Lesson typology: a lesson in learning new knowledge and ways of doing things

Type of lesson: combined

Lesson Objectives:

• didactic:
explain the physical nature of the thermal expansion of bodies; to teach students to calculate linear and volumetric changes in solid and liquid bodies when their temperature changes;
• educational:
to improve the ability of students to apply the acquired theoretical knowledge to solving practical problems; arouse interest in the process under study;
• developing:
to develop in students the thinking of the use and significance of thermal expansion in nature and technology; be able to explain the mechanism of thermal expansion of bodies on the basis of molecular kinetic theory.

Lesson plan

1. Organization of the beginning of the lesson
2. Repetition of the studied material
3. Learning new material
4. Intermediate fastening of the material
5. Learning new material (continued) Attachment 1
6. Consolidation of the studied material Appendix 2,
7. Homework Appendix 4

Plan for studying the topic.

Equipment: a ball with a ring; bimetallic plate; thermal relay; a flask with a rubber and glass tube inserted into the cork; G - a cut glass tube with a drop of water; uncolored water; electric stove; transformer; wire.

Demos:

1. Thermal expansion of solids.
2. Thermal expansion of liquids.
3. The action and purpose of the bimetallic thermal regulator.

Message:

Features of thermal expansion of water.

Motivation of cognitive activity of students

It is well known that a substance usually expands when heated and contracts when cooled, i.e. thermal deformation of the body occurs under the action of molecular forces in the process of heating and cooling. This phenomenon is explained by the fact that an increase in temperature is associated with an increase in the speed of movement of molecules, and this leads to an increase in intermolecular distances and, in turn, to an expansion of the body.

Thermal expansion must be taken into account in heat treatment and in the thermal method of manufacturing parts and equipment, in the construction of machines, pipelines, electrical lines, bridges, buildings subject to temperature changes.

STUDY PROCESS

I. Organization of the beginning of the lesson

Greeting, wording of the topic, objectives of the lesson, indication of the upcoming scope of work. Motivation of cognitive activity.

II. Repetition of the studied material

1. Checking homework

Check the solution of qualitative physical problems on the topic “Solid bodies and their properties” (frontal survey of students).

2. Preparation for the perception of new material

1. Repeat the formulas from the mathematics course (a + c) 3, and 3 + in 3;
2. Repeat the topic “Thermal expansion of gases” (Gay-Lussac law)
3. Repeat the topic “Deformation of solid bodies”.

III. Learning new material

1. Students are asked to answer the following questions:
1. What happens to bodies when they cool and expand?
2. Why do bodies expand? What changes in the body in the process of expansion?

During the discussion, the concept of thermal expansion of bodies, examples of the expansion of bodies, types of thermal expansion are introduced.

Thermal expansion is an increase in the linear dimensions of the body and its volume, occurring with an increase in temperature.

When the body expands, its volume increases, and they talk about volumetric expansion of the body. But sometimes we are only interested in changing one dimension, such as the length of a railroad track or a metal rod. In that case, one speaks of linear expansion. Automobile designers are interested in expanding the surface of the metal sheets used in the construction of the car. Here the question is about surface expansion.

Setting up experiments:

1. expansion of liquids when heated (increase in the water level in a flask with a tube);
2. expansion of solids when heated (a ball with a ring, an increase in the length of stretched wires);
3. the action of a bimetallic regulator (thermal relay).

Question: Do bodies expand in the same way when heated by the same number of degrees?

Answer: no, because different substances have different molecules. A change in temperature by the same number of degrees characterizes the same root-mean-square velocity of the molecules. E k = molecules with a smaller mass will be less than molecules with a large mass. Therefore, the intermolecular spaces of various substances change differently at the same temperature, which leads to unequal expansion.

2. Consider the linear expansion of rigid bodies and its features

The expansion of a rigid body along one of its dimensions is called linear.

To characterize the degree of linear expansion of various solids, the concept of the coefficient of linear expansion is introduced.

