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The internal energy of the body can change due to the work of external forces. To characterize the change in internal energy during heat transfer, a quantity called the amount of heat and denoted by Q is introduced.
In the international system, the unit of the amount of heat, as well as work and energy, is the joule: = = = 1 J.
In practice, an off-system unit of the amount of heat is sometimes used - a calorie. 1 cal. = 4.2 J.
It should be noted that the term "quantity of heat" is unfortunate. It was introduced at a time when it was believed that bodies contained some weightless, elusive liquid - caloric. The process of heat transfer allegedly consists in the fact that caloric, pouring from one body into another, carries with it a certain amount of heat. Now, knowing the basics of the molecular-kinetic theory of the structure of matter, we understand that there is no caloric in bodies, the mechanism for changing the internal energy of a body is different. However, the power of tradition is great and we continue to use the term, introduced on the basis of incorrect ideas about the nature of heat. At the same time, understanding the nature of heat transfer, one should not completely ignore misconceptions about it. On the contrary, by drawing an analogy between the flow of heat and the flow of a hypothetical fluid of caloric, the amount of heat and the amount of caloric, it is possible, when solving some classes of problems, to visualize the ongoing processes and solve problems correctly. In the end, the correct equations describing the processes of heat transfer were obtained at one time on the basis of incorrect ideas about caloric as a heat carrier.
Let us consider in more detail the processes that can occur as a result of heat transfer.
Pour some water into a test tube and close it with a cork. Hang the test tube to a rod fixed in a tripod and bring an open flame under it. From the flame, the test tube receives a certain amount of heat and the temperature of the liquid in it rises. As the temperature rises, the internal energy of the liquid increases. There is an intensive process of its vaporization. The expanding liquid vapors do mechanical work to push the stopper out of the tube.
Let's conduct another experiment with a model of a cannon made from a piece of brass tube, which is mounted on a trolley. On one side, the tube is tightly closed with an ebonite plug, through which a pin is passed. Wires are soldered to the stud and tube, ending in terminals that can be energized from the lighting network. The gun model is thus a kind of electric boiler.
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Pour some water into the cannon barrel and close the tube with a rubber stopper. Connect the gun to a power source. An electric current passing through water heats it up. Water boils, which leads to its intense vaporization. The pressure of water vapor increases and, finally, they do the work of pushing the cork out of the gun barrel.
The gun, due to recoil, rolls back in the direction opposite to the cork launch.
Both experiences are united by the following circumstances. In the process of heating the liquid in various ways, the temperature of the liquid and, accordingly, its internal energy increased. In order for the liquid to boil and evaporate intensively, it was necessary to continue heating it.
The vapors of the liquid, due to their internal energy, performed mechanical work.
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We investigate the dependence of the amount of heat necessary to heat the body on its mass, temperature changes and the type of substance. To study these dependencies, we will use water and oil. (To measure the temperature in the experiment, an electric thermometer is used, made of a thermocouple connected to a mirror galvanometer. One thermocouple junction is lowered into a vessel with cold water to ensure its temperature is constant. The other thermocouple junction measures the temperature of the liquid under study).
The experience consists of three series. In the first series, for a constant mass of a particular liquid (in our case, water), the dependence of the amount of heat required to heat it on temperature change is studied. The amount of heat received by the liquid from the heater (electric stove) will be judged by the heating time, assuming that there is a directly proportional relationship between them. In order for the result of the experiment to correspond to this assumption, it is necessary to ensure a steady flow of heat from the electric stove to the heated body. To do this, the electric stove was connected to the network in advance, so that by the beginning of the experiment the temperature of its surface would cease to change. For more uniform heating of the liquid during the experiment, we will stir it with the help of the thermocouple itself. We will record the readings of the thermometer at regular intervals until the light spot reaches the edge of the scale.
Let us conclude: there is a direct proportional relationship between the amount of heat required to heat a body and a change in its temperature.
In the second series of experiments, we will compare the amount of heat required to heat the same liquids of different masses when their temperature changes by the same amount.
For the convenience of comparing the obtained values, the mass of water for the second experiment will be taken two times less than in the first experiment.
Again, we will record the thermometer readings at regular intervals.
Comparing the results of the first and second experiments, we can draw the following conclusions.
In the third series of experiments, we will compare the amounts of heat required to heat equal masses of different liquids when their temperature changes by the same amount.
We will heat oil on an electric stove, the mass of which is equal to the mass of water in the first experiment. We will record the thermometer readings at regular intervals.
The result of the experiment confirms the conclusion that the amount of heat necessary to heat the body is directly proportional to the change in its temperature and, in addition, indicates the dependence of this amount of heat on the type of substance.
Since oil was used in the experiment, the density of which is less than the density of water, and a smaller amount of heat was required to heat the oil to a certain temperature than to heat water, it can be assumed that the amount of heat required to heat the body depends on its density.
