The amount of heat by volume. Amount of heat: concept, calculations, application
>>Physics: Amount of heat
You can 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 therefore its internal energy are increasing.
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 a body gives off during heat exchange.
Molecular picture of heat transfer
During heat exchange, energy is not converted from one form to another; part of the internal energy of the hot body is transferred to the cold body.
Amount of heat and heat capacity. You already know that to heat a body of mass m on temperature t 1 up to temperature t 2 it is necessary to transfer the amount of heat to it:
When a body cools down, its final temperature is t 2 turns out to be less than the initial temperature t 1 and the amount of heat given off by the body is negative.
Coefficient c in formula (13.5) is called specific heat capacity substances. Specific heat capacity is a value numerically equal to the amount of heat that a substance weighing 1 kg receives or releases when its temperature changes by 1 K.
Specific heat capacity depends not only on the properties of the substance, but also on the process by which heat transfer occurs. 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 transform a liquid into steam during the boiling process, a certain amount of heat must be transferred to it. The temperature of a liquid does not change when it boils. Conversion of liquid into vapor when 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.
A quantity numerically equal to the amount of heat required to convert a liquid weighing 1 kg into steam at a constant temperature is called specific heat of vaporization. This value is denoted by the letter r and are expressed in joules per kilogram (J/kg).
The specific heat of vaporization of water is very high: r H2O=2.256 10 6 J/kg at a temperature of 100°C. For other liquids, for example alcohol, ether, mercury, kerosene, the specific heat of vaporization is 3-10 times less than that of water.
To transform liquid into 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 transform a crystalline substance weighing 1 kg at the melting point into a liquid is called specific heat of fusion.
When a substance weighing 1 kg crystallizes, exactly the same amount of heat is released as is absorbed during melting.
The specific heat of melting of ice is quite 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 the spring the entire mass of ice should have melted in a few minutes or seconds, since heat is continuously transferred to ice from the air. The consequences of this would be dire; after all, even in the current situation, large floods and strong flows of water arise when large masses of ice or snow melt.”
In order to melt a crystalline body weighing m, the required amount of heat is equal to:
The amount of heat released during crystallization of a 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 the specific heat of fusion called?
5. In what cases is the amount of heat a positive quantity, and in what cases is it negative?
G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade
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HEAT EXCHANGE.
1. Heat exchange.
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- This is heat exchange between bodies during their direct contact.
2) Convection- This is heat exchange in which heat is transferred by gas or liquid flows.
3) Radiation– This is heat exchange through electromagnetic radiation.
2. 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 the letter Q.
Unit for measuring the amount of heat = 1 J.
The amount of heat received by a body from another body as a result of heat exchange can be spent on increasing temperature (increasing the kinetic energy of molecules) or changing the state of aggregation (increasing potential energy).
3.Specific heat capacity of the 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 mass of the body m and the temperature difference (T 2 - T 1), i.e.
Q = cm(T 2 - T 1 ) = smΔ T,
With 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 to heat it by 1 K.
Unit of measurement of specific heat capacity =.
The heat capacity values for 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 steam is proportional to the mass of the liquid, i.e.
Q = Lm,
where is the proportionality coefficient L is called the specific heat of vaporization.
The specific heat of vaporization is equal to the amount of heat required to convert 1 kg of liquid at boiling point into steam.
A unit of measurement for the specific heat of vaporization.
During the reverse process, steam condensation, heat is released in the same amount that was spent on steam formation.
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 proportionality coefficient λ is called the specific heat of fusion.
The specific heat of fusion is equal to the amount of heat that is necessary to transform a solid body weighing 1 kg into a liquid at the melting point.
A unit of measurement for the specific heat of fusion.
During the reverse process, crystallization of the 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 complete combustion of fuel is proportional to the mass of the fuel, i.e.
Q = qm,
Where the proportionality coefficient q is called the specific heat of combustion.
The specific heat of combustion is equal to the amount of heat released during complete combustion of 1 kg of fuel.
Unit of measurement of specific heat of combustion.
