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 capital Latin letter WITH.

What does the heat capacity of a body depend on? 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 type of substance? Let's do an experiment. Let's take two identical vessels and, having poured water weighing 400 g into one of them, and vegetable oil weighing 400 g into the other, we will begin to heat 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, different amounts of heat are required to heat the same mass of different substances to the same temperature. 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.

A 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 the internal structure of 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 the 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. Due to the strong interaction between closely located 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 . A liquid, unlike gases, does not fill the entire volume of the container into which it is poured. An interface is formed between the liquid and gas (or vapor), which is in special conditions compared to the rest of the liquid. 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 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 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 of a 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 the molecular structure of matter. 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 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, the average speed of the 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

The concept of the amount of heat was formed on early stages development of modern physics, when there were no clear ideas about internal structure substances, what energy is, what forms of energy exist in nature and 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 outdated unit 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 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 is the body mass, 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, clean mechanical work. For example, when a gas heats up, it increases its volume. Let's take a situation like the picture below:

IN in this case 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 is the 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.

Along with mechanical energy, any body (or system) has internal energy. Internal energy is the energy of rest. It consists of the thermal chaotic movement of the molecules that make up the body, the potential energy of their mutual arrangement, the kinetic and potential energy of electrons in atoms, nucleons in nuclei, and so on.

In thermodynamics, it is important to know not the absolute value of internal energy, but its change.

In thermodynamic processes, only the kinetic energy of moving molecules changes (thermal energy is not enough to change the structure of an atom, much less a nucleus). Therefore, in fact under internal energy in thermodynamics we mean energy thermal chaotic molecular movements.

Internal energy U one mole of an ideal gas is equal to:

Thus, internal energy depends only on temperature. The internal energy U is a function of the state of the system, regardless of background.

It is clear that in the general case, a thermodynamic system can have both internal and mechanical energy, and different systems can exchange these types of energy.

Exchange mechanical energy characterized by perfect work A, and the exchange of internal energy – the amount of heat transferred Q.

For example, in winter you threw a hot stone into the snow. Due to the reserve of potential energy, mechanical work was done to compress the snow, and due to the reserve of internal energy, the snow was melted. If the stone was cold, i.e. If the temperature of the stone is equal to the temperature of the medium, then only work will be done, but there will be no exchange of internal energy.

So, work and heat are not special forms of energy. You cannot talk about the reserve of heat or work. This measure of transferred another system of mechanical or internal energy. We can talk about the reserve of these energies. In addition, mechanical energy can be converted into thermal energy and vice versa. For example, if you hit an anvil with a hammer, then after a while the hammer and the anvil will heat up (this is an example dissipation energy).

We can give many more examples of the transformation of one form of energy into another.

Experience shows that in all cases, The conversion of mechanical energy into thermal energy and vice versa always occurs in strictly equivalent quantities. This is the essence of the first law of thermodynamics, which follows from the law of conservation of energy.

The amount of heat imparted to the body goes to increase internal energy and to perform work on the body:

- That's what it is first law of thermodynamics , or law of conservation of energy in thermodynamics.

Sign rule: if heat is transferred from environment this system, and if the system performs work on surrounding bodies, in this case . Taking into account the sign rule, the first law of thermodynamics can be written as:

In this expression U– system state function; d U is its total differential, and δ Q and δ A they are not. In each state, the system has a certain and only this value of internal energy, so we can write:

,

It is important to note that heat Q and work A depend on how the transition from state 1 to state 2 is accomplished (isochorically, adiabatically, etc.), and the internal energy U does not depend. At the same time, it cannot be said that the system has a specific value of heat and work for a given state.

From formula (4.1.2) it follows that the amount of heat is expressed in the same units as work and energy, i.e. in joules (J).

Of particular importance in thermodynamics are circular or cyclic processes in which a system, after passing through a series of states, returns to its original state. Figure 4.1 shows the cyclic process 1– A–2–b–1, and work A was done.

Rice. 4.1

Because U is a state function, then

This is true for any state function.

If then according to the first law of thermodynamics, i.e. It is impossible to build a periodically operating engine that would perform more work than the amount of energy imparted to it from the outside. In other words, perpetual motion machine the first kind is impossible. This is one of the formulations of the first law of thermodynamics.

It should be noted that the first law of thermodynamics does not indicate in which direction the processes of state change occur, which is one of its shortcomings.

1. The change in 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 internal energy of a body during heat transfer is characterized by a quantity called amount of heat.

The amount of heat is the change in the internal energy of a body during the process of heat transfer without doing work.

The amount of heat is denoted by the letter ​\(Q\) ​. Since the amount of heat is a measure of the change in internal energy, its unit is the joule (1 J).

