The amount of heat absorbed by the body when heated formula. Lesson topic: "The amount of heat

Change internal energy by doing work is characterized by the amount of work, i.e. work is a measure of the change in internal energy in a given process. The change in the internal energy of a body during heat transfer is characterized by a quantity called the amount of heat.

is the change in the internal energy of the body in the process of heat transfer without doing work. The amount of heat is denoted by the letter Q .

Work, internal energy and quantity of heat measured in the same units - joules ( J), like any other form of energy.

In thermal measurements, a special unit of energy, the calorie ( feces), equal to the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius (more precisely, from 19.5 to 20.5 ° C). This unit, in particular, is currently used in calculating the consumption of heat (thermal energy) in apartment buildings. Empirically, the mechanical equivalent of heat has been established - the ratio between calories and joules: 1 cal = 4.2 J.

When a body transfers a certain amount of heat without doing work, its internal energy increases, if a body gives off a certain amount of heat, then its internal energy decreases.

If you pour 100 g of water into two identical vessels, and 400 g into another at the same temperature and put them on the same burners, then the water in the first vessel will boil earlier. Thus, the more body mass, topics large quantity It needs heat to warm up. The same goes for cooling.

The amount of heat required to heat a body also depends on the kind of substance from which this body is made. This dependence of the amount of heat required to heat the body on the type of substance is characterized by a physical quantity called specific heat capacity substances.

- this is a physical quantity equal to the amount of heat that must be reported to 1 kg of a substance to heat it by 1 ° C (or 1 K). The same amount of heat is given off by 1 kg of a substance when cooled by 1 °C.

The specific heat capacity is denoted by the letter from. The unit of specific heat capacity is 1 J/kg °C or 1 J/kg °K.

The values of the specific heat capacity of substances are determined experimentally. Liquids have a higher specific heat capacity than metals; Water has the highest specific heat capacity, gold has a very small specific heat capacity.

Since the amount of heat is equal to the change in the internal energy of the body, we can say that the specific heat capacity shows how much the internal energy changes 1 kg substance when its temperature changes 1 °C. In particular, the internal energy of 1 kg of lead, when it is heated by 1 °C, increases by 140 J, and when it is cooled, it decreases by 140 J.

Q required to heat the body mass m temperature t 1 °С up to temperature t 2 °С, is equal to the product of the specific heat capacity of the substance, body mass and the difference between the final and initial temperatures, i.e.

Q \u003d c ∙ m (t 2 - t 1)

According to the same formula, the amount of heat that the body gives off when cooled is also calculated. Only in this case should the final temperature be subtracted from the initial temperature, i.e. Subtract the smaller temperature from the larger temperature.

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You can change the internal energy of the gas in the cylinder not only by doing work, but also by heating the gas (Fig. 43). If the piston is fixed, then the volume of the gas will not change, but the temperature, and hence the internal energy, will increase.
The process of transferring energy from one body to another without doing work is called heat transfer or heat transfer.

The energy transferred to the body as a result of heat transfer is called the 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 at the boundary between bodies, slowly moving molecules of a cold body interact with faster moving molecules of a hot body. As a result, the kinetic energies of the molecules are equalized and the velocities of the molecules of a cold body increase, while those of a hot body decrease.

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. It is known from the class VII physics course that in order to heat a body with mass m from temperature t 1 to temperature t 2, it is necessary to inform it of the amount of heat

Q \u003d cm (t 2 - t 1) \u003d cmΔt. (4.5)

When a body cools, its eternal temperature t 2 is less than the initial t 1 and the amount of heat given off by the body is negative.
The coefficient c in formula (4.5) is called specific heat . Specific heat capacity is the amount of heat that 1 kg of a substance receives or gives off when its temperature changes by 1 K.

Specific heat capacity is expressed in joules per kilogram times kelvin. Different bodies require a different amount of energy to increase the temperature by 1 K. Thus, the specific heat capacity of water is 4190 J/(kg K), and that of copper is 380 J/(kg 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 will need to transfer more heat than to heat it at constant volume.

liquid and solid bodies expand slightly when heated, and their specific heat capacities at constant volume and constant pressure differ little.

Specific heat of vaporization. To convert a liquid into vapor, a certain amount of heat must be transferred to it. The temperature of the liquid does not change during this transformation. The transformation of 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 their potential energy. After all, the average distance between gas molecules is many times greater than between liquid molecules. In addition, the increase in volume during the transition of a substance from liquid state into gaseous requires work to be done against the forces of external pressure.

The amount of heat required to convert 1 kg of liquid to vapor at a constant temperature is called specific heat vaporization. This value is denoted by the letter r and expressed in joules per kilogram.

The specific heat of vaporization of water is very high: 2.256 · 10 6 J/kg at 100°C. For other liquids (alcohol, ether, mercury, kerosene, etc.), the specific heat of vaporization is 3-10 times less.

