Thermal expansion of solids. What happens to water when heated

(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.

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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 volume coefficients (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

The change in the linear dimensions of the body when heated is proportional to the change in temperature.

Most substances expand when heated. This is easily explained from the standpoint of the mechanical theory of heat, since when heated, the molecules or atoms of a substance begin to move faster. In solids, atoms begin to oscillate with greater amplitude around their average position in the crystal lattice, and they require more free space. As a result, the body expands. Similarly, liquids and gases, for the most part, expand with increasing temperature due to an increase in speed. thermal motion free molecules ( cm. Boyle's law - Mariotte, Charles's law, Equation of state of an ideal gas).

The basic law of thermal expansion states that a body with a linear dimension L in the corresponding dimension with an increase in its temperature by Δ T expands by Δ L equal to:

Δ L = aLΔ T

where α — 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.

For engineers, thermal expansion is a vital phenomenon. Designing a steel bridge across a river in a city with continental climate, it is impossible not to take into account the possible temperature difference ranging from -40°C to +40°C during the year. Such differences will cause a change in the total length of the bridge up to several meters, and so that the bridge does not rear up in summer and does not experience powerful rupture loads in winter, the designers make up the bridge from separate sections, connecting them with special thermal buffer joints, which are engaged, but not rigidly connected, rows of teeth that close tightly in the heat and diverge quite widely in the cold. On the long bridge there may be quite a few such buffers.

However, not all materials, especially crystalline solids, expand uniformly in all directions. And not all materials expand equally when different temperatures. Most a prime example the last kind is water. When cooled, water first contracts, like most substances. However, from +4°C to the freezing point of 0°C, water begins to expand when cooled and contract when heated (in terms of the above formula, we can say that in the temperature range from 0°C to +4°C, the coefficient of thermal expansion water α accepts negative meaning). It is thanks to this rare effect that the earth's seas and oceans do not freeze to the bottom even in the most severe frosts: water colder than +4°C becomes less dense than warmer water, and floats to the surface, displacing water with a temperature above +4°C to the bottom.

What ice has specific gravity below the density of water is another (though not related to the previous one) anomalous property of water, to which we owe the existence of life on our planet. If not for this effect, the ice would go to the bottom of rivers, lakes and oceans, and they, again, would freeze to the bottom, killing all life.

Most substances tend to expand when heated. Which is quite easy to explain from the standpoint of the mechanical theory of heat. According to her, when heated, the atoms and molecules of a substance begin to move faster. In solids, atomic vibrations reach a greater amplitude, and they need more free space. As a result, the body expands.

The same process occurs with liquids and gases. That is, due to the increase in temperature, the speed of thermal movement of free molecules increases, and the body expands. When cooled, the body contracts accordingly. This is true for almost all substances. Except for water.

When cooled in the range from 0 to 4°C, water expands. And shrinks when heated. When the water temperature mark reaches 4°C, at that moment the water has a maximum density, which is 1000 kg/m3. If the temperature is below or above this mark, then the density is always slightly less.

Due to this property, when the air temperature drops in autumn and winter in deep waters, interesting process. When the water cools, it sinks lower to the bottom, but only until its temperature reaches +4oC. It is for this reason that in large bodies of water, colder water is closer to the surface, and warmer water sinks to the bottom. So when the surface of the water freezes in winter, the deeper layers continue to maintain a temperature of 4oC. Thanks to this moment, the fish can safely winter in the depths of ice-covered reservoirs.

Impact of water expansion on climate

Thus, the layers of water are continuously changing, which leads to uniform heating until the temperature corresponding to the maximum density is reached. Then, as it heats up, the upper layers become less dense and no longer sink down, but remain at the top and just gradually become warmer. Due to this process, huge layers of water are quite easily warmed up by the sun's rays.

From the previous paragraphs, we know that all substances are composed of particles (atoms, molecules). These particles are constantly moving randomly. When a substance is heated, the movement of its particles becomes faster. In this case, the distances between the particles increase, which leads to an increase in the size of the body.

The change in the size of a body when it is heated is called thermal expansion..

Thermal expansion of solids is easy to confirm by experiment. A steel ball (Fig. 87, a, b, c), freely passing through the ring, after heating on a spirit lamp, expands and gets stuck in the ring. After cooling, the ball again freely passes through the ring. It follows from experience that the dimensions of a solid body increase when heated, and decrease when cooled.

Rice. 87

The thermal expansion of different solids is not the same.

During the thermal expansion of solids, huge forces appear that can destroy bridges, bend railroad tracks, and break wires. To prevent this from happening, the factor of thermal expansion is taken into account when designing a structure. The wires of power lines sag (Fig. 88) so that in winter, when shortened, they do not break.

Rice. 88

Rice. 89

The rails at the joints have a gap (Fig. 89). Bearing parts of bridges are placed on rollers that can move when the length of the bridge changes in winter and summer (Fig. 90).

Rice. 90

Do liquids expand when heated? Thermal expansion of liquids can also be confirmed experimentally. Pour into identical flasks: in one - water, and in the other - the same volume of alcohol. We close the flasks with stoppers with tubes. We mark the initial levels of water and alcohol in the tubes with rubber rings (Fig. 91, a). Put the flasks in a container with hot water. The water level in the pipes will become higher (Fig. 91, b). Water and alcohol expand when heated. But the level in the tube of the flask with alcohol is higher. So the alcohol expands more. Consequently, thermal expansion of different liquids, like solids, unequally.