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

2. If you pour 100 g of water into two identical vessels, one and 400 g into the other at the same temperature and place them on identical burners, then the water in the first vessel will boil earlier. Thus, the greater the mass of a body, the greater the amount of heat it requires to heat up. The same is true with cooling: when a body of greater mass is cooled, it gives off a greater amount of heat. These bodies are made of the same substance and they heat up or cool down by the same number of degrees.

​3. If we now heat 100 g of water from 30 to 60 °C, i.e. at 30 °C, and then up to 100 °C, i.e. by 70 °C, then in the first case it will take less time to heat up than in the second, and, accordingly, heating water by 30 °C will require less heat than heating water by 70 °C. Thus, the amount of heat is directly proportional to the difference between the final ​\((t_2\,^\circ C) \) ​ and initial \((t_1\,^\circ C) \) temperatures: ​\(Q\sim(t_2- t_1) \) ​.

4. If you now pour 100 g of water into one vessel, and pour a little water into another identical vessel and put in it a metal body such that its mass and the mass of water are 100 g, and heat the vessels on identical tiles, then you will notice that in a vessel containing only water will have a lower temperature than one containing water and a metal body. Therefore, in order for the temperature of the contents in both vessels to be the same, it is necessary to transfer more heat to the water than to the water and the metal body. Thus, the amount of heat required to heat a body depends on the type of substance from which the body is made.

5. The dependence of the amount of heat required to heat a body on the type of substance is characterized physical quantity, called specific heat capacity of a substance.

A physical quantity equal to the amount of heat that must be imparted to 1 kg of a substance to heat it by 1 ° C (or 1 K) is called the specific heat capacity of the substance.

1 kg of substance releases the same amount of heat when cooled by 1 °C.

Specific heat capacity is denoted by the letter ​\(c\) ​. Unit specific heat capacity is 1 J/kg °C or 1 J/kg K.

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

The specific heat of lead is 140 J/kg °C. This means that to heat 1 kg of lead by 1 °C it is necessary to expend an amount of heat of 140 J. The same amount of heat will be released when 1 kg of water cools by 1 °C.

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

The amount of heat ​\(Q \) ​ required to heat a body of mass ​\(m \) ​ from temperature \((t_1\,^\circ C) \) to temperature \((t_2\,^\circ C) \) 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.

\[ Q=cm(t_2()^\circ-t_1()^\circ) \]

​The same formula is used to calculate the amount of heat that a body gives off when cooling. Only in this case should the final temperature be subtracted from the initial temperature, i.e. Subtract the smaller temperature from the larger temperature.

6. Example of problem solution. 100 g of water at a temperature of 20 °C is poured into a glass containing 200 g of water at a temperature of 80 °C. After which the temperature in the vessel reached 60 °C. How much heat did the cold water receive and how much heat did the hot water give out?

When solving a problem, you must perform the following sequence of actions:

1. write down briefly the conditions of the problem;
2. convert the values ​​of quantities to SI;
3. analyze the problem, establish which bodies are involved in heat exchange, which bodies give off energy and which receive;
4. solve the problem in general view;
5. perform calculations;
6. analyze the received answer.

1. The task.

Given:
​\(m_1 \) ​ = 200 g
​\(m_2\) ​ = 100 g
​\(t_1 \) ​ = 80 °C
​\(t_2 \) ​ = 20 °C
​\(t\) ​ = 60 °C
______________

​\(Q_1 \) ​ — ? ​\(Q_2 \) ​ — ?
​\(c_1 \) ​ = 4200 J/kg °C

2. SI:​\(m_1\) ​ = 0.2 kg; ​\(m_2\) ​ = 0.1 kg.

3. Task analysis. The problem describes the process of heat exchange between hot and cold water. Hot water gives off an amount of heat ​\(Q_1 \) ​ and cools from temperature ​\(t_1 \) ​ to temperature ​\(t \) ​. Cold water receives the amount of heat ​\(Q_2 \) ​ and is heated from temperature ​\(t_2 \) ​ to temperature ​\(t \) ​.

4. Solution of the problem in general form. The amount of heat given hot water, is calculated by the formula: ​\(Q_1=c_1m_1(t_1-t) \) ​.

The amount of heat received by cold water is calculated by the formula: \(Q_2=c_2m_2(t-t_2) \) .

5. Computations.
​\(Q_1 \) ​ = 4200 J/kg · °С · 0.2 kg · 20 °С = 16800 J
\(Q_2\) = 4200 J/kg °C 0.1 kg 40 °C = 16800 J

6. The answer is that the amount of heat given off by hot water is equal to the amount of heat received by cold water. In this case, an idealized situation was considered and it was not taken into account that a certain amount of heat was used to heat the glass in which the water was located and the surrounding air. In reality, the amount of heat given off by hot water is greater than the amount of heat received by cold water.