The value showing by what fraction of the initial length, taken at 0 0 C, the length of the body increases from heating it by 1 0 C, is called linear expansion coefficient and is denoted by .

K -1 = or = 0 C -1 =

Let's introduce the notation: t 0 – initial temperature; t is the final temperature; l 0 - body length at t 0 \u003d 0 0 С; l t - body length at t 0 С; l - change in body length; t is the change in temperature.

Suppose that the wire was heated by 60 0 C. At the beginning, the wire had a length of 100 cm, and when heated, its length increased by 0.24 cm.

From here, it is possible to calculate the increase in the length of the wire when heated by 1 0 C.

The total elongation (0.024 cm) is divided by the length of the wire and the change in temperature: \u003d 0.000004 0 С -1 \u003d (4 * 10 -6) 0 С -1.

Then = or = (1)

3. a) To calculate the length of the body depending on the temperature t, we transform the formula (2)

l t -l 0 \u003d l 0 t l t \u003d l 0 + l 0 t l t \u003d l 0 (1+ t)

The binomial (1+t) is called binomial linear expansion . It shows how many times the length of the body increased when it was heated from 0 0 to t 0 С.

So, the final length of the body is equal to the initial length multiplied by the linear expansion binomial.

The formula l t \u003d l 0 (1+? t) is approximate and can be used at not very high temperatures (200 0 C-300 0 C).

For large temperature changes, this formula cannot be applied.

b) Often, when solving problems, they use another approximate formula that simplifies calculations. For example, if it is necessary to calculate the length of a body when heated from temperature t 1 to temperature t 2, then use the formula:

l 2 ~ l 1 , coefficient of linear expansion ~

IV. Intermediate fastening of the material

Let's go for a walk along the railroad tracks. If the weather is cold, then we will notice that the ends of two adjacent rails are separated from each other by intervals of 0.6-1.2 cm, in hot weather these ends almost converge closely. Hence the conclusion that the rails expand when heated, shrink when cooled. Consequently, if the road was built in winter, then some margin had to be left to allow the rails to expand freely in the hot season. The question arises, how much margin is required for this expansion?

Let's assume that in our area the temperature change per year is from -30 0 C to -35 0 C and the rail length is 12.5 m. What gap should be left between the rails?

Answer: so it is necessary to leave a gap of 1 cm if the rails are laid at low temperatures or the rails should be laid butt-to-butt if the rails are laid in the hottest weather.

V. Learning new material (continued)

4. Consider the volumetric expansion of solids and its features

The increase in the volume of a body when heated is called bulk expansion.

Volumetric expansion is characterized by the coefficient of volumetric expansion and is denoted by? .

Task: by analogy with linear expansion, define the coefficient of volumetric expansion and derive the formula =.

Students independently implement the solution of this issue and enter the designations: V 0 - initial volume at 0 0 С; V t is the final volume at t 0 С; V - change in body volume; t 0 - initial temperature; t is the final temperature.

The value showing by what fraction of the initial volume, taken at 0 0 C, the volume of the body increases from heating by 1 0 C, is called volume expansion coefficient .

a) Let us find the dependence of the volume of a solid body on temperature. From the formula = we find the final volume V t .

V t -V 0 \u003d V 0 t, V t \u003d V 0 + V 0 t, V t \u003d V 0 (1+ t).

The binomial (1+? t) is called volume expansion binomial . It shows how many times the volume of the body increased when it was heated from 0 to t 0 C.

So, the final volume of the body is equal to the initial volume multiplied by the volume expansion binomial.

If the volume of the body V 1 at temperature t 1 is known, then the volume V 2 at temperature t 2 can be found by the approximate formula V 2 ~V 1, and the volume expansion coefficient ~.

The derivation and recording of formulas is implemented by students independently.

6. The value of the coefficient of volumetric expansion? very small value.

However, if we turn to the tables, we will see that the meaning? for solids there is none. It turns out that there is a relationship between the coefficients of linear and volumetric expansion? =3? .