7. Heat balance equation.
Heat exchange involves two or more bodies. Some bodies give off heat, while others receive it. Heat exchange occurs until the temperatures of the bodies become equal. According to the law of conservation of energy, the amount of heat that is given out is equal to the amount that is received. On this basis, the heat balance equation is written.
Let's look at an example.
A body of mass m 1, the heat capacity of which is c 1, has a temperature T 1, and a body of mass m 2, the heat capacity of which is c 2, has a 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 the hot body is transferred to the cold one, and the temperatures are equalized. Let us denote the final overall 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's open the brackets and express the value of the total steady-state temperature θ.
In this case, we obtain the temperature value θ in kelvins.
However, since Q is passed in the expressions. and Q is received. is the difference between two temperatures, and it is the same both in Kelvin and in degrees Celsius, then the calculation can be carried out in degrees Celsius. Then
In this case, we obtain the temperature value θ in degrees Celsius.
The equalization of temperatures as a result of thermal conductivity can be explained on the basis of molecular kinetic theory as the exchange of kinetic energy between molecules upon collision in the process of thermal chaotic motion.
This example can be illustrated with a graph. 
The concept of the amount of heat was formed on early stages development modern physics, when there were no clear ideas about the internal structure of 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 a material body in the process of heat exchange.
The obsolete unit of heat is the calorie, equal to 4.2 J, today this unit is practically not used, and its place has been taken by the joule.
Initially, it was assumed that the carrier of thermal energy was some completely weightless medium with 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 hypothetical caloric was 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 based on incorrect physical concepts. There is a known law according to which the amount of heat is directly proportional to the mass of the body participating in heat exchange and the temperature gradient:
Where Q is the amount of heat, m body mass, and the coefficient With– a quantity called specific heat capacity. Specific heat capacity is a characteristic of a substance involved in a process.
Work in thermodynamics
As a result of thermal processes, clean mechanical work. For example, when a gas heats up, it increases its volume. Let's take a situation like the picture below:
In this case, the mechanical work will be equal to the force of gas pressure on the piston multiplied by the path traveled by the piston under pressure. Of course this simplest case. But even in it one can notice one difficulty: the pressure force will depend on the volume of the gas, which means that we are not dealing with constants, but with variable quantities. Since all three variables: pressure, temperature and volume are related to each other, calculating work becomes significantly more complicated. There are some ideal, infinitely slow processes: isobaric, isothermal, adiabatic and isochoric - for which such calculations can be performed relatively simply. A graph of pressure versus volume is plotted and the work is calculated as an integral of the form.
What will heat up faster on the stove - a kettle or a bucket of water? The answer is obvious - a teapot. Then the second question is why?
The answer is no less obvious - because the mass of water in the kettle is less. Great. And now you can do the real thing yourself physical experience at home. To do this you will need two identical small saucepans, equal amount water and vegetable oil, for example, half a liter and a stove. Place saucepans with oil and water on the same heat. Now just watch what will heat up faster. If you have a thermometer for liquids, you can use it; if not, you can simply test the temperature with your finger from time to time, just be careful not to get burned. In any case, you will soon see that the oil heats up significantly faster than water. And one more question, which can also be implemented in the form of experience. Which will boil faster - warm water or cold? Everything is obvious again - the warm one will be first at the finish line. Why all these strange questions and experiments? To determine the physical quantity called “amount of heat”.
Quantity of heat
The amount of heat is the energy that a body loses or gains during heat transfer. This is clear from the name. When cooling, the body will lose a certain amount of heat, and when heating, it will absorb. And the answers to our questions showed us What does the amount of heat depend on? Firstly, the greater the mass of a body, the greater the amount of heat that must be expended to change its temperature by one degree. Secondly, the amount of heat required to heat a body depends on the substance of which it consists, that is, on the type of substance. And thirdly, the difference in body temperature before and after heat transfer is also important for our calculations. Based on the above, we can determine the amount of heat using the formula:
Q=cm(t_2-t_1) ,
where Q is the amount of heat,
m - body weight,
(t_2-t_1) - difference between initial and final body temperatures,
c is the specific heat capacity of the substance, found from the corresponding tables.
