovery of the phenomenon of heat
Initially, the phenomenon of heat transfer was described very simply and clearly: if the temperature of a substance rises, it receives heat, and in the case of cooling, it releases it into the environment. However, heat is not an integral part of the liquid or body under consideration, as was thought three centuries ago. People naively believed that matter consists of two parts: its own molecules and heat. Now, few people remember that the term "temperature" in Latin means "mixture", and, for example, they spoke of bronze as "the temperature of tin and copper."
In the 17th century, two hypotheses appeared thatcould clearly explain the phenomenon of heat and heat transfer. The first was proposed in 1613 by Galileo. His wording was: "Heat is an unusual substance that can penetrate into and out of any body." Galileo called this substance caloric. He argued that caloric cannot disappear or collapse, but is only capable of passing from one body to another. Accordingly, the more caloric in the substance, the higher its temperature.
The second hypothesis appeared in 1620, and was proposed by the philosopher Bacon. He noticed that under the strong blows of the hammer, the iron heated up. This principle also operated when kindling a fire by friction, which led Bacon to think about the molecular nature of heat. He argued that when a body is mechanically affected, its molecules begin to beat against each other, increase the speed of movement and thereby raise the temperature.
The result of the second hypothesis was the conclusion that heat is the result of the mechanical action of the molecules of a substance with each other. For a long period of time, Lomonosov tried to substantiate and experimentally prove this theory.
Heat is a measure of the internal energy of matter
Modern scientists have come to the following conclusion: thermal energy is the result of the interaction of substance molecules, i.e., the internal energy of the body. The speed of movement of particles depends on temperature, and the amount of heat is directly proportional to the mass of the substance. So, a bucket of water has more thermal energy than a filled cup. However, a saucer of hot liquidmay have less warmth than a cold basin.
The theory of caloric, which was proposed in the 17th century by Galileo, was refuted by scientists J. Joule and B. Rumford. They proved that thermal energy does not have any mass and is characterized solely by the mechanical movement of molecules.
How much heat will be released during the combustion of a substance? Specific calorific value
Today, peat, oil, coal, natural gas or wood are universal and widely used energy sources. When these substances are burned, a certain amount of heat is released, which is used for heating, starting mechanisms, etc. How can this value be calculated in practice?
For this, the concept of specific heat of combustion is introduced. This value depends on the amount of heat that is released during the combustion of 1 kg of a certain substance. It is denoted by the letter q and is measured in J / kg. Below is a table of q values for some of the most common fuels.
When building and calculating engines, an engineer needs to know how much heat will be released when a certain amount of substance is burned. To do this, you can use indirect measurements using the formula Q=qm, where Q is the heat of combustion of the substance, q is the specific heat of combustion (table value), and m is the given mass.
The formation of heat during combustion is based on the phenomenon of energy release during the formation of chemical bonds. The simplest example is the combustion of carbon, which is containedin any type of modern fuel. Carbon burns in the presence of atmospheric air and combines with oxygen to form carbon dioxide. The formation of a chemical bond proceeds with the release of thermal energy into the environment, and man has adapted to use this energy for his own purposes.
Unfortunately, the thoughtless spending of such valuable resources as oil or peat may soon lead to the depletion of sources for the production of these fuels. Already today, electrical appliances and even new models of cars are appearing, the operation of which is based on alternative energy sources such as sunlight, water or the energy of the earth's crust.
Heat transfer
The ability to exchange thermal energy within a body or from one body to another is called heat transfer. This phenomenon does not occur spontaneously and occurs only with a temperature difference. In the simplest case, thermal energy is transferred from a hotter body to a cooler one until equilibrium is established.
Bodies do not have to be in contact for the phenomenon of heat transfer to occur. In any case, the establishment of equilibrium can also occur at a small distance between the objects under consideration, but at a slower speed than when they come into contact.
Heat transfer can be divided into three types:
1. Thermal conductivity.
2. Convection.
3. Radiant exchange.
Thermal conductivity
This phenomenon is based on the transfer of thermal energy between atoms or molecules of matter. Causetransmission - the chaotic movement of molecules and their constant collision. Due to this, heat passes from one molecule to another along the chain.
The phenomenon of thermal conductivity can be observed when any iron material is calcined, when the redness on the surface spreads smoothly and gradually fades (a certain amount of heat is released into the environment).
F. Fourier derived a formula for heat flow, which collected all the quantities that affect the degree of thermal conductivity of a substance (see figure below).
In this formula, Q/t is the heat flux, λ is the thermal conductivity coefficient, S is the cross-sectional area, T/X is the ratio of the temperature difference between the ends of the body located at a certain distance.
Thermal conductivity is a tabular value. It is of practical importance when insulating a residential building or thermal insulation of equipment.
Radiant heat transfer
Another way of heat transfer, which is based on the phenomenon of electromagnetic radiation. Its difference from convection and heat conduction lies in the fact that energy transfer can also occur in vacuum space. However, as in the first case, a temperature difference is required.
Radiant exchange is an example of the transfer of thermal energy from the Sun to the Earth's surface, which is mainly responsible for infrared radiation. To determine how much heat reaches the earth's surface, numerous stations have been built, whichmonitor the change in this indicator.
Convection
Convective movement of air flows is directly related to the phenomenon of heat transfer. Regardless of how much heat we imparted to a liquid or gas, the molecules of the substance begin to move faster. Because of this, the pressure of the entire system decreases, and the volume, on the contrary, increases. This is the reason for the movement of warm air currents or other gases upwards.
The simplest example of using the phenomenon of convection in everyday life can be called heating a room with batteries. They are located at the bottom of the room for a reason, but so that the heated air has room to rise, which leads to the circulation of flows around the room.
How can heat be measured?
The heat of heating or cooling is calculated mathematically using a special device - a calorimeter. The installation is represented by a large heat-insulated vessel filled with water. A thermometer is lowered into the liquid to measure the initial temperature of the medium. Then a heated body is lowered into the water to calculate the change in temperature of the liquid after equilibrium is established.
By increasing or decreasing t, the environment determines how much heat to heat the body should be spent. The calorimeter is the simplest device that can register temperature changes.
Also, using a calorimeter, you can calculate how much heat will be released during combustionsubstances. To do this, a “bomb” is placed in a vessel filled with water. This "bomb" is a closed vessel in which the test substance is located. Special electrodes for arson are connected to it, and the chamber is filled with oxygen. After the complete combustion of the substance, a change in the temperature of the water is recorded.
In the course of such experiments, it was established that the sources of thermal energy are chemical and nuclear reactions. Nuclear reactions take place in the deep layers of the Earth, forming the main source of heat for the entire planet. They are also used by humans to generate energy through nuclear fusion.
Examples of chemical reactions are the combustion of substances and the breakdown of polymers into monomers in the human digestive system. The quality and quantity of chemical bonds in a molecule determines how much heat is ultimately released.
How is heat measured?
The unit of heat in the international SI system is the joule (J). Also in everyday life are used off-system units - calories. 1 calorie equals 4.1868 J according to the international standard and 4.184 J based on thermochemistry. Previously, there was a British thermal unit Btu, which is rarely used by scientists. 1 BTU=1.055 J.