To test this assumption, we will simultaneously heat identical masses of water, paraffin and copper on a heater of constant power.
After the same time, the temperature of copper is about 10 times, and paraffin is about 2 times higher than the temperature of water.
But copper has a greater and paraffin less density than water.
Experience shows that the quantity that characterizes the rate of change in the temperature of the substances from which the bodies involved in heat exchange are made is not the density. This quantity is called the specific heat capacity of the substance and is denoted by the letter c.
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A special device is used to compare the specific heat capacities of various substances. The device consists of racks in which a thin paraffin plate and a bar with rods passed through it are attached. Aluminum, steel and brass cylinders of equal mass are fixed at the ends of the rods.
We heat the cylinders to the same temperature by immersing them in a vessel of water standing on a hot electric stove. Let's fix the hot cylinders on the racks and release them from the fasteners. The cylinders simultaneously touch the paraffin plate and, melting the paraffin, begin to sink into it. The depth of immersion of cylinders of the same mass into a paraffin plate, when their temperature changes by the same amount, turns out to be different.
Experience shows that the specific heat capacities of aluminum, steel and brass are different.
Having done the corresponding experiments with the melting of solids, the vaporization of liquids, and the combustion of fuel, we obtain the following quantitative dependences.
To obtain units of specific quantities, they must be expressed from the corresponding formulas and the units of heat - 1 J, mass - 1 kg, and for specific heat - and 1 K should be substituted into the resulting expressions.
We get units: specific heat capacity - 1 J / kg K, other specific heats: 1 J / kg.
In this lesson, we will continue to study the internal energy of the body, and more specifically, ways to change it. And the subject of our attention this time will be heat transfer. We will remember what types it is divided into, what it is measured in, and by what ratios it is possible to calculate the amount of heat transferred as a result of heat transfer, we will also give a definition of the specific heat capacity of a body.
Topic: Fundamentals of thermodynamics
Lesson: The amount of heat. Specific heat
As we already know from elementary grades, and as we recalled in the last lesson, there are two ways to change the internal energy of a body: to do work on it or to transfer a certain amount of heat to it. We already know about the first method from, again, the last lesson, but we also talked a lot about the second in the eighth grade course.
The process of transferring heat (the amount of heat or energy) without doing work is called heat transfer or heat transfer. It is divided according to the transmission mechanisms, as we know, into three types:
- Thermal conductivity
- Convection
- Radiation
As a result of one of these processes, a certain amount of heat is transferred to the body, by the value of which, in fact, the internal energy changes. Let's characterize this value.
Definition. Quantity of heat. Designation - Q. Units of measurement - J. When the body temperature changes (which is equivalent to a change in internal energy), the amount of heat spent on this change can be calculated by the formula:
Here: - body weight; - specific heat capacity of the body; - change in body temperature.
Moreover, if, that is, during cooling, they say that the body gave off a certain amount of heat, or a negative amount of heat was transferred to the body. If , that is, heating of the body is observed, the amount of heat transferred, of course, will be positive.
Particular attention should be paid to the value of the specific heat capacity of the body.
Definition. Specific heat- a value numerically equal to the amount of heat that must be transferred in order to heat one kilogram of a substance by one degree. Specific heat capacity is an individual value for each individual substance. Therefore, this is a tabular value, known for sure, provided that we know which portion of the substance heat is transferred.
The SI unit for specific heat capacity can be obtained from the above equation:
In this way:
Let us now consider the cases when the transfer of a certain amount of heat leads to a change in the state of aggregation of the substance. Recall that such transitions are called melting, crystallization, evaporation and condensation.
When moving from a liquid to a solid body and vice versa, the amount of heat is calculated by the formula:
Here: - body weight; - specific heat of fusion of the body (the amount of heat required for the complete melting of one kilogram of a substance).
In order to melt a body, it needs to transfer a certain amount of heat, and during condensation, the body itself gives off a certain amount of heat to the environment.
During the transition from a liquid to a gaseous body and vice versa, the amount of heat is calculated by the formula:
Here: - body weight; - specific heat of vaporization of the body (the amount of heat required for the complete evaporation of one kilogram of a substance).
In order to evaporate a liquid, it needs to transfer a certain amount of heat, and during condensation, the vapor itself gives off a certain amount of heat to the environment.
It should also be emphasized that both melting with crystallization and evaporation with condensation proceed at a constant temperature (melting and boiling points, respectively) (Fig. 1).
Rice. 1. Graph of the dependence of temperature (in degrees Celsius) on the amount of substance received ()
Separately, it is worth noting the calculation of the amount of heat released during the combustion of a certain mass of fuel:
Here: - mass of fuel; - specific heat of combustion of fuel (the amount of heat released during the combustion of one kilogram of fuel).