Rice. 91

Do gases experience thermal expansion? We will answer the question with the help of experience. We close the flask with air with a cork with a curved tube. In the tube (Fig. 92, a) there is a drop of liquid. It is enough to bring the hands closer to the flask, as the drop begins to move to the right (Fig. 92, b). This confirms the thermal expansion of air when it is even slightly heated. Moreover, which is very important, all gases, unlike solids and liquids, when heated expand equally.

Rice. 92

Think and answer 1. What is called the thermal expansion of bodies? 2. Give examples of thermal expansion (compression) of solids, liquids, gases. 3. How does the thermal expansion of gases differ from the thermal expansion of solids and liquids?

Do it yourself at home

Using plastic bottle and a thin tube for juice, conduct an experiment at home on the thermal expansion of air and water. Describe the results of the experiment in a notebook.

Interesting to know!

Do not drink immediately after hot tea cold water. A sudden change in temperature often leads to tooth decay. This is due to the fact that the main substance of the tooth - dentin - and the enamel covering the tooth expand differently at the same temperature change.

When bodies are heated, the average kinetic energy forward movement molecules and the average distance between molecules. Therefore, all substances expand when heated, and contract when cooled. Distinguish between linear and volumetric expansion.

The change in one specific size of a solid with changes in temperature is called linear expansion (or contraction).

Where is the length of the rod at 0 0,

Linear expansion coefficient. Dimension = O C -1.

body length at any temperature t: ;

With volume expansion the volume increases: , where: is the volume of the body at 0 0 C.

body volume at any temperature t: , where:

Volume expansion coefficient;

It has been experimentally established that . That's why .

Similarly for surface area solid body: .

In liquids, there is one remarkable exception: when heated from 0 0 C to +4 0 C, water contracts, and when cooled from +4 0 C to 0 0 C, it expands. The volumetric expansion coefficient of water varies greatly with temperature.

Examples of thermal expansions:

Freezing water expands and breaks rocks, metal pipes and other technical structures.

In automation, bimetallic plates are used, using the difference in the coefficients of linear expansion of each of the two plates. When heated, the bimetallic plate loses stability, presses the switch, as a result of which the actuator is triggered.

Thermal expansion is important to consider when laying rails, pulling wires, building bridges, etc. Conclusions from electric lamps and radio lamps are made from a material whose coefficient of linear expansion is close to the coefficient of linear expansion of glass.

melting and crystallization.
Phase diagram

The transition of a substance from a solid state to a liquid state is called melting and the transition from liquid to solid solidification or crystallization. Melting and solidification occur at the same temperature, called melting temperature. Pressure has practically no effect on the melting temperature. The melting point of a substance at normal atmospheric pressure called melting point.

When a solid is melted, the distances between the particles that form crystal lattice, and the lattice itself is destroyed. In the vast majority of substances, the volume increases during melting, and decreases during solidification.

The region in which matter is homogeneous in all physical and chemical properties, is called phase state of this substance. The liquid and solid phases of a substance at the same temperature can remain in equilibrium arbitrarily for a long time(ice and water at 0 0 C). Therefore, until all the substance is melted, its temperature remains unchanged., equal to the melting point.

heat of fusion called the amount of heat that must be brought to a body of mass m, located at melting point to melt it down.

Where is the specific heat of fusion.

1 J/kg.

Figure 34 shows graphs of the change in the temperature of a substance during melting and solidification. The segment (Figure 34a) expresses the amount of heat, received substance when heated in the solid state (from to T PL), cut - when melting and cut - when heated in the liquid state. The segment (Figure 34b) expresses the amount of heat, given substance when cooled in a liquid state (from to), cut - when solidified and cut - when cooled in a solid state.

Figure 34. Graphs of changes in the temperature of a substance during melting and solidification

Many solids have an odor. This proves that solids can pass into a gaseous state, bypassing the liquid state. Evaporation of solids is called sublimation or sublimation(from the Latin "sublimate" - to lift up). AT Food Industry“dry ice” (CO 2) having this property is used. The reverse process is also possible - the growth of crystals from a gaseous substance (ice on the windows, overgrowth of ROM jumpers).

For each substance, one can state diagram in coordinates P and T (Figure 35), on the basis of which it is easy to determine in what state this substance will be under certain external conditions. Each point of the diagram corresponds to the equilibrium state of the substance, in which it can be arbitrarily long.

The KC curve is the dependence of the saturation vapor pressure on the temperature. Point K is a critical point.

Curve CA - temperature dependence of the pressure of saturated vapors in equilibrium state with the surface of a solid body.

The KC curve is the line of equilibrium between the liquid and gaseous phases. Straight line BC is the line of equilibrium of the liquid and solid phases. The AC curve is the equilibrium line for the solid and gaseous phases.

Point C represents the equilibrium between all three phases and is called the triple point. Helium does not have a triple point.

Test questions:

1. Tell us about the thermal expansion of solids.

2. What is melting and crystallization? What is the heat of fusion?

3. What is the sublimation of a substance?

4. Tell us about the state diagram of matter.