Using this formula, you can calculate the amount of heat that is necessary to heat any body or that this body will release when cooling.
The amount of heat is measured in joules (1 J), like any type of energy. However, this value was introduced not so long ago, and people began measuring the amount of heat much earlier. And they used a unit that is widely used in our time - calorie (1 cal). 1 calorie is the amount of heat required to heat 1 gram of water by 1 degree Celsius. Guided by these data, those who like to count calories in the food they eat can, just for fun, calculate how many liters of water can be boiled with the energy they consume with food during the day.
Heat capacity- this is the amount of heat absorbed by the body when heated by 1 degree.
The heat capacity of a body is indicated by a capital Latin letter WITH.
What does the heat capacity of a body depend on? First of all, from its mass. It is clear that to heat, for example, 1 kilogram of water will be required more heat than for heating 200 grams.
What about the type of substance? Let's do an experiment. Let's take two identical vessels and pour water weighing 400 g into one of them, and into the other - vegetable oil weighing 400 g, let's start heating them using identical burners. By observing the thermometer readings, 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 is required different quantities warmth. The amount of heat required to heat a body and, therefore, its heat capacity depend on the type of substance of which the body is composed.
So, for example, to increase the temperature of water weighing 1 kg by 1°C, an amount of heat equal to 4200 J is required, and to heat the same mass by 1°C sunflower oil the amount of heat required is 1700 J.
Physical quantity showing how much heat is required to heat 1 kg of a substance by 1 ºС is called specific heat capacity of this substance.
Each substance has its own specific heat capacity, which is denoted by the Latin letter c and measured in joules per kilogram degree (J/(kg °C)).
The specific heat capacity of the same substance in different states of aggregation (solid, liquid and gaseous) is different. For example, the specific heat capacity of water is 4200 J/(kg °C), and the specific heat capacity of ice is 2100 J/(kg °C); 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, water in the seas and oceans, heating up in summer, absorbs from the air a large number of heat. Thanks to this, in those places that are located near large bodies of water, summer is not as hot as in places far from the water.
Calculation of the amount of heat required to heat a body or released by it during cooling.
From the above it is clear that the amount of heat required to heat a 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 body temperature.
So, to determine the amount of heat required to heat a body or released by it during cooling, you need to multiply the specific heat capacity of the body by its mass and by 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, t 1- initial temperature, t 2- final temperature.
When the body heats up t 2> t 1 and therefore Q >0 . When the body cools down t 2i< t 1 and therefore Q< 0 .
If the heat capacity of the entire body is known WITH, Q determined by the formula: Q = C (t 2 - t 1).
22) Melting: definition, calculation of the amount of heat for melting or solidification, specific heat of fusion, graph of t 0 (Q).
Thermodynamics
A branch of molecular physics that studies the transfer of energy, the patterns of transformation of one type of energy into another. Unlike molecular kinetic theory, thermodynamics does not take into account internal structure substances and microparameters.
Thermodynamic system
It 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 kettle cools down, and heat is exchanged between the water and the kettle and the heat of the kettle with the environment. A cylinder with gas under the piston: the piston performs work, as a result of which the gas receives energy and its macroparameters change.
Quantity of heat
This energy, which the system receives or releases during the heat exchange process. Denoted by the symbol Q, it is measured, like any energy, in Joules.
As a result of various heat exchange 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
Specific heat capacity of a substance with measured by the amount of heat required to warm up units of mass of this substance by 1K. To heat 1 kg of glass or 1 kg of water is required different quantity energy. Specific heat capacity is a known quantity, 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 a 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 that is 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 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 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 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 may move to a nearby vacant location. Such jumps in liquids occur quite often; therefore, the molecules are not tied to specific centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Because of strong interaction between closely spaced molecules they can form local (unstable) ordered groups containing several molecules. This phenomenon is called close order(Fig. 3.5.1).