Let's derive this ratio.

Suppose we have a cube whose edge length at 0 0 C is 1 cm. Let's heat the cube by 1 0 C, then the length of its edge will be l t \u003d 1+? *1 0 =1+? . Volume of heated cube V t =(1+?) 3 . On the other hand, the volume of the same cube can be calculated using the formula V t =1+? *1 0 =1+? .

From the last equalities we get 1+? =(1+?) 3 , hence 1+? =1+3? +3? 2+? 3.

So how are the numeric values? very small - of the order of millionths, then 3? 2 and? 3 are even more extremely small quantities. On this basis, neglecting the values ​​of 3? 2 and? 3 , get what? =3? .

The coefficient of volumetric expansion of a solid body is equal to three times the coefficient of linear expansion.

7. Find out how the density of bodies changes with temperature. Body density at 0 0 С.

p, whence m=p 0 *V 0 , where m is the body mass; V 0 - volume at 0 0 С;

m = const when the temperature changes, but the volume of the body changes, which means that the density also changes.

On this basis, we can write that the density of the body at a temperature t = 0 0 C , because V t = V 0 (1+? t), then .

When calculating, it must be taken into account that the tables indicate the density of a substance at 0 0 C. The density at other temperatures is calculated by the formula? t .

When heated, p t - decreases, when cooled, p t - increases.

1. Tell about the device, purpose and principle of operation of a bimetallic thermal relay, demonstrate its actions. Give examples of the beneficial and harmful effects of thermal deformation in engineering, transport, construction, etc.
2. Briefly describe the features of thermal expansion of liquids.
3. Message “Peculiarities of thermal expansion of water”.

VI. Consolidation of the studied material.

1. A short survey-conversation is conducted for a deeper understanding and consolidation of the studied material on the issues.
2. Independent work students. Solve problems on the topic.

1. P.I. Samoilenko, A.V. Sergeev.
Physics. –M.: 2002.
2. A.A. Pinsky, G.Yu. Grakovsky.
Physics. –M.: 2002.
3. V.F. Dmitriev.
Physics.-M.: 2000.
4. G.I. Ryabovodov, P.I. Samoilenko, E.I. Ogorodnikov.
Planning educational process in physics.-M.: graduate School, 1988.
5. A.A. Gladkov
. Collection of tasks and questions for secondary school in physics. -M.: Science. 1996.

T.I.RADCHENKO(school № 26, Vladikavkaz),
I.V. SILAEV(North Ossetian State University)
[email protected] ,
Vladikavkaz, Rep. North Ossetia Alania)

Thermal expansion of solids

Will the diameter of the hole in the round plate change when it is heated?

(The question was proposed by the newspaper "Physics" in No. 11/06.)

Engineering examples

The diameter of the hole increases when heated. It finds application in technology. For example, in the engines of cars VAZ-1111, Tavria ZAZ-1102, etc., each piston is connected to the upper head of its connecting rod pivotally, using a piston pin (steel tube), which is inserted into the corresponding holes of the piston and connecting rod. In this case, the pin is fixed in the upper head of the connecting rod by a hot fit, heating the upper part of the connecting rod. When cooling, the diameter of the hole in the head decreases, and the pin is tightly clamped, which eliminates its longitudinal movement and the formation of scoring on the cylinder walls when the pistons reciprocate.

Similarly, a preheated clamping ring is attached to the axle shafts connecting the differential to the drive wheels, for example, on Volga and Zhiguli cars. (A differential is a device that allows the driving wheels of a car to rotate at different frequencies, for example, when cornering, when the inner wheel closest to the center of the turn goes in a circle of a smaller radius than the outer one.) The outer end of the axle shaft (with the car wheel) is mounted on a ball bearing , the outer ring of which is tightly clamped. The axle shaft rotates together with the inner ring of the bearing. So that the axle shaft does not come out of the bearing due to longitudinal displacements, it is held with a clamping ring. This ring, being put on the axle shaft, rotates with it. It is closed by the casing of the axle shaft and rests against the fixed bearing through the spring ring, which prevents the axle shaft with the wheel from moving away from the longitudinal axis of the car.

