Relative humidity indoors. Air humidity. Methods for determining air humidity Using a spruce cone

For this task you can get 1 point on the exam in 2020

Task 10 of the USE in physics is devoted to thermal equilibrium and everything connected with it. The tickets are structured in such a way that approximately half of them contain questions on humidity (a typical example of such a task is “How many times has the concentration of vapor molecules increased if the volume of vapor is isothermally halved”), the rest concern the heat capacity of substances. Questions on heat capacity almost always contain a graph that must first be studied in order to correctly answer the question.

Task 10 of the USE in physics usually causes difficulties for students, except for a few options that are devoted to determining the relative humidity of the air using psychrometric tables. Most often, students begin the tasks with this question, the solution of which usually takes one to two minutes. Giving a student a ticket with just this type of task No. 10 of the Unified State Exam in physics will greatly facilitate the whole test, since the time to complete it is limited to a certain number of minutes.

Some water was poured into a glass flask and closed with a cork. The water gradually evaporated. At the end of the process, only a few drops of water remained on the walls of the flask. The figure shows a plot of concentration versus time n water vapor molecules inside the flask. Which statement can be considered correct?

o 1) in section 1, steam is saturated, and in section 2 - unsaturated

o 2) in section 1, steam is unsaturated, and in section 2 - saturated

o 3) in both sections, steam is saturated

2. Task #D3360E

The relative humidity of the air in a closed vessel is 60%. What will be the relative humidity if the volume of the vessel at a constant temperature is reduced by 1.5 times?

5. Task №4aa3e9

Relative humidity of the air in the room at a temperature of 20 °C
is equal to 70%. Using the vapor pressure table, determine the vapor pressure of the room.

o 1) 21.1 mm Hg. Art.

o 2) 25 mm Hg. Art.

o 3) 17.5 mm Hg. Art.

o 4) 12.25 mm Hg. Art.

32. Quest №e430b9

The relative humidity of the air in the room at a temperature of 20°C is 70%. Using the table of saturated water vapor density, determine the mass of water per cubic meter of the room.

o 3)1.73⋅10 -2 kg

o 4)1.21⋅10 -2 kg

33. Task №DFF058

On the ri-sun-ke of the image-ra-zhe-na: dot-dir-noy li-ni-her - graph for-vi-si-mo-sti pressure-of-saturated vapors water from tem-pe-ra-tu-ry, and a continuous li-ni-her - process 1-2 from-me-not-pair-qi-al-no-go vapor pressure water.

To the extent of such a change from the par-qi-al-no-go pressure of water vapor, the absolute humidity of the air-du-ha

1) uve-li-chi-va-et-sya

2) reduce-sha-et-sya

3) not from me

4) can both increase and decrease

34. Quest №e430b9

To determine-de-le-niya from-but-si-tel-noy humidity-no-sti air-du-ha use-pol-zu-yut difference in-ka-za-ny su-ho-go and wet-but-go ter-mo-meters (see ri-su-nok). Using the data of ri-sun-ka and psi-chro-met-ri-che-table-tsu, define-de-li-te, what kind of pe-ra-tu-ru ( in gra-du-sah Tsel-siya) ka-zy-va-et dry ter-mo-meter, if from-no-si-tel-naya humidity of the air-du-ha in a better place -nii 60%.

35. Task №DFF034

In co-su-de, under the piston, on-ho-dit-sya is not-on-saturated steam. It can be re-re-ve-sti in the rich,

1) iso-bar-but-you-shay-pe-ra-tu-ru

2) adding another gas to the vessel

3) increase the volume of steam

4) reduce the volume of steam

36. Task #9C5165

From-no-si-tel-naya humidity of the air-du-ha in someone-on-one is 40%. Ka-ko-in co-from-no-she-nie con-centr-tra-tion n mo-le-cool of water in the air of the room-on-you and concentration of mo-le-cool of water in saturated water vapor at the same dark per-ra-tu-re?

