Units for measuring the amount of heat and work. Thermal energy: units of measurement and their correct use

This lesson discusses the concept of heat quantity.

If up to this point we have considered general properties and phenomena associated with heat, energy or their transfer, now it's time to get acquainted with the quantitative characteristics of these concepts. More precisely, introduce the concept of the amount of heat. All further calculations related to the transformation of energy and heat will be based on this concept.

Definition

Quantity of heat is the energy that is transferred by heat transfer.

Let us consider the question: what quantity shall we express this quantity of heat?

The amount of heat is related to internal energy bodies, therefore, when the body receives energy, its internal energy increases, and when it gives it away, it decreases (Fig. 1).

Rice. 1. The relationship between the amount of heat and internal energy

Similar conclusions can be drawn about body temperature (Fig. 2).

Rice. 2. The relationship between the amount of heat and temperature

Internal energy is expressed in joules (J). This means that the amount of heat is also measured in joules (in SI):

The standard notation for the amount of heat.

To find out: what depends on, we will conduct 3 experiments.

Experiment #1

Let's take two identical bodies, but different masses. For example, let's take two identical pans and pour into them different amount water (same temperature).

Obviously, in order to boil the pot in which there is more water, it will take more time. That is, she must be informed large quantity warmth.

From this we can conclude that the amount of heat depends on the mass (directly proportional - the greater the mass, the greater the amount of heat).

Rice. 3. Experiment #1

Experiment #2

In the second experiment, we will heat bodies of the same mass to different temperatures. That is, let's take two pots of water of the same mass and heat one of them to , and the second, for example, to .

Obviously, in order to heat the pan to a higher temperature, it will take more time, that is, it will need to impart more heat.

From this we can conclude that the amount of heat depends on the temperature difference (directly proportional - the greater the temperature difference, the greater the amount of heat).

Rice. 4. Experiment #2

Experiment #3

In the third experiment, we consider the dependence of the amount of heat on the characteristics of the substance. To do this, take two pans and pour water into one of them, and sunflower oil into the other. In this case, the temperatures and masses of water and oil must be the same. We will heat both pans to the same temperature.

In order to heat a pot of water, it will take more time, that is, it will need to impart more heat.

From this we can conclude that the amount of heat depends on the type of substance (we will talk more about how exactly in the next lesson).

Rice. 5. Experiment #3

After the experiments, we can conclude that it depends:

• from body weight;
• changes in its temperature;
• kind of substance.

Note that in all the cases we have considered, we are not talking about phase transitions (i.e., changes in state of aggregation substances).

At the same time, the numerical value of the amount of heat may also depend on its units of measurement. In addition to the joule, which is an SI unit, another unit for measuring the amount of heat is used - calorie(translated as "heat", "warmth").

This is enough small value, so the concept of kilocalorie is more often used: . This value corresponds to the amount of heat that must be transferred to water in order to heat it by.

In the next lesson, we will consider the concept of specific heat capacity, which relates a substance and the amount of heat.

Bibliography

1. Gendenstein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
3. Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.

1. Internet portal "festival.1september.ru" ()
2. Internet portal "class-fizika.narod.ru" ()
3. Internet portal "school.xvatit.com" ()

Homework

1. Page 20, paragraph 7, questions 1-6. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
2. Why does the water in the lake cool down much less overnight than the sand on the beach?
3. Why is the climate, which is characterized by sharp temperature changes between day and night, called sharply continental?

As we already know, the internal energy of a body can change both when doing work and by heat transfer (without doing work). The main difference between work and the amount of heat is that work determines the process of converting the internal energy of the system, which is accompanied by the transformation of energy from one type to another.

In the event that the change in internal energy proceeds with the help of heat transfer, the transfer of energy from one body to another is carried out due to thermal conductivity, radiation, or convection.

The energy that a body loses or gains during heat transfer is called the amount of warmth.

When calculating the amount of heat, you need to know what quantities affect it.