Examples could go on...

Physics of thermal expansion

Let us now consider the question from the point of view of physics. Imagine that the hole is formed by eight atoms or molecules (further we will talk about particles). Particles of a solid body mainly oscillate around their equilibrium positions and jump to other places quite rarely - the time of their “sedentary” life is 0.1–0.001 s even near the melting point, and at lower temperatures it is already hours and days (recall about diffusion rates in solids). Thus, the number of particles framing the hole will remain unchanged until the transition to the liquid phase begins. As the temperature rises, the range of oscillations of each particle will increase, it will take up more space in space, therefore, the diameter of the hole will increase. Particles cannot approach each other, because at the same time they will begin to "overlap".

To give scientific explanations, you have to remember the graph of the dependence of the interaction strength F particles from distance r between these particles. It is obtained by adding the ordinates of the corresponding points of the upper curve II, describing the repulsive force, and the lower curve I, describing the force of attraction. The resulting curve III has a rather complex shape, since the repulsive force is inversely proportional to the thirteenth power of the distance, and the attractive force is the seventh. Curve IV looks similar, showing the distance dependence of the potential energy Ep. In a position of balance r 0, curve III passes through zero (the resultant of the applied forces is zero), and curve IV passes through a minimum (potential well). This is a position of stable equilibrium, and as the distance between the particles decreases, work will be done against the repulsive forces, which will lead to a decrease in the kinetic energy of the particle to zero, so that the “impact” of one particle on another, like the impact of billiard balls, will not occur.

On the whole, the thermal motion of particles is considered as their oscillations near centers located at an equilibrium distance from each other, which is different for different substances. The free volume in liquids is approximately 29% of the total volume, and in solids up to 26%. "The molecules (atoms) of solids are so densely packed that their electron shells touch and sometimes overlap." So, apparently, it is more correct to speak about the position not of the molecules themselves, but of their centers.

Let's look again at curve IV. The depth of the potential well determines the binding energy of molecules. Note that the curve is not symmetrical about its minimum. “For this reason, only very small vibrations of particles around the equilibrium position will have a harmonic character. With an increase in the amplitude of oscillations (which occurs with an increase in temperature), anharmonicity (i.e., the deviation of oscillations from harmonic ones) will become more and more pronounced. This leads to an increase in the average distances between particles and, consequently, to an increase in volume. “At a lower temperature, the molecule oscillates around the point BUT within the segment BUT 1 BUT 2. The average distance between interacting molecules (we mentally placed the second molecule at the origin of coordinates) is r 0 . As the temperature rises, the vibrational energy increases; now the molecule oscillates within the segment AT 1 AT 2. The equilibrium position corresponds to the middle of the segment AT 1 AT 2 , i.e. dot AT» . Thus, although the amplitudes of oscillations are small, due to anharmonicity, the individual oscillations are not independent, but are related to each other. That's why r 0 (the distance at which the sum of the forces of attraction and repulsion of two molecules is equal to zero) begins to increase with increasing temperature.

Accounting for thermal conductivity and thermal expansion of solids for an internal combustion engine of a car

Thermal expansion in technology has to be reckoned with all the time. If we take the mentioned pistons in automobile engines, then there will already be several options at once. So, for example, the piston head (its upper part) has a slightly smaller diameter than the skirt (lower part), because. the head is in direct contact with heated gases. It heats up more and expands more. In this case, engineers must comply with two mutually exclusive requirements. On the one hand, it is necessary to ensure a good seal between the piston and the cylinder, and on the other hand, to avoid piston jamming when heated. For this purpose, grooves are made around the circumference of the head, into which special rings are placed: compression and oil scraper.