1) n less than 2.5 times

2) n more than 2.5 times

3) n less than 40%

4) n more by 40%

37. Task №DFF058

The relative humidity of the air in the cylinder under the piston is 60%. The air iso-ter-mi-che-ski was compressed, reducing its volume by half. From-no-si-tel-naya humidity air-du-ha has become

38. Task №1BE1AA

In the closed qi-lin-dri-che-so-su-de, humid air is on-ho-dit at a temperature of 100 ° C. In order for you-pa-la dew to be on the walls of this so-su-da, you need iso-ter-mi-che-ski from me-thread the volume of so-su-da is 25 once. What is approximately equal to the first-in-the-initial ab-co-lute humidity of the air-du-ha in so-su-de? Answer with-ve-di-te in g / m 3, district-whether to whole.

39. Task №0B1D50

In a cylindrical vessel under the piston for a long time there are water and its steam. The piston begins to move out of the vessel. At the same time, the temperature of water and steam remains unchanged. How will the mass of the liquid in the vessel change in this case? Explain your answer by indicating what physical laws you used to explain

40. Task №C32A09

In a cylindrical vessel under the piston for a long time there are water and its steam. The piston is pushed into the vessel. At the same time, the temperature of water and steam remains unchanged. How will the mass of the liquid in the vessel change in this case? Explain your answer by indicating what physical patterns you used to explain.

41. Task №AB4432

In an experiment illustrating the dependence of the boiling point on air pressure (Fig. a ), boiling water under the bell of the air pump occurs already at room temperature, if the pressure is low enough.

Using a pressure plot saturated steam on temperature (Fig. b ), indicate how much air pressure must be created under the pump bell so that the water boils at 40 ° C. Explain your answer by indicating what phenomena and patterns you used to explain.


(a)	(b)

42. Quest #E6295D

Relative humidity at t= 36 o C is 80%. Saturated vapor pressure at this temperature p n = 5945 Pa. What mass of vapor is contained in 1 m 3 of this air?

43. Task #9C5165

A man with glasses entered the warm room from the street and found that his glasses were fogged up. What should be the outside temperature for this phenomenon to occur? The air temperature in the room is 22°C and the relative humidity is 50%. Explain how you got the answer. (Use the table for saturated vapor pressure of water when answering this question.)

44. Quest #E6295D

In the closed so-su-de, on-ho-dyat-sya-dya-noy steam and not-something-swarm amount of water. How from-me-nyat-sya with an iso-ter-mi-che-sky decrease in volume-e-ma co-su-yes the following three things-li-chi-na: giving -le-nie in so-su-de, mass of water, mass of steam? For each ve-li-chi-ny, define-de-li-te co-from-vet-stvo-u-char-ter from-me-non-niya:

1) increase-li-chit-sya;

2) reduce;

3) not from-me-nit-Xia.

For-pi-shi-te in the table-li-tsu selected numbers for each fi-zi-che-ve-li-chi-ny. The numbers in the from-ve-those may be repeated.

45. Task #8BE996

The absolute humidity of the air-du-ha, on-ho-dya-sche-go-xia in qi-lin-dri-che-so-su-de under the piston, is equal to. The temperature of the gas in co-su-de is 100 ° C. How and how many times tre-bu-et-sya iso-ter-mi-che-ski from-me-thread the volume of co-su-da in order to form on its walls about-ra-zo-va did the dew fall?

1) reduce-sew near-bli-zi-tel-but 2 times 2) increase-li-chit near-zi-tel-but 20 times
3) reduce-sew near-bli-zi-tel-but 20 times 4) increase-li-chit near-zi-tel-but 2 times

46. Task №8BE999

In the ex-pe-ri-men-te, we establish-new-le-but, that at the same time-pe-ra-tu-re air-du-ha in someone on the wall-ke hundred-ka-on with cold water na-chi-na-et-sya con-den-sa-tion of water vapor from the air-du-ha, if you reduce the-pe-ra-tu-ru hundred-ka-na to . According to the rezul-ta-there of these ex-pe-ri-men-tov, determine the de-li-te from-no-si-tel-nuyu humidity of the air-du-ha. For solving for-da-chi, use the table-li-tsey. Is it due to-no-si-tel-naya humidity when the temperature rises-pe-ra-tu-ry air-du-ha in someone-on-those, if con-den-sa-tion of water vapor from air-du-ha will be na-chi-na-et-sya at the same te-pe-ra-tu-re hundred-ka-na? Pressure and density of saturated water-no-go steam at different temperatures-pe-ra-tu-re in-ka-for-but in tab -whether:



7,7	8,8	10,0	10,7	11,4	12,11	12,8	13,6	16,3	18,4	20,6	23,0	25,8	28,7	51,2	130,5

« Physics - Grade 10 "

When solving problems, it must be borne in mind that the pressure and density of saturated vapor do not depend on its volume, but depend only on temperature. The ideal gas equation of state is also approximately applicable to the description of saturated steam. But when saturated steam is compressed or heated, its mass does not remain constant.

Some applications may require saturation vapor pressures at certain temperatures. These data must be taken from the table.

Task 1.

A closed vessel with a volume V 1 = 0.5 m 3 contains water weighing m = 0.5 kg. The vessel was heated to a temperature t = 147 °C. By how much should the volume of the vessel be changed so that it contains only saturated steam? Saturated steam pressure p. p at a temperature of t = 147 ° C is equal to 4.7 10 5 Pa.

Decision.

Saturated steam at a pressure of pH. n occupies a volume equal to where M \u003d 0.018 kg / mol is the molar mass of water. The volume of the vessel is V 1 > V, which means that the steam is not saturated. In order for the steam to become saturated, the volume of the vessel must be reduced by

Task 2.

The relative humidity of air in a closed vessel at a temperature of t 1 = 5 ° C is equal to φ 1 = 84%, and at a temperature of t 2 = 22 ° C is equal to φ 2 = 30%. How many times is the saturated vapor pressure of water at temperature t 2 greater than at temperature t 1 ?

Decision.

The water vapor pressure in the vessel at T 1 \u003d 278 K is where r n. n1 - pressure of saturated steam at temperature T 1 . At a temperature T 2 \u003d 295 K, the pressure

Since the volume is constant, according to Charles's law

From here

Task 3.

In a room with a volume of 40 m 3, the air temperature is 20 ° C, its relative humidity φ 1 \u003d 20%. How much water must be evaporated so that the relative humidity φ 2 reaches 50%? It is known that at 20 °C the pressure of saturating vapors is рнп = 2330 Pa.

Decision.

Relative Humidity from here

Steam pressure at relative humidity φ 1 and φ 2

Density is related to pressure by the equation ρ = Mp/RT, whence

Masses of water in the room at humidity φ 1 and φ 2

Mass of water to be evaporated:

Task 4.

In a room with closed windows at a temperature of 15 °C relative humidity φ = 10%. What will be the relative humidity if the temperature in the room rises by 10°C? Saturated vapor pressure at 15 °C p.m. n1 = 12.8 mm Hg. Art., and at 25 ° C p n p2 \u003d 23.8 mm Hg. Art.

Since the steam is unsaturated, the partial pressure of the vapor changes according to Charles's law p 1 /T 1 = p 2 /T 2. From this equation, you can determine the pressure of unsaturated vapor p 2 at T 2: p 2 \u003d p 1 T 2 /T 1. Relative humidity at T 1 is equal.

Saturated steam.

If a vessel with liquid tightly, then the amount of liquid will first decrease, and then will remain constant. If not menn th temperature, the liquid - vapor system will come to a state of thermal equilibrium and will remain in it for an arbitrarily long time. Simultaneously with the evaporation process, condensation also occurs, both processes on average compenergize each other. At the first moment, after the liquid is poured into the vessel and closed, the liquid willevaporate and the vapor density above it will increase. However, at the same time, the number of molecules returning to the liquid will also increase. The greater the vapor density, the greater the number of its molecules returned to the liquid. As a result, a dynamic (mobile) equilibrium between liquid and vapor is established in a closed vessel at a constant temperature, i.e., the number of molecules leaving the liquid surface for some R th period of time, will be equal on average to the number of vapor molecules returning in the same time to the liquid b. Steam, nah which is in dynamic equilibrium with its liquid is called saturated vapor. This is the definition of underscoreIt means that a given volume at a given temperature cannot contain a greater amount of vapor.