The coefficient β is called temperature coefficient of volumetric expansion . This coefficient for liquids is tens of 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 - 1 .
The thermal expansion of water has an interesting and important anomaly for life on Earth. At temperatures below 4 °C, water expands as the temperature decreases (β< 0). Максимум плотности ρ в = 10 3 кг/м 3 вода имеет при температуре 4 °С.
When water freezes, it expands, so ice remains floating on the surface of a freezing body of water. The temperature of freezing water under the ice is 0 °C. In denser layers of water at 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 container into which it is poured. An interface is formed between liquid and 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 a 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 depths of the liquid to the surface (i.e., increase the surface area of the liquid), external forces must perform positive work Δ A external, proportional to the change Δ S surface area:
It is known from mechanics that the equilibrium states of the system correspond to minimum value 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 liquid behaves as if forces acting tangentially to its surface are contracting (pulling) this surface. These forces are called surface tension forces .
The presence of surface tension forces makes the surface of a liquid 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. Well-known soap bubbles have a regular spherical shape - this also shows the effect of surface tension forces. If in soap solution lower the wire frame, one of the sides of which is movable, then the entire frame will be covered with a film of liquid (Fig. 3.5.3).
Surface tension forces tend to reduce the surface of the film. To balance the movable side of the frame, an external force must be applied to it. If, under the influence of force, the crossbar moves by Δ x, then work Δ will be performed A vn = F vn Δ x = Δ E p = σΔ S, where Δ S = 2LΔ x– 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 drops of liquid and inside soap bubbles, excess pressure Δ arises p. If you 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 cut boundary of length 2π R and excess pressure 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. In this case, the liquid approaches the surface of the solid at a certain acute angle θ, characteristic of a given liquid-solid pair. The angle θ is called contact angle . If the forces of interaction 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 they say that the liquid does not wet surface of a solid. At complete wettingθ = 0, at complete non-wettingθ = 180°.
Capillary phenomena called the rise or fall of liquid in small diameter tubes - capillaries. Wetting liquids rise through the capillaries, non-wetting liquids descend.
In Fig. 3.5.6 shows a capillary tube of a certain radius r, lowered at the lower end into a wetting liquid of density ρ. The upper end of the capillary is open. The rise of liquid in the capillary continues until the force of gravity acting on the column of liquid in the capillary becomes equal in magnitude to the resultant 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 non-wetting 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. On the contrary, mercury does not completely wet the glass surface. Therefore, the level of mercury in the glass capillary drops 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 molecular structure substances. Specific heat of vaporization. Its units.
The phenomenon of turning a liquid into vapor is called vaporization.
Evaporation - the process of vaporization occurring from an open surface.
Liquid molecules move with at different speeds. If any molecule ends up at the surface of a liquid, it can overcome the attraction of neighboring molecules and fly out of the liquid. The ejected molecules form steam. The remaining molecules of the liquid change speed upon collision. At the same time, some molecules acquire a speed sufficient to fly out of the liquid. This process continues so the liquids evaporate slowly.
*The rate of evaporation depends on the type of liquid. Those liquids whose molecules are attracted with less force evaporate faster.
*Evaporation can occur at any temperature. But when high temperatures evaporation occurs faster .
*The rate of evaporation depends on its surface area.
*With wind (air flow), evaporation occurs faster.
During evaporation, the internal energy decreases, because During evaporation, the liquid leaves fast molecules, therefore, average speed remaining molecules decreases. This means that if there is no influx of energy from outside, then the temperature of the liquid decreases.
The phenomenon of vapor turning into liquid is called condensation.
It is accompanied by the release of energy.
Steam 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 value showing how much heat is needed to convert a liquid weighing 1 kg into steam without changing temperature.
Ud. heat of vaporization denoted by the letter L and measured in J/kg
Ud. heat of vaporization of water: L=2.3×10 6 J/kg, alcohol L=0.9×10 6
Amount of heat required to convert liquid into vapor: Q = Lm