Compression rings have cuts called locks, which allow sealing the gap without jamming the piston. Jamming is also prevented by the special shape of the piston skirt - in the form of an ellipse, the major axis of which is perpendicular to the axis of the piston pin and lies in the plane of action of the lateral forces. As a result, both knocking when the engine is cold and skirt sticking when heated are eliminated: the ellipse becomes a circle, and the piston continues to move freely inside the cylinder.

You can also prevent jamming by making compensation cuts in the skirt: oblique, T-shaped, U-shaped, due to which the expansion of the metal when heated does not lead to an increase in the piston diameter. It is possible to reduce the heating of the upper piston compression ring due to a groove machined in the piston or a fire belt that prevents additional heat from the upper part of the piston head heated by the hot gases in the cylinder.

For better heat dissipation from the pistons and cylinders, both the pistons themselves and the cylinder head are made of aluminum alloy, which has good thermal conductivity. There are engines where the entire cylinder block is cast from aluminum alloy. In addition, a special cooling system (air or liquid) is provided. For example, the so-called cooling jacket The fluid system provides heat removal from both the cylinders and the combustion chambers.

Literature

1. Plekhanov I.P. Automobile. – M.: Enlightenment, 1984.

2. Shestopalov K.S.,Demikhovsky S.F. Cars. – M.: DOSAAF, 1989.

3. Podgornova I.I. Molecular physics in high school. - M .: Education, 1970.

4. Berger N.M. The study of thermal phenomena in the course of high school physics. – M.: Enlightenment, 1981.

5. Shamash S.Ya. Methods of teaching physics in high school. - M .: Education, 1975.

6. Bludov M.I. Conversations on physics. – M.: Enlightenment, 1992.

7. Saveliev A.V. Course of General Physics: T. 1. - M .: Nauka, 1970.

8. Physical Encyclopedic Dictionary: Ed. Prokhorova A.M. - M.: Soviet Encyclopedia, 1984.

It is known that under the influence of heat particles accelerate their chaotic motion. If you heat a gas, then the molecules that make it up will simply scatter from each other. The heated liquid will first increase in volume, and then begin to evaporate. What will happen to solids? Not each of them can change its state of aggregation.

Thermal expansion: definition

Thermal expansion is a change in the size and shape of bodies with a change in temperature. Mathematically, it is possible to calculate the volumetric expansion coefficient, which makes it possible to predict the behavior of gases and liquids in changing external conditions. To obtain the same results for solids, it is necessary to take into account. Physicists have singled out a whole section for this kind of research and called it dilatometry.

Engineers and architects need knowledge about the behavior of different materials under the influence of high and low temperatures for the design of buildings, laying roads and pipes.

Expansion of gases

The thermal expansion of gases is accompanied by the expansion of their volume in space. This was noticed by natural philosophers in ancient times, but only modern physicists managed to build mathematical calculations.

First of all, scientists became interested in the expansion of air, since it seemed to them a feasible task. They got down to business so zealously that they got rather contradictory results. Naturally, the scientific community was not satisfied with such an outcome. The accuracy of the measurement depended on which thermometer was used, the pressure, and a variety of other conditions. Some physicists have even come to the conclusion that the expansion of gases does not depend on changes in temperature. Or is this relationship incomplete?

Works by Dalton and Gay-Lussac

Physicists would have continued to argue until they were hoarse or would have abandoned measurements, if not for He and another physicist, Gay-Lussac, at the same time, independently of each other, could obtain the same measurement results.

Lussac tried to find the reason for so many different results and noticed that some of the devices at the time of the experiment had water. Naturally, in the process of heating, it turned into steam and changed the amount and composition of the studied gases. Therefore, the first thing the scientist did was to thoroughly dry all the instruments that he used to conduct the experiment, and to exclude even the minimum percentage of moisture from the gas under study. After all these manipulations, the first few experiments turned out to be more reliable.