Saturated steam pressure .

What will happen to saturated steam if the volume occupied by it is reduced? For example, if you compress vapor that is in equilibrium with a liquid in a cylinder under a piston, keeping the temperature of the contents of the cylinder constant. When the vapor is compressed, the equilibrium will begin to be disturbed. The vapor density at the first moment will increase slightly, and more molecules will begin to pass from gas to liquid than from liquid to gas. After all, the number of molecules leaving the liquid per unit time depends only on the temperature, and the compression of the vapor does not change this number. The process continues until the dynamic equilibrium and vapor density are again established, and hence the concentration of its molecules will not take their previous values. Consequently, the concentration of saturated vapor molecules at a constant temperature does not depend on its volume. Since the pressure is proportional to the concentration of molecules (p=nkT), it follows from this definition that the pressure of saturated vapor does not depend on the volume it occupies. Pressure p n.p. the vapor at which the liquid is in equilibrium with its vapor is called the saturation vapor pressure.

Dependence of pressure of saturated vapor on temperature.

The state of saturated steam, as experience shows, is approximately described by the equation of state of an ideal gas, and its pressure is determined by the formula P = nkT With increasing temperature, the pressure increases. Since the saturation vapor pressure does not depend on volume, it therefore depends only on temperature. However, the dependence of рn.p. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. With an increase in temperature, the pressure of a real saturated vapor increases faster than the pressure of an ideal gas (Fig.curve sink 12). Why is this happening? When a liquid is heated in a closed vessel, part of the liquid turns into vapor. As a result, according to the formula P = nkT, the saturated vapor pressure increases not only due to an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor. Basically, the increase in pressure with increasing temperature is determined precisely by the increase in concentration center ii. (The main difference in behavior andideal gas and saturated steam lies in the fact that when the temperature of the vapor in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the vapor changes. The liquid partially turns into vapor, or, conversely, the vapor partially condensestsya. Nothing like this happens with an ideal gas.) When all the liquid has evaporated, the vapor, upon further heating, will cease to be saturated and its pressure at constant volume will increasebe directly proportional to the absolute temperature (see Fig., curve section 23).

Boiling.

Boiling is an intense transition of a substance from a liquid to a gaseous state, occurring throughout the entire volume of the liquid (and not just from its surface). (Condensation is the reverse process.) As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles form throughout the volume of the liquid, which float to the surface. The boiling point of a liquid remains constant. This is because all the energy supplied to the liquid is spent on turning it into steam. Under what conditions does boiling begin?

Dissolved gases are always present in the liquid, which are released on the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are the centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the vapor pressure increases and the bubbles increase in size. Under the action of the buoyant force, they float up. If the upper layers of the liquid have a lower temperature, then vapor condenses in the bubbles in these layers. The pressure drops rapidly and the bubbles collapse. The collapse is so fast that the walls of the bubble, colliding, produce something like an explosion. Many of these microexplosions create a characteristic noise. When the liquid warms up enough, the bubbles stop collapsing and float to the surface. The liquid will boil. Watch the kettle on the stove carefully. You will find that it almost stops making noise before boiling. The dependence of saturation vapor pressure on temperature explains why the boiling point of a liquid depends on the pressure on its surface. A vapor bubble can grow when the pressure of the saturated vapor inside it slightly exceeds the pressure in the liquid, which is the sum of the air pressure on the surface of the liquid (external pressure) and the hydrostatic pressure of the liquid column. Boiling begins at a temperature at which the pressure of saturated vapor in the bubbles is equal to the pressure in the liquid. The greater the external pressure, the higher the boiling point. Conversely, by reducing the external pressure, we thereby lower the boiling point. By pumping out air and water vapor from the flask, you can make the water boil at room temperature. Each liquid has its own boiling point (which remains constant until the entire liquid boils off), which depends on its saturated vapor pressure. The higher the saturation vapor pressure, the lower the boiling point of the liquid.