From two identical burners we will heat two vessels. In one vessel 1 kg of water, in the other - 2 kg. The temperature of the water in the two vessels is initially the same. We can see that in the same time the water in one of the vessels heats up faster, although both vessels receive the same amount of heat.

Thus, we conclude: the greater the mass given body, the more heat must be expended in order to lower or raise its temperature by the same number of degrees.

When the body cools down, it gives off to neighboring objects the greater the amount of heat, the greater its mass.

We all know that if we need to heat a full kettle of water to a temperature of 50°C, we will spend less time on this action than to heat a kettle with the same volume of water, but only up to 100°C. In case number one, less heat will be given to the water than in the second.

Thus, the amount of heat required for heating is directly dependent on how many degrees the body can warm up. We can conclude: the amount of heat directly depends on the temperature difference of the body.

But is it possible to determine the amount of heat required not for heating water, but for some other substance, say, oil, lead or iron.

Fill one vessel with water and the other with vegetable oil. The masses of water and oil are equal. Both vessels will be evenly heated on the same burners. Let's start the experiment at equal initial temperature of vegetable oil and water. Five minutes later, by measuring the temperatures of the heated oil and water, we will notice that the temperature of the oil is much higher than the temperature of the water, although both fluids received the same amount of heat.

The obvious conclusion is: When heating equal masses of oil and water at the same temperature, different amounts of heat are needed.

And we immediately draw another conclusion: the amount of heat that is required to heat the body directly depends on the substance that the body itself consists of (the kind of substance).

Thus, the amount of heat needed to heat the body (or released during cooling) directly depends on the mass of the given body, the variability of its temperature, and the type of substance.

The amount of heat is denoted by the symbol Q. Like other different kinds energy, the amount of heat is measured in joules (J) or in kilojoules (kJ).

1 kJ = 1000 J

However, history shows that scientists began to measure the amount of heat long before such a concept as energy appeared in physics. At that time, a special unit was developed for measuring the amount of heat - a calorie (cal) or a kilocalorie (kcal). The word has Latin roots, calorus - heat.

1 kcal = 1000 cal

Calorie is the amount of heat required to raise the temperature of 1 g of water by 1°C

1 cal = 4.19 J ≈ 4.2 J

1 kcal = 4190 J ≈ 4200 J ≈ 4.2 kJ

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The thermal energy (amount of heat) of a body can be measured directly with a so-called calorimeter; A simple version of such a device is shown in Fig. 5. This is a carefully insulated closed vessel, equipped with devices for measuring the temperature inside it and sometimes filled with a working fluid with known properties, such as water. To measure the amount of heat in a small heated body, it is placed in a calorimeter and waiting for the system to come into thermal equilibrium. The amount of heat transferred to the calorimeter (more precisely, to the water filling it) is determined by the increase in water temperature.(14.86 Kb)

The amount of heat released during chemical reaction, such as combustion, can be measured by placing a small "bomb" in the calorimeter. The "bomb" contains a sample, to which electrical wires are connected for ignition, and the corresponding amount of oxygen. After the sample burns out completely and thermal equilibrium is established, it is determined how much the temperature of the water in the calorimeter has increased, and hence the amount of heat released.