Particular attention should be paid to the fact that in addition to the fact that specific heat capacities take on different values for different substances, this parameter can be different for the same substance under different conditions. For example, different values of specific heat capacities are distinguished for heating processes occurring at a constant volume () and for processes occurring at a constant pressure ().
A distinction is also made between molar heat capacity and simply heat capacity.
Definition. Molar heat capacity () is the amount of heat required to raise the temperature of one mole of a substance by one degree.
Heat capacity (C) - the amount of heat required to raise a portion of a substance of a certain mass by one degree. Relationship between heat capacity and specific heat capacity:
In the next lesson, we will consider such an important law as the first law of thermodynamics, which relates the change in internal energy to the work of the gas and the amount of heat transferred.
Bibliography
- Myakishev G.Ya., Sinyakov A.Z. Molecular physics. Thermodynamics. - M.: Bustard, 2010.
- Gendenstein L.E., Dick Yu.I. Physics grade 10. - M.: Ileksa, 2005.
- Kasyanov V.A. Physics grade 10. - M.: Bustard, 2010.
- Dictionaries and encyclopedias on Academician ().
- Tt.pstu.ru ().
- Elementy.ru ().
Homework
- Page 83: No. 643-646. Physics. Task book. 10-11 grades. Rymkevich A.P. - M.: Bustard, 2013. ()
- How are molar and specific heat capacities related?
- Why do window surfaces sometimes fog up? Which side of the window is this on?
- In what weather do puddles dry out faster: in calm or windy weather?
- * What is the heat received by the body during melting spent on?
HEAT EXCHANGE.
1.Heat transfer.
Heat exchange or heat transfer is the process of transferring the internal energy of one body to another without doing work.
There are three types of heat transfer.
1) Thermal conductivity is the heat exchange between bodies in direct contact.
2) Convection is heat transfer in which heat is transferred by gas or liquid flows.
3) Radiation is heat transfer by means of electromagnetic radiation.
2. The amount of heat.
The amount of heat is a measure of the change in the internal energy of a body during heat exchange. Denoted by letter Q.
The unit of measurement of the amount of heat = 1 J.
The amount of heat received by a body from another body as a result of heat transfer can be spent on increasing the temperature (increasing the kinetic energy of molecules) or on changing the state of aggregation (increasing potential energy).
3. Specific heat capacity of a substance.
Experience shows that the amount of heat required to heat a body of mass m from temperature T 1 to temperature T 2 is proportional to the body mass m and the temperature difference (T 2 - T 1), i.e.
Q = cm(T 2 - T 1 ) = withmΔ T,
from is called the specific heat capacity of the substance of the heated body.
The specific heat capacity of a substance is equal to the amount of heat that must be imparted to 1 kg of the substance in order to heat it by 1 K.
Unit of specific heat capacity =.
The heat capacity values of various substances can be found in physical tables.
Exactly the same amount of heat Q will be released when the body is cooled by ΔT.
4. Specific heat of vaporization.
Experience shows that the amount of heat required to convert a liquid into vapor is proportional to the mass of the liquid, i.e.
Q = lm,
where is the coefficient of proportionality L is called the specific heat of vaporization.
The specific heat of vaporization is equal to the amount of heat that is necessary to convert 1 kg of liquid at the boiling point into steam.
Unit of measure for the specific heat of vaporization.
In the reverse process, the condensation of steam, heat is released in the same amount that was spent on vaporization.
5. Specific heat of fusion.
Experience shows that the amount of heat required to transform a solid into a liquid is proportional to the mass of the body, i.e.
Q = λ m,
where the coefficient of proportionality λ is called the specific heat of fusion.
The specific heat of fusion is equal to the amount of heat that is necessary to turn a solid body weighing 1 kg into a liquid at the melting point.
Unit of measure for specific heat of fusion.
In the reverse process, the crystallization of a liquid, heat is released in the same amount that was spent on melting.
6. Specific heat of combustion.
Experience shows that the amount of heat released during the complete combustion of the fuel is proportional to the mass of the fuel, i.e.
Q = qm,
Where the proportionality factor q is called the specific heat of combustion.
The specific heat of combustion is equal to the amount of heat that is released during the complete combustion of 1 kg of fuel.
Unit of measure for specific heat of combustion.
7. Heat balance equation.
Two or more bodies are involved in heat exchange. Some bodies give off heat, while others receive it. Heat transfer occurs until the temperatures of the bodies become equal. According to the law of conservation of energy, the amount of heat that is given off is equal to the amount that is received. On this basis, the heat balance equation is written.
Consider an example.
A body of mass m 1 , whose heat capacity is c 1 , has temperature T 1 , and a body of mass m 2 , whose heat capacity is c 2 , has temperature T 2 . Moreover, T 1 is greater than T 2. These bodies are brought into contact. Experience shows that a cold body (m 2) begins to heat up, and a hot body (m 1) begins to cool. This suggests that part of the internal energy of a hot body is transferred to a cold one, and the temperatures even out. Let us denote the final total temperature by θ. 
