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 of 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.
The 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 the 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 fluid or body in question, as was thought three centuries ago. People naively believed that matter consists of two parts: its own molecules and heat. Nowadays, 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 that could 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 leave any body.” Galileo called this substance a calorific value. He argued that the caloric can not disappear or collapse, but only able to move from one body to another. Accordingly, the greater the calorific value in a substance, the higher its temperature.
The second hypothesis appeared in 1620, and was proposed by its philosopher Bacon. He noticed that under the strong blows of the hammer, the iron was heating up. This principle also acted during the bonfire kindling by friction, which led Bacon to think about the molecular nature of heat. He argued that with mechanical action on the body, his 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 justify this theory and experimentally prove it.
Heat is a measure of the internal energy of a substance.
Modern scientists have come to the following conclusion: thermal energy is the result of the interaction of molecules of a substance, i.e., the internal energy of the body. The speed of particles depends on temperature, and the magnitude 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 with hot liquid may have less heat than a basin with cold.
The theory of calorific, which was proposed by Galileo in the 17th century, 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 is released during the combustion of a substance? Specific heat of combustion
Today, universal and widely used sources of energy are peat, oil, coal, natural gas or wood. 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 when 1 kg of a certain substance is burned. It is indicated 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 constructing and calculating engines, an engineer needs to know how much heat will be released when a certain amount of substance is burned. For this, we can use indirect measurements according to the formula Q = qm, where Q is the heat of combustion of the substance, q is the specific heat of combustion (tabular 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 contained in 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 adapted to use this energy for his own purposes.
Unfortunately, the thoughtless spending of such valuable resources as oil or peat can soon lead to the depletion of the sources of extraction of these fuels. Already today there are electrical appliances and even new car models whose work is based on such alternative energy sources as sunlight, water or the energy of the earth's crust.
Heat transfer
The ability to exchange thermal energy within the body or from one body to another is called heat transfer. This phenomenon does not occur spontaneously and occurs only at a temperature difference. In the simplest case, thermal energy is transferred from a warmer body to a less heated body until an equilibrium is established.
The bodies do not have to touch in order for the heat transfer phenomenon to occur. In any case, the establishment of equilibrium can occur at a small distance between the objects in question, but at a lower speed than when they are in 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 a substance. The cause of transmission is the random movement of molecules and their constant collision. Due to this, heat passes from one molecule to another along the chain.
It is possible to observe the phenomenon of thermal conductivity when calcining any iron material, when the redness on the surface gradually spreads and gradually fades (a certain amount of heat is released into the environment).
J. Fourier derived a formula for heat flux, which collected all 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 heat 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 has practical value when warming a house or thermal insulation of equipment.
Radiant heat transfer
Another method of heat transfer, which is based on the phenomenon of electromagnetic radiation. Its difference from convection and thermal conductivity is that energy transfer can occur in a vacuum space. However, as in the first case, a temperature difference is necessary.
Radiant exchange is an example of the transfer of solar thermal energy to the Earth's surface, for which infrared radiation is primarily responsible. To determine how much heat falls on the earth's surface, numerous stations have been built that monitor the change in this indicator.
Convection
Convection movement of air flows is directly related to the phenomenon of heat transfer. Regardless of how much heat we reported 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 or other gases up.
The simplest example of the use of the phenomenon of convection in everyday life can be called heating the room with batteries. They are located at the bottom of the room for a reason, but so that the heated air has a place to rise, which leads to the circulation of flows around the room.
How to measure the amount of heat?
The heat of heating or cooling is calculated mathematically using a special device - a calorimeter. The installation is represented by a large insulated vessel, which is filled with water. A thermometer is lowered into the liquid to measure the initial temperature of the medium. Then the heated body is lowered into the water to calculate the change in the temperature of the liquid after equilibrium is established.
By increasing or decreasing the medium’s t, it is determined how much heat should be spent for heating the body. The calorimeter is the simplest device that can detect temperature changes.
Also, using a calorimeter, you can calculate how much heat will be released during the combustion of substances. For 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 were brought to him, and the chamber was filled with oxygen. After complete combustion of the substance, a change in water temperature is recorded.
In the course of such experiments, it was found that the sources of thermal energy are chemical and nuclear reactions. Nuclear reactions occur in the deep layers of the Earth, forming the main reserve of heat of the entire planet. They are also used by humans to generate energy during fusion.
Examples of chemical reactions are the burning of substances and the breakdown of polymers to 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, off-system units - calories are used in everyday life. 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.