Dalton dealt with this issue longer than his colleague and published the results in the very early XIX century. He dried the air with sulfuric acid vapor and then heated it. After a series of experiments, John came to the conclusion that all gases and vapors expand by a factor of 0.376. Lussac came up with the number 0.375. This was the official result of the study.

Water vapor pressure

The thermal expansion of gases depends on their elasticity, that is, the ability to return to their original volume. Ziegler was the first to investigate this issue in the middle of the eighteenth century. But the results of his experiments were too different. More reliable figures were obtained by using a boiler for high temperatures, and a barometer for low temperatures.

AT late XVIII century, the French physicist Prony attempted to derive a single formula that would describe the elasticity of gases, but it turned out to be too cumbersome and difficult to use. Dalton decided to test all the calculations empirically, using a siphon barometer for this. Despite the fact that the temperature was not the same in all experiments, the results were very accurate. So he published them as a table in his physics textbook.

Evaporation theory

The thermal expansion of gases (as a physical theory) has undergone various changes. Scientists tried to get to the bottom of the processes by which steam is produced. Here again, the well-known physicist Dalton distinguished himself. He hypothesized that any space is saturated with gas vapor, regardless of whether any other gas or vapor is present in this reservoir (room). Therefore, it can be concluded that the liquid will not evaporate simply by coming into contact with atmospheric air.

The pressure of the air column on the surface of the liquid increases the space between the atoms, tearing them apart and evaporating, that is, it contributes to the formation of vapor. But gravity continues to act on the vapor molecules, so scientists considered that atmospheric pressure does not affect the evaporation of liquids in any way.

Expansion of liquids

The thermal expansion of liquids was studied in parallel with the expansion of gases. The same scientists were engaged in scientific research. To do this, they used thermometers, aerometers, communicating vessels and other instruments.

All experiments together and each separately refuted Dalton's theory that homogeneous liquids expand in proportion to the square of the temperature to which they are heated. Of course, the higher the temperature, the greater the volume of the liquid, but there was no direct relationship between it. Yes, and the expansion rate of all liquids was different.

The thermal expansion of water, for example, starts at zero degrees Celsius and continues as the temperature drops. Previously, such results of experiments were associated with the fact that it is not the water itself that expands, but the container in which it is located narrows. But some time later, the physicist Deluca nevertheless came to the conclusion that the cause should be sought in the liquid itself. He decided to find the temperature of its greatest density. However, he did not succeed due to the neglect of some details. Rumfort, who studied this phenomenon, found that the maximum density of water is observed in the range from 4 to 5 degrees Celsius.

Thermal expansion of bodies

In solids, the main expansion mechanism is a change in the amplitude of vibrations of the crystal lattice. If to speak in simple terms, then the atoms that make up the material and are rigidly linked to each other begin to “tremble”.

The law of thermal expansion of bodies is formulated as follows: any body with a linear size L in the process of heating by dT (delta T is the difference between the initial temperature and the final temperature), expands by dL (delta L is the derivative of the coefficient of linear thermal expansion by the length of the object and by the difference temperature). This is the simplest version of this law, which by default takes into account that the body expands in all directions at once. But for practical work they use much more cumbersome calculations, since in reality materials behave differently from those modeled by physicists and mathematicians.

Thermal expansion of the rail

Physicists are always involved in laying the railway track, as they can accurately calculate what distance should be between the joints of the rails so that the tracks do not deform when heated or cooled.

As mentioned above, thermal linear expansion is applicable to all solids. And the rail is no exception. But there is one detail. Linear change occurs freely if the body is not affected by the friction force. The rails are rigidly attached to the sleepers and welded to adjacent rails, so the law that describes the change in length takes into account the overcoming of obstacles in the form of linear and butt resistances.

If the rail cannot change its length, then with a change in temperature, thermal stress increases in it, which can both stretch and compress it. This phenomenon is described by Hooke's law.

thermal expansion

thermal expansion- change in the linear dimensions and shape of the body with a change in its temperature. Quantitatively, the thermal expansion of liquids and gases at constant pressure is characterized by an isobaric expansion coefficient (volumetric coefficient of thermal expansion). To characterize the thermal expansion of solids, the coefficient of linear thermal expansion is additionally introduced.