see also CALORIMETRY.Heat units. Heat is a form of energy and therefore must be measured in units of energy. In the international SI system, the unit of energy is the joule (J). It is also allowed to use non-systemic units of the amount of heat - calories: an international calorie is 4.1868 J, a thermochemical calorie is 4.1840 J. In foreign laboratories, research results are often expressed using the so-called. 15-degree calorie, equal to 4.1855 J. The non-systemic British thermal unit(BTU): BTU avg = 1.055 J. The main sources of heat are chemical and nuclear reactions, as well as various energy conversion processes. Examples of chemical reactions with the release of heat are combustion and the breakdown of food components. Almost all the heat received by the Earth is provided by nuclear reactions flowing in the bowels of the Sun. Mankind has learned how to obtain heat with the help of controlled processes of nuclear fission, and now it is trying to use thermonuclear fusion reactions for the same purpose. Other types of energy can also be converted into heat, such as mechanical work and electrical energy. It is important to remember that thermal energy (like any other) can only be transformed into another form, but it can neither be obtained "out of nothing" nor destroyed. This is one of the basic principles of the science called thermodynamics. THERMODYNAMICS Thermodynamics is the science of the relationship between heat, work and matter. Modern views about these relationships were formed on the basis of the works of such great scientists of the past as Carnot, Clausius, Gibbs, Joule, Kelvin, and others. Thermodynamics explains the meaning of the heat capacity and thermal conductivity of a substance, the thermal expansion of bodies, and the heat of phase transitions. This science is based on several experimentally established laws - principles.Principles of thermodynamics. The zeroth law of thermodynamics formulated above introduces the concepts of thermal equilibrium, temperature and thermometry. The first law of thermodynamics is a statement that is of key importance for all science as a whole: energy can neither be destroyed nor obtained "out of nothing", so the total energy of the Universe is a constant value. In its simplest form, the first law of thermodynamics can be stated as follows: the energy that the system receives, minus the energy that it gives up, is equal to the energy remaining in the system. At first glance, this statement seems obvious, but not in such, for example, situations like the combustion of gasoline in the cylinders of an automobile engine: here the energy received is chemical, the energy given off is mechanical (work), and the energy remaining in the system is thermal.

So, it is clear that energy can change from one form to another and that such transformations are constantly taking place in nature and technology. More than a hundred years ago, J. Joule proved this for the case of the conversion of mechanical energy into thermal energy using the device shown in fig. 6,

a . In this device, descending and rising weights rotated a shaft with blades in a calorimeter filled with water, as a result of which the water was heated. Precise measurements allowed Joule to determine that one calorie of heat is equivalent to 4.186 J of mechanical work. The device shown in fig. 6, b , was used to determine the thermal equivalent of electrical energy.

The first law of thermodynamics explains many common phenomena. For example, it becomes clear why it is impossible to cool the kitchen with an open refrigerator. Let's assume that we have thermally insulated the kitchen from the environment. Energy is continuously supplied to the system through the power wire of the refrigerator, but the system does not give off any energy. Thus, its total energy increases, and the kitchen becomes warmer: just touch the tubes of the heat exchanger (condenser) on the back of the refrigerator, and you will understand its uselessness as a "cooling" device. But if these pipes were brought out of the system (for example, out of the window), then the kitchen would give out more energy than it received, i.e. would be cooled, and the refrigerator worked as a window air conditioner.

The first law of thermodynamics is a law of nature that precludes the creation or destruction of energy. However, it says nothing about how the processes of energy transfer proceed in nature. Thus, we know that a hot body will heat a cold one if these bodies are brought into contact. But can a cold body by itself transfer its heat reserve to a hot one? The latter possibility is categorically rejected by the second law of thermodynamics.

The first law also excludes the possibility of creating an engine with a coefficient of performance (COP) of more than 100% (similar

" eternal " the engine could give off more energy for an arbitrarily long time than it consumes). It is impossible to build an engine even with an efficiency equal to 100%, since some part of the energy supplied to it must necessarily be lost by it in the form of less useful thermal energy. So, the wheel will not spin for an arbitrarily long time without energy supply, because due to friction in the bearings, the energy mechanical movement will gradually turn into heat until the wheel stops.

The tendency to convert "useful" work into less useful energy - heat - can be compared with another process that occurs when two vessels containing different gases are connected. After waiting long enough, we find in both vessels a homogeneous mixture of gases - nature works so that the order of the system decreases. The thermodynamic measure of this disorder is called entropy, and the second law of thermodynamics can be formulated differently: processes in nature always proceed in such a way that the entropy of the system and its environment increases. Thus, the energy of the Universe remains constant, while its entropy is continuously growing.

