The formula for the amount of heat when a body is cooled. How to calculate the amount of heat, thermal effect and heat of formation
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,
With is called the specific heat capacity of the substance of the heated body.
Specific heat of a substance is equal to the amount of heat that must be imparted to 1 kg of a 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 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. Equation heat balance.
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 thermal conduction can be explained on the basis of molecular kinetic theory as an exchange kinetic energy between molecules when colliding in the process of thermal chaotic motion.
This example can be illustrated with a graph. 
>>Physics: Calculation of the amount of heat required to heat the body and released by it during cooling
To learn how to calculate the amount of heat that is necessary to heat the body, we first establish on what quantities it depends.
From the previous paragraph, we already know that this amount of heat depends on the kind of substance that the body consists of (i.e., its specific heat capacity):
Q depends on c
But that's not all.
If we want to heat the water in the kettle so that it becomes only warm, then we will not heat it for long. And in order for the water to become hot, we will heat it longer. But the longer the kettle is in contact with the heater, the more heat it will receive from it.
Therefore, the more the temperature of the body changes during heating, the more heat must be transferred to it.
Let the initial temperature of the body be equal to tini, and the final temperature - tfin. Then the change in body temperature will be expressed by the difference:
Finally, everyone knows that for heating, for example, 2 kg of water takes more time (and therefore more heat) than it takes to heat 1 kg of water. This means that the amount of heat required to heat up a body depends on the mass of that body:
So, to calculate the amount of heat, you need to know the specific heat capacity of the substance from which the body is made, the mass of this body and the difference between its final and initial temperatures.
Let, for example, it is required to determine how much heat is needed to heat an iron part with a mass of 5 kg, provided that its initial temperature is 20 °C, and the final temperature should be 620 °C.
From table 8 we find that the specific heat capacity of iron is c = 460 J/(kg°C). This means that it takes 460 J to heat 1 kg of iron by 1 °C.
To heat 5 kg of iron by 1 ° C, it will take 5 times more quantity heat, i.e. 460 J * 5 = 2300 J.
To heat iron not by 1 °C, but by A t \u003d 600 ° C, another 600 times more heat will be required, i.e. 2300 J X 600 \u003d 1 380 000 J. Exactly the same (modulo) amount of heat will be released when this iron cools from 620 to 20 ° C.
So, to find the amount of heat necessary to heat the body or released by it during cooling, you need to multiply the specific heat of the body by its mass and by the difference between its final and initial temperatures: 
??? 1. Give examples showing that the amount of heat received by a body when heated depends on its mass and temperature changes. 2. By what formula is the amount of heat required to heat the body or released by it when cooling?
S.V. Gromov, N.A. Motherland, Physics Grade 8
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The heat capacity of a body is indicated by capital letters Latin letter FROM.
What determines the heat capacity of a body? First of all, from its mass. It is clear that heating, for example, 1 kilogram of water will require more heat than heating 200 grams.
What about the kind of substance? Let's do an experiment. Let's take two identical vessels and, pouring 400 g of water into one of them, and into the other - vegetable oil weighing 400 g, we will start heating them with the help of identical burners. By observing the readings of thermometers, we will see that the oil heats up quickly. To heat water and oil to the same temperature, the water must be heated longer. But the longer we heat the water, the more heat it receives from the burner.
Thus, to heat the same mass of different substances to the same temperature, it takes different amount warmth. The amount of heat required to heat a body and, consequently, its heat capacity depend on the kind of substance of which this body is composed.
So, for example, to increase the temperature of 1 kg water by 1°C, an amount of heat equal to 4200 J is required, and to heat the same mass of sunflower oil by 1°C, an amount of heat equal to 1700 J is required.
The physical quantity showing how much heat is required to heat 1 kg of a substance by 1 ºС is called specific heat this substance.
Each substance has its own specific heat capacity, which is denoted by the Latin letter c and is measured in joules per kilogram-degree (J / (kg ° C)).
The specific heat capacity of the same substance in different aggregate states (solid, liquid and gaseous) is different. For example, the specific heat capacity of water is 4200 J/(kg ºС), and the specific heat capacity of ice is 2100 J/(kg ºС); aluminum in the solid state has a specific heat capacity of 920 J / (kg - ° C), and in the liquid state - 1080 J / (kg - ° C).
Note that water has a very high specific heat capacity. Therefore, the water in the seas and oceans, heating up in summer, absorbs from the air a large number of heat. Due to this, in those places that are located near large bodies of water, summer is not as hot as in places far from water.
Calculation of the amount of heat required to heat the body or released by it during cooling.