The branch of physics that studies this property is called dilatometry.

The thermal expansion of bodies is taken into account in the design of all installations, instruments and machines operating in variable temperature conditions.

Basic law of thermal expansion states that a body with a linear dimension in the corresponding dimension, with an increase in its temperature, expands by an amount equal to:

,

where is the so-called coefficient of linear thermal expansion. Similar formulas are available for calculating changes in the area and volume of a body. In the simplest case presented, when the coefficient of thermal expansion does not depend on either the temperature or the direction of expansion, the substance will expand uniformly in all directions in strict accordance with the above formula.

see also

Links

Wikimedia Foundation. 2010 .

See what "Thermal Expansion" is in other dictionaries:

Change in the size of the body in the process of heating it. Quantitatively T. r. at constant pressure p is characterized by an isobaric coefficient. expansion (coefficient of volumetric T. p.) a \u003d 1 / VX (dV / dT) p, where V is the volume of the body (solid, liquid or gaseous), T its ... ... Physical Encyclopedia

Thermal expansion, change in the size and shape of the body with a change in its temperature. It is characterized by the coefficients of volumetric (for solids and linear) thermal expansion, i.e. a change in the volume (linear dimensions) of the body when it changes ... ... Modern Encyclopedia

Change in the size of the body when it is heated; characterized by the coefficient of volumetric expansion, and for solids and the coefficient of linear expansion, where l is the change in linear size, ?V body volume, ?T temperature, the index indicates ... ... Big Encyclopedic Dictionary

thermal expansion- — [Ya.N. Luginsky, M.S. Fezi Zhilinskaya, Yu.S. Kabirov. English Russian Dictionary of Electrical Engineering and Power Engineering, Moscow, 1999] Electrical engineering topics, basic concepts EN heat expansionthermal expansion ... Technical Translator's Handbook

THERMAL EXPANSION- change in the size and shape of bodies when they are heated. The difference in the forces of adhesion between the molecules of the body in its various aggregate (see) affects the value of T. r. Solids, whose molecules interact strongly, expand little, liquids ... ... Great Polytechnic Encyclopedia

Change in the size of the body in the process of heating it. Quantitatively T. r. at constant pressure, it is characterized by an isobaric expansion coefficient (volume coefficient T. R.) T2 > T1, V the initial volume of the body (temperature difference T2 T1 ... ... Great Soviet Encyclopedia

thermal expansion- šiluminis plėtimasis statusas T sritis Standartizacija ir metrologija apibrėžtis Kaitinamo kūno matmenų padidėjimas. atitikmenys: engl. heat expansion; thermal expansion vok. thermische Ausdehnung, f; Wärmeausdehnung, f rus. thermal expansion, ... ... Penkiakalbis aiskinamasis metrologijos terminų žodynas

thermal expansion- šiluminis plėtimasis statusas T sritis chemija apibrėžtis Kaitinamo kūno matmenų padidėjimas. atitikmenys: engl. heat expansion; thermal expansion eng. thermal expansion; thermal expansion... Chemijos terminų aiskinamasis žodynas

thermal expansion- šiluminis plėtimasis statusas T sritis fizika atitikmenys: engl. heat expansion; thermal expansion vok. thermische Ausdehnung, f; Wärmeausdehnung, f rus. thermal expansion, n; thermal expansion, n pranc. dilatation thermal, f; expansion… … Fizikos terminų žodynas

Change in the size of the body when it is heated; characterized by the coefficient of volumetric expansion αυ = 1/V (ΔV/VT)Ξ, and for solids and the coefficient of linear expansion αl = 1/l(Δl/ΔТ)Ξ, where Δl is the change in the linear size, ΔV of the volume of the body, ΔТ … … encyclopedic Dictionary