Air humidity and its measurement.

The air around us almost always contains some amount of water vapor. The humidity of the air depends on the amount of water vapor it contains. Raw air contains a higher percentage of water molecules than dry air. Pain Of great importance is the relative humidity of the air, reports of which are heard every day in weather forecast reports.

RelativeHumidity is the ratio of the density of water vapor contained in the air to the density of saturated vapor at a given temperature, expressed as a percentage (shows how close the water vapor in the air is to saturation).

Dew point

The dryness or humidity of the air depends on how close its water vapor is to saturation. If moist air is cooled, then the vapor in it can be brought to saturation, and then it will condense. A sign that the steam is saturated is the appearance of the first drops of condensed liquid - dew. The temperature at which the vapor in the air becomes saturated is called the dew point. The dew point also characterizes the humidity of the air. Examples: dew in the morning, fogging of cold glass if you breathe on it, the formation of a drop of water on a cold water pipe, dampness in the basements of houses. Hygrometers are used to measure air humidity. There are several types of hygrometers, but the main ones are hair and psychrometric.

In this lesson, the concept of absolute and relative humidity will be introduced, the terms and quantities associated with these concepts will be discussed: saturated steam, dew point, devices for measuring humidity. During the lesson, we will get acquainted with the density and pressure tables of saturated steam and the psychrometric table.

For a person, the value of humidity is a very important parameter of the environment, since our body reacts very actively to its changes. For example, such a mechanism for regulating the functioning of the body as sweating is directly related to the temperature and humidity of the environment. At high humidity, the processes of evaporation of moisture from the surface of the skin are practically compensated by the processes of its condensation and the removal of heat from the body is disturbed, which leads to violations of thermoregulation. At low humidity, the processes of evaporation of moisture prevail over the processes of condensation and the body loses too much fluid, which can lead to dehydration.

The value of humidity is important not only for humans and other living organisms, but also for the flow of technological processes. For example, due to the known property of water to conduct electricity, its content in the air can seriously affect the correct operation of most electrical appliances.

In addition, the concept of humidity is the most important criterion for evaluating weather conditions, which is known to all from weather forecasts. It should be noted that if we compare the humidity at different times of the year in our usual climatic conditions, then it is higher in summer and lower in winter, which is associated, in particular, with the intensity of evaporation processes at different temperatures.

The main characteristics of humid air are:

density of water vapor in air;
relative humidity.

Air is a compound gas, it contains many different gases, including water vapour. To estimate its amount in the air, it is necessary to determine what mass the water vapor has in a certain allocated volume - this value characterizes the density. The density of water vapor in air is called absolute humidity.

Definition.Absolute air humidity- the amount of moisture contained in one cubic meter of air.

Designationabsolute humidity: (as well as the usual notation for density).

Unitsabsolute humidity: (in SI) or (for the convenience of measuring the small amount of water vapor in the air).

Formula calculations absolute humidity:

Designations:

Mass of steam (water) in air, kg (in SI) or g;

The volume of air in which the indicated mass of vapor is contained, .

On the one hand, the absolute humidity of the air is an understandable and convenient value, since it gives an idea of the specific water content in the air by mass, on the other hand, this value is inconvenient from the point of view of the susceptibility of humidity by living organisms. It turns out that, for example, a person feels not the mass content of water in the air, but its content relative to the maximum possible value.

To describe this perception, a quantity such as relative humidity.

Definition.Relative humidity- a value showing how far the steam is from saturation.

That is, the value of relative humidity, in simple words, shows the following: if the steam is far from saturation, then the humidity is low, if it is close, it is high.

Designationrelative humidity: .

Unitsrelative humidity: %.

Formula calculations relative humidity:

Notation:

Water vapor density (absolute humidity), (in SI) or ;

Density of saturated water vapor at a given temperature, (in SI) or .