From the foregoing, it is clear that the amount of heat necessary to heat the body depends on the type of substance of which the body consists (i.e., its specific heat capacity) and on the mass of the body. It is also clear that the amount of heat depends on how many degrees we are going to increase the temperature of the body.
So, to determine the amount of heat required to heat the body or released by it during cooling, you need to multiply the specific heat of the body by its mass and the difference between its final and initial temperatures:
Q= cm (t 2 -t 1),
where Q- quantity of heat, c- specific heat capacity, m- body mass, t1- initial temperature, t2- final temperature.
When the body is heated t2> t1 and hence Q >0 . When the body is cooled t 2and< t1 and hence Q< 0 .
If the heat capacity of the whole body is known FROM, Q is determined by the formula: Q \u003d C (t 2 - t1).
22) Melting: definition, calculation of the amount of heat for melting or solidification, specific heat of melting, graph of t 0 (Q).
Thermodynamics
Chapter molecular physics, which studies the transfer of energy, the patterns of transformation of some types of energy into others. Unlike the molecular-kinetic theory, thermodynamics does not take into account the internal structure of substances and microparameters.
Thermodynamic system
This is a collection of bodies that exchange energy (in the form of work or heat) with each other or with environment. For example, the water in the teapot cools down, the exchange of heat of the water with the teapot and of the teapot with the environment takes place. Cylinder with gas under the piston: the piston performs work, as a result of which the gas receives energy and its macro parameters change.
Quantity of heat
it energy, which is received or given by the system in the process of heat exchange. Denoted by the symbol Q, measured, like any energy, in Joules.
As a result of various heat transfer processes, the energy that is transferred is determined in its own way.
Heating and cooling
This process is characterized by a change in the temperature of the system. The amount of heat is determined by the formula
The specific heat capacity of a substance with measured by the amount of heat required to heat up mass units of this substance by 1K. To heat 1kg of glass or 1kg of water, different quantity energy. Specific heat capacity is a known value already calculated for all substances, see the value in physical tables.
Heat capacity of substance C- this is the amount of heat that is necessary to heat the body without taking into account its mass by 1K.
Melting and crystallization
Melting is the transition of a substance from a solid to a liquid state. The reverse transition is called crystallization.
The energy spent on the destruction of the crystal lattice of a substance is determined by the formula
The specific heat of fusion is a known value for each substance, see the value in the physical tables.
Vaporization (evaporation or boiling) and condensation
Vaporization is the transition of a substance from a liquid (solid) state to a gaseous state. The reverse process is called condensation.
The specific heat of vaporization is a known value for each substance, see the value in the physical tables.
Combustion
The amount of heat released when a substance burns
The specific heat of combustion is a known value for each substance, see the value in the physical tables.
For a closed and adiabatically isolated system of bodies, the heat balance equation is satisfied. The algebraic sum of the amounts of heat given and received by all bodies participating in heat exchange is equal to zero:
Q 1 +Q 2 +...+Q n =0
23) The structure of liquids. surface layer. Surface tension force: examples of manifestation, calculation, surface tension coefficient.
From time to time, any molecule can move to an adjacent vacancy. Such jumps in liquids occur quite frequently; therefore, the molecules are not tied to certain centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely spaced molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called short-range order(Fig. 3.5.1).
The coefficient β is called temperature coefficient volume expansion . This coefficient for liquids is ten times greater than for solids. For water, for example, at a temperature of 20 ° C, β in ≈ 2 10 - 4 K - 1, for steel β st ≈ 3.6 10 - 5 K - 1, for quartz glass β kv ≈ 9 10 - 6 K - one .
The thermal expansion of water has an interesting and important anomaly for life on Earth. At temperatures below 4 °C, water expands with decreasing temperature (β< 0). Максимум плотности ρ в = 10 3 кг/м 3 вода имеет при температуре 4 °С.
When water freezes, it expands, so the ice remains floating on the surface of the freezing body of water. The temperature of freezing water under ice is 0°C. In denser layers of water near the bottom of the reservoir, the temperature is about 4 °C. Thanks to this, life can exist in the water of freezing reservoirs.
Most interesting feature liquids is the presence free surface . Liquid, unlike gases, does not fill the entire volume of the vessel into which it is poured. An interface is formed between a liquid and a gas (or vapor), which is located in special conditions compared to the rest of the liquid mass. It should be borne in mind that, due to the extremely low compressibility, the presence of a more densely packed surface layer does not lead to any noticeable change in the volume of the liquid. If the molecule moves from the surface into the liquid, the forces of intermolecular interaction will do positive work. On the contrary, in order to pull a certain number of molecules from the depth of the liquid to the surface (i.e., increase the surface area of the liquid), external forces must do a positive work Δ A external, proportional to the change Δ S surface area:
It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy. It follows that the free surface of the liquid tends to reduce its area. For this reason, a free drop of liquid takes on a spherical shape. The fluid behaves as if forces are acting tangentially to its surface, reducing (contracting) this surface. These forces are called surface tension forces .
