The internal energy of a body can be changed by work 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 international system the unit of heat quantity, 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 liquid 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.

Pour some water into the cannon barrel and close the tube with a rubber stopper. Connect the gun to a power source. Electricity, passing through the water, heats it. 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. During the heating of the liquid different 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.

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 junction of the thermocouple is lowered into a vessel with cold water to keep its temperature constant. The other junction of the thermocouple measures the temperature of the test liquid.)

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 changes 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 necessary to heat the body and the 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 the experiment used oil, the density of which less density water and heating oil to a certain temperature required a smaller amount of heat than heating water, it can be assumed that the amount of heat required to heat a 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.

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 are reinforced at the ends of the rods equal mass.

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.

Learning objective: Introduce the concepts of heat quantity and specific heat capacity.

Developmental goal: To cultivate mindfulness; learn to think, draw conclusions.

1. Topic update

2. Explanation of new material. 50 min.

You already know that the internal energy of a body can change both by doing work and by transferring heat (without doing work).

The energy that a body receives or loses during heat transfer is called the amount of heat. (notebook entry)

This means that the units of measurement of the amount of heat are also Joules ( J).

We conduct an experiment: two glasses in one 300 g of water, and in the other 150 g, and an iron cylinder weighing 150 g. Both glasses are placed on the same tile. After some time, thermometers will show that the water in the vessel in which the body is located heats up faster.

This means that less heat is required to heat 150 g of iron than to heat 150 g of water.

The amount of heat transferred to the body depends on the kind of substance from which the body is made. (notebook entry)

We propose the question: is the same amount of heat required to heat to the same temperature bodies of equal mass, but consisting of different substances?

We conduct an experiment with the Tyndall device to determine the specific heat capacity.

We conclude: bodies of different substances, but of the same mass, give off when cooled and demand when heated by the same number of degrees different amount warmth.

We draw conclusions:

1. To heat bodies of equal mass, consisting of different substances, to the same temperature, a different amount of heat is required.

2. Bodies of equal mass, consisting of different substances and heated to the same temperature. When cooled by the same number of degrees, they give off a different amount of heat.

We make the conclusion that the amount of heat required to raise one degree of unit mass of different substances will be different.

We give the definition of specific heat capacity.

The physical quantity, numerically equal to the amount of heat that must be transferred to a body of mass 1 kg in order for its temperature to change by 1 degree, is called the specific heat of the substance.

We introduce the unit of measurement of specific heat capacity: 1J / kg * degree.

The physical meaning of the term : specific heat capacity shows how much the internal energy of 1 g (kg.) of a substance changes when it is heated or cooled by 1 degree.

Consider the table of specific heat capacities of some substances.

We solve the problem analytically

How much heat is required to heat a glass of water (200 g) from 20 0 to 70 0 C.

For heating 1 g per 1 g. Required - 4.2 J.

And to heat 200 g per 1 g, it will take 200 more - 200 * 4.2 J.

And to heat 200 g by (70 0 -20 0) it will take another (70-20) more - 200 * (70-20) * 4.2 J

Substituting the data, we get Q = 200 * 50 * 4.2 J = 42000 J.

We write the resulting formula in terms of the corresponding quantities

4. What determines the amount of heat received by the body when heated?

Please note that the amount of heat required to heat a body is proportional to the mass of the body and the change in its temperature.,

There are two cylinders of the same mass: iron and brass. Is the same amount of heat needed to heat them up by the same number of degrees? Why?

How much heat is needed to heat 250 g of water from 20 o to 60 0 C.

What is the relationship between calories and joules?

A calorie is the amount of heat required to raise the temperature of 1 gram of water by 1 degree.

1 cal = 4.19=4.2 J

1kcal=1000cal

1kcal=4190J=4200J

3. Problem solving. 28 min.

If cylinders of lead, tin and steel heated in boiling water with a mass of 1 kg are placed on ice, they will cool, and part of the ice under them will melt. How will the internal energy of the cylinders change? Under which of the cylinders will melt more ice, under which - less?