Part 1

1. The specific heat capacity of silver is 250 J/(kg °C). What does this mean?

1) when 1 kg of silver cools at 250 °C, an amount of heat of 1 J is released
2) when 250 kg of silver cools by 1 °C, an amount of heat of 1 J is released
3) when 250 kg of silver cools by 1 °C, an amount of heat of 1 J is absorbed
4) when 1 kg of silver cools by 1 °C, an amount of heat of 250 J is released

2. The specific heat capacity of zinc is 400 J/(kg °C). It means that

1) when 1 kg of zinc is heated by 400 °C, its internal energy increases by 1 J
2) when 400 kg of zinc is heated by 1 °C, its internal energy increases by 1 J
3) to heat 400 kg of zinc by 1 °C it is necessary to expend 1 J of energy
4) when 1 kg of zinc is heated by 1 °C, its internal energy increases by 400 J

3. When transferring the amount of heat ​\(Q \) ​ to a solid body with mass ​\(m \) ​, the body temperature increased by ​\(\Delta t^\circ \) ​. Which of the following expressions determines the specific heat capacity of the substance of this body?

1) ​\(\frac(m\Delta t^\circ)(Q) \)​
2) \(\frac(Q)(m\Delta t^\circ) \)​
3) \(\frac(Q)(\Delta t^\circ) \) ​
4) \(Qm\Delta t^\circ \) ​

4. The figure shows a graph of the dependence of the amount of heat required to heat two bodies (1 and 2) of the same mass on temperature. Compare the specific heat capacity values ​​(​\(c_1 \) ​ and ​\(c_2 \) ​) of the substances from which these bodies are made.

1) ​\(c_1=c_2 \) ​
2) ​\(c_1>c_2 \) ​
3)\(c_1 4) the answer depends on the value of the mass of the bodies

5. The diagram shows the amount of heat transferred to two bodies of equal mass when their temperature changes by the same number of degrees. Which relationship is correct for the specific heat capacities of the substances from which bodies are made?

1) \(c_1=c_2\)
2) \(c_1=3c_2\)
3) \(c_2=3c_1\)
4) \(c_2=2c_1\)

6. The figure shows a graph of the temperature of a solid body depending on the amount of heat it gives off. Body weight 4 kg. What is the specific heat capacity of the substance of this body?

1) 500 J/(kg °C)
2) 250 J/(kg °C)
3) 125 J/(kg °C)
4) 100 J/(kg °C)

7. When heating a crystalline substance weighing 100 g, the temperature of the substance and the amount of heat imparted to the substance were measured. The measurement data was presented in table form. Assuming that energy losses can be neglected, determine the specific heat capacity of the substance in the solid state.

1) 192 J/(kg °C)
2) 240 J/(kg °C)
3) 576 J/(kg °C)
4) 480 J/(kg °C)

8. To heat 192 g of molybdenum by 1 K, you need to transfer an amount of heat of 48 J to it. What is the specific heat of this substance?

1) 250 J/(kg K)
2) 24 J/(kg K)
3) 4·10 -3 J/(kg K)
4) 0.92 J/(kg K)

9. What amount of heat is needed to heat 100 g of lead from 27 to 47 °C?

1) 390 J
2) 26 kJ
3) 260 J
4) 390 kJ

10. Heating a brick from 20 to 85 °C requires the same amount of heat as heating water of the same mass by 13 °C. The specific heat capacity of the brick is

1) 840 J/(kg K)
2) 21000 J/(kg K)
3) 2100 J/(kg K)
4) 1680 J/(kg K)

11. From the list of statements below, select two correct ones and write their numbers in the table.

1) The amount of heat that a body receives when its temperature increases by a certain number of degrees is equal to the amount of heat that this body gives off when its temperature decreases by the same number of degrees.
2) When a substance cools, its internal energy increases.
3) The amount of heat that a substance receives when heated is used mainly to increase the kinetic energy of its molecules.
4) The amount of heat that a substance receives when heated is used mainly to increase the potential energy of interaction of its molecules
5) The internal energy of a body can be changed only by imparting a certain amount of heat to it

12. The table presents the results of measurements of mass ​\(m\) ​, temperature changes ​\(\Delta t\) ​ and the amount of heat ​\(Q\) ​ released when cooling cylinders made of copper or aluminum.

Which statements correspond to the results of the experiment? Select two correct ones from the list provided. Indicate their numbers. Based on the measurements taken, it can be argued that the amount of heat released during cooling

1) depends on the substance from which the cylinder is made.
2) does not depend on the substance from which the cylinder is made.
3) increases with increasing cylinder mass.
4) increases with increasing temperature difference.
5) the specific heat capacity of aluminum is 4 times greater than the specific heat capacity of tin.