The presence of surface tension forces makes the liquid surface look like an elastic stretched film, with the only difference that the elastic forces in the film depend on its surface area (i.e., on how the film is deformed), and the surface tension forces do not depend on the surface area of the liquid.
Some liquids, such as soapy water, have the ability to form thin films. All well-known soap bubbles have the correct spherical shape - this also manifests the action of surface tension forces. If a wire frame is lowered into the soapy solution, one of the sides of which is movable, then the whole of it will be covered with a film of liquid (Fig. 3.5.3).
Surface tension forces tend to shorten the surface of the film. To balance the movable side of the frame, you need to attach to it external force If, under the action of a force, the crossbar moves Δ x, then the work Δ A ext = F ext Δ x = Δ Ep = σΔ S, where ∆ S = 2LΔ x is the increment in the surface area of both sides of the soap film. Since the moduli of forces and are the same, we can write:
|
Thus, the surface tension coefficient σ can be defined as modulus of the surface tension force acting per unit length of the line bounding the surface.
Due to the action of surface tension forces in liquid droplets and inside soap bubbles overpressure occurs Δ p. If we mentally cut a spherical drop of radius R into two halves, then each of them must be in equilibrium under the action of surface tension forces applied to the boundary of the cut with a length of 2π R and overpressure forces acting on the area π R 2 sections (Fig. 3.5.4). The equilibrium condition is written as
If these forces are greater than the forces of interaction between the molecules of the liquid itself, then the liquid wets surface solid body. In this case, the liquid approaches the surface of the solid under some acute angleθ, characteristic for a given pair of liquid - solid. The angle θ is called contact angle . If the interaction forces between liquid molecules exceed the forces of their interaction with solid molecules, then the contact angle θ turns out to be obtuse (Fig. 3.5.5). In this case, the liquid is said to does not wet the surface of a solid body. At complete wettingθ = 0, at complete non-wettingθ = 180°.
capillary phenomena called the rise or fall of fluid in small diameter tubes - capillaries. Wetting liquids rise through the capillaries, non-wetting liquids descend.
On fig. 3.5.6 shows a capillary tube of a certain radius r lowered by the lower end into a wetting liquid of density ρ. The upper end of the capillary is open. The rise of the liquid in the capillary continues until the force of gravity acting on the liquid column in the capillary becomes equal in absolute value to the resulting F n surface tension forces acting along the boundary of contact of the liquid with the surface of the capillary: F t = F n, where F t = mg = ρ hπ r 2 g, F n = σ2π r cos θ.
This implies:
With complete nonwetting, θ = 180°, cos θ = –1 and, therefore, h < 0. Уровень несмачивающей жидкости в капилляре опускается ниже уровня жидкости в сосуде, в которую опущен капилляр.
Water almost completely wets the clean glass surface. Conversely, mercury does not completely wet the glass surface. Therefore, the level of mercury in the glass capillary falls below the level in the vessel.
24) Vaporization: definition, types (evaporation, boiling), calculation of the amount of heat for vaporization and condensation, specific heat of vaporization.
Evaporation and condensation. Explanation of the phenomenon of evaporation based on ideas about the molecular structure of matter. Specific heat of vaporization. Her units.
The phenomenon of liquid turning into vapor is called vaporization.
Evaporation - the process of vaporization occurring from an open surface.
Molecules in a liquid move with different speeds. If any molecule is at the surface of the liquid, it can overcome the attraction of neighboring molecules and fly out of the liquid. The escaping molecules form vapor. The velocities of the remaining liquid molecules change upon collision. In this case, some molecules acquire a speed sufficient to fly out of the liquid. This process continues, so liquids evaporate slowly.
*Evaporation rate depends on the type of liquid. Those liquids evaporate faster, in which the molecules are attracted with less force.
*Evaporation can occur at any temperature. But at high temperatures evaporation is faster .
*Evaporation rate depends on its surface area.
*With wind (air flow), evaporation occurs faster.
During evaporation, the internal energy decreases, because. when the liquid evaporates, fast molecules leave, therefore, average speed other molecules decreases. This means that if there is no influx of energy from outside, then the temperature of the liquid decreases.
The phenomenon of the transformation of vapor into liquid is called condensation.
It is accompanied by the release of energy.