To convert a liquid of mass m into vapor requires an amount of heat equal to:

When steam condenses, the same amount of heat is released

Q k = –rm. (4.7)

The amount of heat λ (lambda) required to convert 1 kg of a crystalline substance at a melting point into a liquid of the same temperature is called the specific heat of fusion.

During the crystallization of 1 kg of a substance, exactly the same amount of heat is released. The specific heat of ice melting is rather high: 3.4 10 5 J/kg.

In order to melt a crystalline body of mass m, an amount of heat is required equal to:

Qpl \u003d λm. (4.8)

The amount of heat released during the crystallization of the body is equal to:

Q cr = - λm. (4.9)

1. What is called the amount of heat? 2. What determines the specific heat capacity of substances? 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 transferred heat negative?

>>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, more heat must be transferred to it 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:

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 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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The process of transferring energy from one body to another without doing work is called heat transfer or heat transfer.

The energy transferred to the body as a result of heat transfer is called the amount of heat. The amount of heat is also called the energy that the body gives off in the process of heat transfer.

molecules are aligned and the velocities of the molecules of a cold body increase, and those of a hot one decrease.

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. From the class VII physics course, it is known that in order to heat a body with a mass from temperature to temperature, it is necessary to inform it of the amount of heat

When the body cools, its final temperature is less than the initial one and the amount of heat given off by the body is negative.

The coefficient c in formula (4.5) is called the specific heat capacity. Specific heat capacity is the amount of heat that 1 kg of a substance receives or gives off when its temperature changes by 1 K -

Specific heat capacity is expressed in joules per kilogram times kelvin. Different bodies require an unequal amount of energy to increase the temperature by I K. Thus, the specific heat capacity of water and copper

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 will need to transfer more heat than to heat it at constant volume.

Liquids and solids expand slightly when heated, and their specific heat capacities at constant volume and constant pressure differ little.

Specific heat of vaporization. To convert a liquid into vapor, a certain amount of heat must be transferred to it. The temperature of the liquid does not change during this transformation. The transformation of 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 their potential energy. After all, the average distance between gas molecules is many times greater than between liquid molecules. In addition, an increase in volume during the transition of a substance from a liquid to a gaseous state requires work to be performed against the forces of external pressure.

The amount of heat required to turn 1 kg of liquid into vapor at a constant temperature is called

specific heat of vaporization. This value is denoted by a letter and expressed in joules per kilogram.

The specific heat of vaporization of water is very high: at a temperature of 100°C. For other liquids (alcohol, ether, mercury, kerosene, etc.), the specific heat of vaporization is 3-10 times less.

To convert a liquid mass into vapor requires an amount of heat equal to:

When steam condenses, the same amount of heat is released:

The amount of heat A required to convert 1 kg of a crystalline substance at the melting point into a liquid of the same temperature is called the specific heat of fusion.

During the crystallization of 1 kg of a substance, exactly the same amount of heat is released. The specific heat of melting of ice is quite high:

In order to melt a crystalline body with a mass, an amount of heat is required equal to:

The amount of heat released during the crystallization of the body is equal to:

What heats up faster on the stove - a kettle or a bucket of water? The answer is obvious - a kettle. Then the second question is why?

The answer is no less obvious - because the mass of water in the kettle is less. Fine. Now you can make your own real physical experience at home. To do this, you will need two identical small saucepans, an equal amount of water and vegetable oil, for example, half a liter and a stove. Put pots of oil and water on the same fire. And now just watch what will heat up faster. If there is a thermometer for liquids, you can use it, if not, you can just try the temperature from time to time with your finger, just be careful not to burn yourself. 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. What will boil faster - warm water or cold? Everything is obvious again - the warm one will be the first to finish. Why all these strange questions and experiments? In order to define physical quantity, called "the amount of heat".

Quantity of heat

The amount of heat is the energy that the 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 heated, it will absorb. And the answers to our questions showed us what does the amount of heat depend on? First, the greater the mass of the body, the greater the amount of heat that must be expended to change its temperature by one degree. Secondly, the amount of heat necessary to heat a body depends on the substance of which it is composed, that is, on the kind of substance. And thirdly, the difference in body temperature before and after heat transfer is also important for our calculations. Based on the foregoing, we can determine the amount of heat by 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 - specific heat capacity of the substance, is found from the relevant 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 it cools.

The amount of heat is measured in joules (1 J), like any other form of energy. However, this value was introduced not so long ago, and people began to measure the amount of heat much earlier. And they used a unit that is widely used in our time - a calorie (1 cal). 1 calorie is the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. Guided by these data, lovers of counting calories in the food they eat can, for the sake of interest, calculate how many liters of water can be boiled with the energy that they consume with food during the day.

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The amount of heat absorbed by the body when heated formula. Lesson topic: "The amount of heat

Q \u003d c ∙ m (t 2 - t 1)

Quantity of heat

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