Part 2

C1. A solid body weighing 2 kg is placed in a 2 kW furnace and begins to heat up. The figure shows the dependence of the temperature ​\(t\) ​ of this body on the heating time ​\(\tau \) ​. What is the specific heat capacity of the substance?

1) 400 J/(kg °C)
2) 200 J/(kg °C)
3) 40 J/(kg °C)
4) 20 J/(kg °C)

Answers

The internal energy of a body depends on its temperature and external conditions - volume, etc. If external conditions remain unchanged, i.e. volume and other parameters are constant, then the internal energy of the body depends only on its temperature.

You can change the internal energy of a body not only by heating it in a flame or performing mechanical work on it (without changing the position of the body, for example, the work of friction), but also by bringing it into contact with another body that has a temperature different from the temperature of this body, i.e. through heat transfer.

The amount of internal energy that a body gains or loses during heat transfer is called the “amount of heat.” The amount of heat is usually denoted by the letter `Q`. If the internal energy of a body increases during the process of heat transfer, then the heat is assigned a plus sign, and the body is said to have been given heat `Q`. When the internal energy decreases during the process of heat transfer, the heat is considered negative, and it is said that the amount of heat `Q` has been removed (or removed) from the body.

The amount of heat can be measured in the same units in which mechanical energy is measured. In SI it is `1` joule. There is another unit of heat measurement - the calorie. Calorie is the amount of heat required to heat `1` g of water by `1^@ bb"C"`. The relationship between these units was established by Joule: `1` cal `= 4.18` J. This means that due to the work of `4.18` kJ, the temperature of `1` kilogram of water will increase by `1` degree.

The amount of heat required to heat a body by `1^@ bb"C"` is called the heat capacity of the body. The heat capacity of a body is designated by the letter `C`. If the body is given a small amount of heat `Delta Q`, and the body temperature changes to `Delta t` degrees, then

If a body is surrounded by a shell that does not conduct heat well, then the temperature of the body, if left to its own devices, will remain practically constant for a long time. Such ideal shells, of course, do not exist in nature, but it is possible to create shells that are close to such in their properties.

Examples include the lining of spaceships and Dewar flasks used in physics and technology. A Dewar flask is a glass or metal cylinder with double mirror walls, between which a high vacuum is created. The glass flask of a home thermos is also a Dewar flask.

The shell is insulating calorimeter- a device that allows you to measure the amount of heat. The calorimeter is a large thin-walled glass, placed on pieces of cork inside another large glass so that a layer of air remains between the walls, and closed on top with a heat-insulating lid.

If two or more bodies having different temperatures are brought into thermal contact in a calorimeter and wait, then after some time thermal equilibrium will be established inside the calorimeter. In the process of transition to thermal equilibrium, some bodies will give off heat (total amount of heat `Q_(sf"floor")`), others will receive heat (total amount of heat `Q_(sf"floor")`). And since the calorimeter and the bodies contained in it do not exchange heat with the surrounding space, but only with each other, we can write down a relationship, also called heat balance equation:

In a number of thermal processes, heat can be absorbed or released by a body without changing its temperature. Such thermal processes occur when the aggregate state of a substance changes - melting, crystallization, evaporation, condensation and boiling. Let us briefly look at the main characteristics of these processes.

Melting- the process of turning a crystalline solid into a liquid. The melting process occurs at a constant temperature, while heat is absorbed.

The specific heat of fusion `lambda` is equal to the amount of heat required to melt `1` kg of a crystalline substance taken at its melting point. The amount of heat `Q_(sf"pl")` that is required to convert a solid body of mass `m` at the melting point into a liquid state is equal to

Since the melting point remains constant, the amount of heat imparted to the body goes to increase the potential energy of interaction between molecules, and the crystal lattice is destroyed.

Process crystallization- This is a process reverse to the melting process. During crystallization, the liquid turns into a solid and an amount of heat is released, also determined by formula (1.5).

Evaporation is the process of converting liquid into vapor. Evaporation occurs from the open surface of the liquid. During the process of evaporation, the fastest molecules leave the liquid, i.e., molecules that can overcome the attractive forces exerted by the liquid molecules. As a result, if the liquid is thermally insulated, it cools during the evaporation process.

The specific heat of vaporization `L` is equal to the amount of heat required to turn `1` kg of liquid into steam. The amount of heat `Q_(sf"use")` that is required to convert a liquid of mass `m` into a vapor state is equal to

Condensation- a process reverse to the evaporation process. When condensation occurs, steam turns into liquid. This generates heat. The amount of heat released during steam condensation is determined by formula (1.6).

Boiling- a process in which the saturated vapor pressure of a liquid is equal to atmospheric pressure, so evaporation occurs not only from the surface, but throughout the entire volume (there are always air bubbles in the liquid; when boiling, the vapor pressure in them reaches atmospheric pressure, and the bubbles rise upward).