Vapor condensation explains the formation of clouds. Water vapor rising above the ground forms clouds in the upper cold layers of air, which consist of tiny drops of water.
Specific heat of vaporization - physical. a quantity indicating how much heat is required to turn a liquid of mass 1 kg into vapor without changing the temperature.
Oud. heat of vaporization denoted by the letter L and is measured in J / kg
Oud. heat of vaporization of water: L=2.3×10 6 J/kg, alcohol L=0.9×10 6
The amount of heat required to turn a liquid into steam: Q = Lm
To learn how to calculate the amount of heat that is necessary to heat the body, we first establish on what quantities it depends.
From the previous paragraph, we already know that this amount of heat depends on the kind of substance of which the body consists (i.e., its specific heat capacity):
Q depends on c .
But that's not all.
If we want to heat the water in the kettle so that it becomes only warm, then we will not heat it for long. And in order for the water to become hot, we will heat it longer. But the longer the kettle is in contact with the heater, the more heat it will receive from it. Therefore, the more the temperature of the body changes during heating, the more heat must be transferred to it.
Let the initial temperature of the body be equal to t initial, and the final temperature - t final. Then the change in body temperature will be expressed by the difference
Δt = t end - t start,
and the amount of heat will depend on this value:
Q depends on Δt.
Finally, everyone knows that heating, for example, 2 kg of water takes more time (and, therefore, more heat) than heating 1 kg of water. This means that the amount of heat required to heat up a body depends on the mass of that body:
Q depends on m.
So, to calculate the amount of heat, you need to know the specific heat capacity of the substance from which the body is made, the mass of this body and the difference between its final and initial temperatures.
Let, for example, it is required to determine how much heat is needed to heat an iron part with a mass of 5 kg, provided that its initial temperature is 20 °C, and the final temperature should be 620 °C.
From table 8 we find that the specific heat capacity of iron is c = 460 J/(kg*°C). This means that it takes 460 J to heat 1 kg of iron by 1 °C.
To heat 5 kg of iron by 1 ° C, it will take 5 times more heat, i.e. 460 J * 5 \u003d 2300 J.
To heat iron not by 1 °C, but by Δt = 600 °C, it will take another 600 times more heat, i.e. 2300 J * 600 = 1,380,000 J. Exactly the same (modulo) amount of heat will be released and when this iron is cooled from 620 to 20 °C.
So, to find the amount of heat required to heat the body or released by it during cooling, you need to multiply the specific heat of the body by its mass and by the difference between its final and initial temperatures:
When the body is heated, tcon > tini and, therefore, Q > 0. When the body is cooled, tcon< t нач и, следовательно, Q < 0.
1. Give examples showing that the amount of heat received by a body when heated depends on its mass and temperature changes. 2. What formula is used to calculate the amount of heat required to heat the body or released by it during cooling?
The concept of the amount of heat was formed in the early stages of the development of modern physics, when there were no clear ideas about internal structure matter, about what energy is, about what forms of energy exist in nature and about energy as a form of movement and transformation of matter.
The amount of heat is understood as a physical quantity equivalent to the energy transferred to the material body in the process of heat exchange.
The obsolete unit of the amount of heat is the calorie, equal to 4.2 J, today this unit is practically not used, and the joule has taken its place.
Initially, it was assumed that the carrier of thermal energy is some completely weightless medium that has the properties of a liquid. Numerous physical problems of heat transfer have been and are still being solved based on this premise. The existence of a hypothetical caloric was taken as the basis for many essentially correct constructions. It was believed that caloric is released and absorbed in the phenomena of heating and cooling, melting and crystallization. The correct equations for heat transfer processes were obtained from incorrect physical concepts. There is a known law according to which the amount of heat is directly proportional to the mass of the body involved in heat exchange and the temperature gradient:
Where Q is the amount of heat, m is the mass of the body, and the coefficient With- a quantity called specific heat capacity. Specific heat capacity is a characteristic of the substance involved in the process.
Work in thermodynamics
As a result of thermal processes, purely mechanical work can be performed. For example, when heated, a gas increases its volume. Let's take a situation as in the figure below:
In this case, the mechanical work will be equal to the gas pressure force on the piston multiplied by the path traveled by the piston under pressure. Of course, this is the simplest case. But even in it one can notice one difficulty: the pressure force will depend on the volume of gas, which means that we are not dealing with constants, but with variables. Since all three variables: pressure, temperature and volume are related to each other, the calculation of work becomes much more complicated. There are some ideal, infinitely slow processes: isobaric, isothermal, adiabatic and isochoric - for which such calculations can be performed relatively simply. A plot of pressure versus volume is plotted, and work is calculated as an integral of the form.