On units of quantity of heat. The unit of the amount of heat - the "small" calorie - we defined above as the amount of heat that is required to raise the temperature of water by 1 K at atmospheric pressure. But since the heat capacity of water at different temperatures different, it is necessary to agree on the temperature at which this one-degree interval is chosen.

In the USSR, the so-called twenty-degree calorie was adopted, for which the interval from 19.5 to 20.5 ° C was adopted. In some countries, a fifteen-degree calorie is used (the interval of the first of them is J, the second - J. Sometimes an average calorie is used, equal to one hundredth of the amount of heat needed to heat water from to

Measurement of the amount of heat. To directly measure the amount of heat given off or received by a body, special devices are used - calorimeters.

In its simplest form, a calorimeter is a vessel filled with a substance whose heat capacity is well known, such as water (specific heat

The measured amount of heat is transferred to the calorimeter in one way or another, as a result of which its temperature changes. By measuring this change in temperature, we get the heat

where c is the specific heat capacity of the substance filling the calorimeter, its mass.

It should be taken into account that heat is transferred not only to the substance of the calorimeter, but also to the vessel and various devices that can be placed in it. Therefore, before the measurement, it is necessary to determine the so-called thermal equivalent of the calorimeter - the amount of heat that heats the "empty" calorimeter by one degree. Sometimes this correction is introduced by adding an additional mass to the mass of water, the heat capacity of which is equal to the heat capacity of the vessel and other parts of the calorimeter. Then we can assume that the heat is transferred to a mass of water equal to The quantity is called the water equivalent of the calorimeter.

Heat capacity measurement. The calorimeter is also used to measure heat capacity. In this case, it is necessary to know exactly the amount of supplied (or removed) heat. If it is known, then the specific heat capacity is calculated from the equality

where is the mass of the body under study, and the change in its temperature caused by heat

Heat is supplied to the body in the calorimeter, which must be designed so that the supplied heat is transferred only to the body under study (and, of course, to the calorimeter), but is not lost in the surrounding space. Meanwhile, such heat losses always occur to some extent, and taking them into account is the main concern in calorimetric measurements.

Measurement of the heat capacity of gases is difficult because, due to their low density, the heat capacity of the mass of gas that can be placed in the calorimeter is small. At ordinary temperatures, it can be comparable to the heat capacity of an empty calorimeter, which inevitably reduces the measurement accuracy. This applies especially to the measurement of heat capacity at constant volume. In determining this difficulty, this difficulty can be circumvented if the gas under investigation is made to flow (at constant pressure) through the calorimeter (see below).

Measurement Almost the only way direct measurement of the heat capacity of a gas at constant volume is the method proposed by Joly (1889). The scheme of this method is shown in fig. 41.

The calorimeter consists of a chamber K, in which two identical hollow copper balls are suspended at the ends of the balance beam, equipped with plates at the bottom and reflectors at the top. One of the balls is evacuated, the other is filled with the investigated gas. In order for the gas to have a noticeable heat capacity, it is injected under considerable pressure. The mass of the injected gas is determined using balances, restoring the balance disturbed by the introduction of gas with weights.

After thermal equilibrium is established between the balls and the chamber, water vapor is let into the chamber (tubes for entering and exiting steam are located on the front and rear walls of the chamber and are not shown in Fig. 41). The steam condenses on both balls, heating them, and flows into the plates. But on a sphere filled with gas, more liquid condenses, since its heat capacity is greater. Due to excess condensate on one of the balls, the balance of the balls will be disturbed again. Having balanced the scales, we find out the excess mass of liquid that has condensed due to the presence of gas in the ball. If this excess mass of water is equal, then, multiplying it by the heat of water condensation, we find the amount of heat that went into heating the gas from the initial temperature to the temperature of water vapor. By measuring this difference with a thermometer, we get:

where specific heat capacity is gas. Knowing the specific heat capacity, we find that the molar heat capacity

Measurement We have already mentioned that in order to measure the heat capacity at constant pressure, the gas under investigation is forced to flow through a calorimeter. This is the only way to ensure the constancy of the gas pressure, despite the supply of heat and heating, without which it is impossible to measure the heat capacity. As an example of such a method, we present here a description of Regnault's classical experiment (The scheme of the apparatus is shown in Fig. 42.