A heated stone with a mass of 5 kg. Cooling in water by 1 degree, it transfers 2.1 kJ of energy to it. What is the specific heat capacity of the stone

When hardening a chisel, it was first heated to 650 0, then lowered into oil, where it cooled to 50 0 C. What amount of heat was released if its mass was 500 g.

How much heat was spent on heating from 20 0 to 1220 0 C. a steel billet for the crankshaft of a compressor weighing 35 kg.

Independent work

What type of heat transfer?

Students complete the table.

  1. The air in the room is heated through the walls.
  2. Through open window, which includes warm air.
  3. Through glass, which transmits the rays of the sun.
  4. The earth is heated by the rays of the sun.
  5. The liquid is heated on the stove.
  6. The steel spoon is heated by the tea.
  7. The air is heated by a candle.
  8. The gas moves around the heat-producing parts of the machine.
  9. Heating the barrel of a machine gun.
  10. Boiling milk.

5. Homework: Peryshkin A.V. “Physics 8” §§7, 8; collection of tasks 7-8 Lukashik V.I. Nos. 778-780, 792,793 2 min.

Hello! A seemingly simple question is what is heat and the amount of heat. However, even a specialist who has been working in the thermal power industry for more than one year, he can confuse. Let's figure it out.

During the interaction of bodies having unequal temperatures, energy can be transferred from a body with more high temperature to a lower temperature body by direct contact and radiation. This form of energy transfer is called heat, and the amount of energy transferred is called the amount of heat.

The amount of heat received or given off by the body depends essentially on the nature of the process, that is, it is a function of the process. It is accepted that the amount of heat supplied to the body is considered positive, and the amount of heat removed from it is negative.

If the amount of heat Q is supplied to the working fluid, which completely transforms into work L, then the work strictly corresponds (equivalent) to the amount of heat. In accordance with this principle of equivalence of heat and work, based on the law of conservation of energy, one can write: Q = L. Here it is assumed that Q and L are measured in the same units (in the SI system in J). If Q and L are measured in different units, then the principle of equivalence of heat and work can be written as:

Q=AL

The coefficient A in this equation is called the thermal equivalent of work. In all processes of transferring heat into work, the coefficient A has the same constant value. In the off-system system of units, Q is usually measured in kcal, L in kgf * cm, then, according to numerous experiments,

A \u003d 1/427 kcal (kgf * m).

This means that in order to obtain 1 kgf * m of work, 1/427 kcal is required for the complete transition of heat into work. On the contrary, to obtain 1 kcal, it is necessary to convert 427 kgf * m of work into heat.

Let us determine, for example, the amount of heat equivalent to the value used in technology - 1 kWh; 1 kW is a unit of power equal to 1 kJ / s = 102 kgf * m / s. 1 kWh (1 kW for an hour) is work:

L \u003d 1 * 3600 \u003d 3600 kJ;

L \u003d 102 * 3600 \u003d 367200 kgf * m.

The amount of heat equivalent to 1 kWh:

Q = L = 3600 kJ;

Q \u003d AL \u003d 1/427 * 367200 \u003d 860 kcal.

So, 1 kWh = 3600 kJ = 367200 kgf * m = 860 kcal.

The amount of heat spent on heating the body or released during its cooling can be found from the formula:

Q = c * m * ΔT;

where Q is the amount of heat, c is the specific heat of the substance that makes up the body, m is the mass of the body, ΔT is the temperature difference.

Thus, the energy that a body receives or loses in the process of heat exchange with environment, and is called the amount of heat, and the form of energy transfer is called heat. The amount of heat is one of the main thermodynamic quantities in technical thermodynamics.

>>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 be transferred large quantity heat than for heating 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:

Specific heat of fusion. When a crystalline body melts, all the heat supplied to it goes to increase the potential energy of the molecules. The kinetic energy of the molecules does not change, since melting occurs at a constant temperature.
A value numerically equal to the amount of heat required for the transformation crystalline substance weighing 1 kg at the melting point into a liquid, is called the 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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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.

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 alignment of temperatures as a result of heat conduction can be explained on the basis of molecular kinetic theory as the exchange of kinetic energy between molecules during collision in the process of thermal chaotic motion.

This example can be illustrated with a graph.