The amount of heat transferred from a hot body to a cold one
Q transferred. = c 1 m 1 (T 1 – θ )
The amount of heat received by a cold body from a hot one
Q received. = c 2 m 2 (θ – T 2 )
According to the law of conservation of energy Q transferred. = Q received., i.e.
c 1 m 1 (T 1 – θ )= c 2 m 2 (θ – T 2 )
Let us open the brackets and express the value of the total steady-state temperature θ.
The temperature value θ in this case will be obtained in kelvins.
However, since in the expressions for Q passed. and Q is received. if there is a difference between two temperatures, and it is the same in both kelvins and degrees Celsius, then the calculation can be carried out in degrees Celsius. Then
In this case, the temperature value θ will be obtained in degrees Celsius.
The equalization of temperatures as a result of heat conduction can be explained on the basis of molecular kinetic theory as an exchange of kinetic energy between molecules during collision in the process of thermal chaotic motion.
This example can be illustrated with a graph. 
>>Physics: Quantity of heat
It is possible to change the internal energy of the gas in the cylinder not only by doing work, but also by heating the gas.
If you fix the piston ( fig.13.5), then the volume of the gas does not change when heated and no work is done. But the temperature of the gas, and hence its internal energy, increases.
The process of transferring energy from one body to another without doing work is called heat exchange or heat transfer.
The quantitative measure of the change in internal energy during heat transfer is called amount of heat. The amount of heat is also called the energy that the body gives off in the process of heat transfer.
Molecular picture of heat transfer
During heat exchange, there is no conversion of energy from one form to another; part of the internal energy of a hot body is transferred to a cold body.
The amount of heat and heat capacity. You already know that to heat a body with a mass m temperature t1 up to temperature t2 it is necessary to transfer the amount of heat to it:
When a body cools, its final temperature t2 is less than the initial temperature t1 and the amount of heat given off by the body is negative.
Coefficient c in formula (13.5) is called specific heat substances. Specific heat capacity is a value numerically equal to the amount of heat that a 1 kg substance receives or gives off when its temperature changes by 1 K.
The specific heat capacity depends not only on the properties of the substance, but also on the process by which heat transfer takes place. If you heat a gas at constant pressure, it will expand and do work. To heat a gas by 1°C at constant pressure, it needs to transfer more heat than to heat it at a constant volume, when the gas will only heat up.
Liquids and solids expand slightly when heated. Their specific heat capacities at constant volume and constant pressure differ little.
Specific heat of vaporization. To convert a liquid into vapor during the boiling process, it is necessary to transfer a certain amount of heat to it. The temperature of a liquid does not change when it boils. The transformation of a liquid into vapor at a constant temperature does not lead to an increase in the kinetic energy of molecules, but is accompanied by an increase in the potential energy of their interaction. After all, the average distance between gas molecules is much greater than between liquid molecules.
The value numerically equal to the amount of heat required to convert a 1 kg liquid into steam at a constant temperature is called specific heat of vaporization. This value is denoted by the letter r and is expressed in joules per kilogram (J/kg).
The specific heat of vaporization of water is very high: rH2O\u003d 2.256 10 6 J / kg at a temperature of 100 ° C. In other liquids, for example, alcohol, ether, mercury, kerosene, the specific heat of vaporization is 3-10 times less than that of water.
To transform a liquid into a mass m steam requires an amount of heat equal to:
When steam condenses, the same amount of heat is released:
A value numerically equal to the amount of heat required to convert a crystalline substance weighing 1 kg at a melting point into a liquid is called specific heat of fusion.
During the crystallization of a substance with a mass of 1 kg, exactly the same amount of heat is released as is absorbed during melting.
The specific heat of melting of ice is rather high: 3.34 10 5 J/kg. “If ice did not have a high heat of fusion,” wrote R. Black back in the 18th century, “then in spring the entire mass of ice would have to melt in a few minutes or seconds, since heat is continuously transferred to ice from the air. The consequences of this would be dire; for even under the present situation great floods and great torrents of water arise from the melting of great masses of ice or snow.”
In order to melt a crystalline body with a mass m, the amount of heat required is:
The amount of heat released during the crystallization of the body is equal to:
The internal energy of a body changes during heating and cooling, during vaporization and condensation, during melting and crystallization. In all cases, a certain amount of heat is transferred to or removed from the body.
???
1. What is called quantity warmth?
2. What does the specific heat capacity of a substance depend on?
3. What is called the specific heat of vaporization?
4. What is called the specific heat of fusion?
5. In what cases is the amount of heat a positive value, and in what cases is it negative?
G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10
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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 heat.
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 of a given body, the greater the amount of heat should be expended in order to lower or increase 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 various types of 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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