The test gas from tank A is passed through a valve through a coil placed in a vessel with oil B, heated by some kind of heat source. The gas pressure is regulated by a valve and its constancy is controlled by a manometer. Passing a long way in the coil, the gas takes on the temperature of the oil, which is measured by a thermometer.

The gas heated in the coil then passes through the water calorimeter, cools down in it to a certain temperature measured by the thermometer and goes outside. By measuring the gas pressure in tank A at the beginning and at the end of the experiment (a pressure gauge is used for this, we will find out the mass of the gas that has passed through the apparatus.

The amount of heat given off by the gas to the calorimeter is equal to the product of the water equivalent of the calorimeter and the change in its temperature, where is the initial temperature of the calorimeter.

When we discuss ways to heat a house, options for reducing heat leakage, we must understand what heat is, in what units it is measured, how it is transmitted and how it is lost. This page will provide the basic information from the course of physics necessary to consider all of the above issues.

Heat is one way to transfer energy

The energy that a body receives or loses in the process of heat exchange with the environment is called the amount of heat or simply heat.

In a strict sense, heat is one way of transferring energy, and physical meaning has only the amount of energy transferred to the system, but the word "heat" is included in such well-established scientific concepts, as heat flux, heat capacity, phase transition heat, chemical reaction heat, thermal conductivity, etc. Therefore, where such word usage is not misleading, the concepts of "heat" and "amount of heat" are synonymous. However, these terms can only be used if they are given precise definition, and in no case can the "amount of heat" be attributed to the number of initial concepts that do not require definition. To avoid mistakes, the term "heat" should be understood precisely as the method of energy transfer, and the amount of energy transferred by this method is denoted by the concept of "amount of heat". It is recommended to avoid the term "thermal energy".

Heat is the kinetic part of the internal energy of a substance, determined by the intense chaotic movement of the molecules and atoms that make up this substance. Temperature is a measure of the intensity of molecular motion. The amount of heat possessed by a body at a given temperature depends on its mass; for example, at the same temperature, more heat is contained in a large cup of water than in a small one, and in a bucket of cold water it can be more than in a cup with hot water(although the temperature of the water in the bucket is lower).

Heat is a form of energy and therefore must be measured in units of energy. In the international SI system, the unit of energy is the joule (J). It is also allowed to use an off-system unit of the amount of heat - calories: an international calorie is 4.1868 J.

Heat transfer and heat transfer

Heat transfer is the process of transferring heat within a body or from one body to another, due to temperature differences. The intensity of heat transfer depends on the properties of the substance, temperature difference and obeys experimentally established laws nature. To create efficient heating or cooling systems, various engines, power plants, thermal insulation systems, you need to know the principles of heat transfer. In some cases, heat transfer is undesirable (thermal insulation of melting furnaces, spaceships etc.), while in others it should be as large as possible (steam boilers, heat exchangers, kitchen utensils). There are three main types of heat transfer: conduction, convection and radiant heat transfer.

Thermal conductivity

If there is a temperature difference inside the body, then thermal energy passes from its hotter part to its colder one. This type of heat transfer, due to thermal movements and collisions of molecules, is called thermal conductivity. The thermal conductivity of the rod is estimated by the value heat flow, which depends on the thermal conductivity coefficient, the cross-sectional area through which heat is transferred and the temperature gradient (the ratio of the temperature difference at the ends of the rod to the distance between them). The unit of heat flow is watt.