Books

• Thermal expansion of solids , S. I. Novikova , Various aspects of existing theories of thermal expansion of solids are considered in detail in the monograph: the relationship between thermal expansion and other properties of solids, the influence ... Category: Solid state physics. Crystallography Publisher: Science,
• Physics. Thermal phenomena. Thermal expansion of solid and liquid bodies. Gases. 9-11 grades. Tasks for preparing for the olympiads,

Ticket number 3

"Thermal expansion of bodies. Thermometer. Temperature scales. The value of thermal expansion of bodies in nature and technology. Features of thermal expansion of water»

thermal expansion- change in the linear dimensions and shape of the body with a change in its temperature.

Cause: body temperature increases -> increases the speed of movement of molecules -> increases the amplitude of oscillations -> increases the distance between the molecules, and hence the size of the body.

Different bodies expand differently when heated, because the masses of the molecules are different, therefore, it differs kinetic energy and intermolecular distances vary in different ways.

Quantitatively, the thermal expansion of liquids and gases at constant pressure is characterized by voluminous thermal expansion coefficient (β).

V=V0(1+β(tfinal-tinitial))

Where V is the volume of the body at the final temperature, V0 is the volume of the body at the initial temperature

To characterize the thermal expansion of solids, the coefficient is additionally introduced linear thermal expansion (α)

l=l0 (1+α(tfinal-tinitial))

Where l is the length of the body at the final temperature, l0 is the length of the body at the initial temperature

Thermometer- temperature measuring device

The action of the thermometer is based on the thermal expansion of the liquid.

Invented by Galileo in 1597.

Types of thermometers:

mercury (from -35 to 750 degrees Celsius)

alcohol (from -80 to 70 degrees Celsius)

Pentane (from -200 to 35 degrees Celsius)

Scales:

Fahrenheit. Fahrenheit in 1732 - filled pipes with alcohol, later switched to mercury. Zero scale - the temperature of the mixture of snow with ammonia or table salt. Freezing water - 32°F. The temperature of a healthy person is 96°F. Water boils at 212°F.

Celsius. Swedish physicist Celsius in 1742 The freezing point of a liquid is 0°C and the boiling point is 100°C

Kelvin scale. In 1848, the English physicist William Thomson (Lord Kelvin). The reference point is "absolute zero" - -273.15°C. At this temperature, the thermal motion of molecules stops. 1°C=1°C

In fact, absolute zero is not reachable.

In everyday life and technology thermal expansion is very great importance. On electric railways it is necessary to maintain a constant tension in the wire supplying energy to electric locomotives in winter and summer. To do this, the tension of the wire is created by a cable, one end of which is connected to the wire, and the other is thrown over the block and a load is suspended from it.

During the construction of the bridge, one end of the truss is placed on the rollers. If this is not done, then when expanding in summer and contracting in winter, the truss will loosen the foundations on which the bridge rests.

In the manufacture of incandescent lamps, part of the wire passing inside the glass must be made of a material whose expansion coefficient is the same as that of glass, otherwise it may crack.

Power line wires are never pulled to avoid breaking.

Steam pipelines are supplied with bends, compensators.

The thermal expansion of air plays a big role role in natural phenomena. Thermal expansion of air creates a movement of air masses in a vertical direction (heated, less dense air rises, cold, less dense air descends). Uneven air heating different parts earth causes wind. The uneven heating of the water creates currents in the oceans.

During heating and cooling of rocks, due to daily and annual temperature fluctuations (if the composition of the rock is heterogeneous), cracks are formed, which contributes to the destruction of rocks.

The most abundant substance on the earth's surface is water- has a feature that distinguishes it from most other liquids. It expands when heated only above 4 °C. From 0 to 4 ° C, the volume of water, on the contrary, decreases when heated. Thus, water has the highest density at 4 °C. These data refer to fresh (chemically pure) water. Sea water has its highest density at about 3°C. An increase in pressure also lowers the temperature of the highest density of water.