As can be seen from the formula, it contains the absolute humidity, with which we are already familiar, and the density of saturated vapor at the same temperature. The question arises, how to determine the last value? For this, there are special devices. We'll consider condensinghygrometer(Fig. 4) - a device that serves to determine the dew point.

Definition.Dew point is the temperature at which the steam becomes saturated.

Rice. 4. Condensation hygrometer ()

Easily evaporating liquid, for example, ether, is poured inside the container of the device, a thermometer (6) is inserted and air is pumped through the container using a pear (5). As a result of increased air circulation, intensive evaporation of the ether begins, the temperature of the container decreases because of this, and dew appears on the mirror (4) (droplets of condensed vapor). At the moment when dew appears on the mirror, the temperature is measured using a thermometer, and this temperature is the dew point.

What to do with the obtained temperature value (dew point)? There is a special table in which data is entered - what density of saturated water vapor corresponds to each specific dew point. It should be noted a useful fact that with an increase in the dew point value, the value of the corresponding saturated vapor density also increases. In other words, the warmer the air, the more moisture it can contain, and vice versa, the colder the air, the lower the maximum vapor content in it.

Let us now consider the principle of operation of other types of hygrometers, devices for measuring humidity characteristics (from the Greek hygros - “wet” and metreo - “I measure”).

Hair hygrometer(Fig. 5) - a device for measuring relative humidity, in which hair, for example, human hair, acts as an active element.

The action of a hair hygrometer is based on the property of fat-free hair to change its length with changes in air humidity (with an increase in humidity, the length of the hair increases, with a decrease, it decreases), which allows measuring relative humidity. The hair is stretched over a metal frame. The change in the length of the hair is transmitted to the arrow moving along the scale. It should be remembered that the hair hygrometer gives inaccurate relative humidity values, and is used mainly for domestic purposes.

More convenient to use and accurate is such a device for measuring relative humidity as a psychrometer (from other Greek ψυχρός - “cold”) (Fig. 6).

The psychrometer consists of two thermometers, which are fixed on a common scale. One of the thermometers is called wet, because it is wrapped in cambric, which is immersed in a water tank located on the back of the device. Water evaporates from the wet tissue, which leads to cooling of the thermometer, the process of reducing its temperature continues until it reaches the stage until the steam near the wet tissue reaches saturation and the thermometer starts to show the dew point temperature. Thus, a wet bulb thermometer indicates a temperature less than or equal to the actual ambient temperature. The second thermometer is called dry and shows the actual temperature.

On the case of the device, as a rule, the so-called psychrometric table is also depicted (Table 2). Using this table, the relative humidity of the ambient air can be determined from the temperature value indicated by the dry bulb and the temperature difference between the dry bulb and the wet bulb.

However, even without such a table at hand, you can roughly determine the amount of humidity using the following principle. If the readings of both thermometers are close to each other, then the evaporation of water from a humid one is almost completely compensated by condensation, i.e., the air humidity is high. If, on the contrary, the difference in thermometer readings is large, then evaporation from the damp tissue prevails over condensation and the air is dry and the humidity is low.

Let's turn to the tables that allow you to determine the characteristics of air humidity.

Temperature,	Pressure, mm rt. Art.	steam density,

Tab. 1. Density and pressure of saturated water vapor

Once again, we note that, as mentioned earlier, the value of the density of saturated vapor increases with its temperature, the same applies to the pressure of saturated vapor.

Tab. 2. Psychometric table

Recall that relative humidity is determined by the value of dry bulb readings (first column) and the difference between dry and wet readings (first row).

In today's lesson, we got acquainted with an important characteristic of air - its humidity. As we have already said, humidity in the cold season (in winter) decreases, and in the warm season (summer) it rises. It is important to be able to regulate these phenomena, for example, if it is necessary to increase the humidity, place several water tanks indoors in winter to enhance evaporation processes, but this method will be effective only at the appropriate temperature, which is higher than outside.

In the next lesson, we will look at what is the work of gas, and the principle of operation of an internal combustion engine.

Bibliography

Gendenstein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.

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