THERMAL CONDUCTIVITY OF SOME SUBSTANCES AND MATERIALS
Substances and materials Thermal conductivity, W/(m^2*K)
Metals
Aluminum ___________________205
Bronze _____________________105
Tungsten ___________________159
Iron ___________________________________67
Copper _______________________389
Nickel ______________________58
Lead ______________________35
Zinc _______________________113
Other materials
Asbestos _______________________0.08
Concrete ________________________0.59
Air _______________________0.024
Eider down (loose) ______0.008
Wood (walnut) ________________0.209
Sawdust _______________________0.059
Rubber (spongy) ____________0.038
Glass _______________________0.75

Convection

Convection is heat transfer due to the movement of masses of air or liquid. When heat is applied to a liquid or gas, the intensity of the movement of molecules increases, and as a result, the pressure increases. If a liquid or gas is not limited in volume, then they expand; the local density of the liquid (gas) becomes less, and due to the buoyancy (Archimedean) forces, the heated part of the medium moves up (which is why the warm air in the room rises from the batteries to the ceiling). AT simple cases fluid flow through a pipe or flow around a flat surface, the coefficient of convective heat transfer can be calculated theoretically. However, it has not yet been possible to find an analytical solution to the problem of convection for a turbulent flow of a medium.

thermal radiation

The third type of heat transfer - radiant heat transfer - differs from heat conduction and convection in that heat in this case can be transferred through a vacuum. Its similarity with other methods of heat transfer is that it is also due to the temperature difference. Thermal radiation is one of the types of electromagnetic radiation.

The Sun is a powerful emitter of thermal energy; it heats the Earth even at a distance of 150 million km. The intensity of solar radiation is approximately 1.37 W/m2.

The rate of heat transfer by conduction and convection is proportional to temperature, and the radiant heat flux is proportional to the fourth power of temperature.

Heat capacity

Different substances have different ability to store heat; it depends on their molecular structure and density. The amount of heat required to raise the temperature of a unit mass of a substance by one degree (1 ° C or 1 K) is called its specific heat capacity. Heat capacity is measured in J/(kg K).

Usually distinguish the heat capacity at a constant volume ( C V) and heat capacity at constant pressure ( C P), if during the heating process the volume of the body or the pressure are kept constant, respectively. For example, to heat one gram of air in a balloon by 1 K, more heat is required than to heat it in the same way in a sealed vessel with rigid walls, since part of the energy imparted to the balloon is spent on expanding the air, and not on heating it. When heated at constant pressure, part of the heat goes to the production of work of expansion of the body, and part - to increase its internal energy, while when heated at a constant volume, all heat is spent on increasing internal energy; concerning C R always more than C V. For liquids and solids difference between C R and C V relatively small.

thermal machines

Heat engines are devices that convert heat into useful work. Examples of such machines are compressors, turbines, steam, gasoline and jet engines. One of the most famous heat engines is the steam turbine used in modern thermal power plants. A simplified diagram of such a power plant is shown in Figure 1.

Rice. 1. Simplified diagram of a steam turbine power plant operating on fossil fuels.

The working fluid - water - is converted into superheated steam in a steam boiler heated by burning fossil fuels (coal, oil or natural gas). Steam high pressure rotates the shaft of a steam turbine, which drives a generator that generates electricity. The exhaust steam condenses when cooled by running water, which absorbs part of the heat. Next, the water is fed into the cooling tower (cooling tower), from where part of the heat is released into the atmosphere. The condensate is pumped back to the steam boiler and the whole cycle is repeated.

Another example of a heat engine is a household refrigerator, the diagram of which is shown in Fig. 2.

In refrigerators and domestic air conditioners, energy is supplied from outside to provide it. The compressor increases the temperature and pressure of the working substance of the refrigerator - freon, ammonia or carbon dioxide. The superheated gas is fed into the condenser, where it cools and condenses, giving off heat environment. The liquid leaving the condenser nozzles passes through the throttling valve to the evaporator, and part of it evaporates, which is accompanied by a sharp drop in temperature. The evaporator takes heat from the refrigerator chamber, which heats the working fluid in the nozzles; this liquid is supplied by the compressor to the condenser, and the cycle repeats again.