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 higher the vapor density, the more its molecules are 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 state 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 more low temperature, then in these layers the vapor condenses in the bubbles. 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 saturation vapor pressure 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.
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 |
Humidity is a measure of the amount of water vapor in the air. Relative humidity is the amount of water contained in the air at a given temperature compared to the maximum amount of water that can be contained in the air at the same temperature as vapor.
In other words, relative humidity shows how much more moisture is not enough to under given conditions environment condensation has started. This value characterizes the degree of air saturation with water vapor. When calculating the optimum humidity in a room, one speaks of relative humidity.
- For example, at a temperature of 21°C, one kilogram of dry air can contain up to 15.8 g of moisture. If 1 kg of dry air contains 15.8 g of water, then the relative humidity is said to be 100%. If the same amount of air contains 7.9 g of water at the same temperature, then, compared to the maximum possible amount of moisture, the ratio will be: 7.9 / 15.8 = 0.50 (50%). Therefore, the relative humidity of such air will be 50%.
What is the optimal humidity
The ideal humidity in a residential area is 40-60%. AT summer months the air is sufficiently humidified (in especially rainy weather, relative humidity can reach 80-90%), so there is no need for additional methods of humidification.
However, in winter, central heating systems and other heating appliances lead to overdrying of the air. This is because strong heating raises the temperature, but does not increase the amount of water vapor. This causes increased evaporation of moisture from everywhere: from your skin and from your body, indoor plants and even furniture. Relative humidity in apartments in winter is usually no more than 15%. This is even less than in the Sahara desert! The relative humidity there is 25%.
Table optimal humidity demonstrates how insufficient the level of 15% is:
How to achieve optimal humidity?
The only advice is to humidify the room.
There are many "folk" ways of moisturizing. You can, for example, hang wet towels and rags in the room. Place a water tank on the heater. Evaporation of water will sooner or later lead to an increase in air humidity. To protect the piano from drying out, it was previously recommended to put a jar of water inside. An option for those who do not spare money is a decorative fountain in the room.
However, these methods are inconvenient and ineffective. Significantly increase the humidity in the room with a jar of water will not work. In addition, a jar on a battery and towels on ropes do not look very aesthetically pleasing.
The most effective and practical way to increase indoor humidity is to install humidifier. This climatic device is able to maintain a precisely set level of humidity, moreover, it is inexpensive and easy to use. A new generation of humidifiers themselves control the optimum humidity.
The air is filled with water vapor to some extent. Its quantity is characterized by such an indicator as humidity. It can be absolute and relative. The first indicator indicates the volume of water contained in one cubic meter of air. The second term is used to define the ratio between the maximum possible amount of steam to the real one. If the humidity in the room is determined, it means a relative indicator.
Why measure and control indoor humidity?
Humidity in the house directly affects the health and well-being of all its inhabitants. If the indicators do not correspond to the norm, not only people suffer, but also houseplants, furniture and other things. The amount of water vapor in the environment is not stable and changes all the time depending on the season.
Why is dry air dangerous?
Low indoor humidity is very common during the heating season. This leads to the fact that a person quickly loses water through the skin and respiratory tract. As a result of such negative phenomena, the following effects are observed:
- decrease in elasticity and dryness of the skin, which is accompanied by the appearance of microcracks, leads to the development of dermatitis;
- drying of the mucous membrane of the eyes leads to their redness, burning, tearing;
- blood loses part of the liquid component, which reduces the speed of its movement, creating an additional load on the heart;
- a person suffers from headaches, feels tired and loses normal working capacity;
- viscosity increases gastric juice that impairs digestion;
- drying of the mucous membranes of the respiratory tract occurs, which weakens local immunity;
- an increase in the concentration of pathogens in the air, which are usually neutralized by air droplets.
To measure the air in an apartment, it is enough to buy the simplest device, which is usually combined with a thermometer or clock. It has a small error of 3-5%, which is not critical.
Using a glass of water
To determine the humidity of the air, it is necessary to draw water into an ordinary glass and send it to the refrigerator for 3 hours so that the liquid cools to 3-5 ° C. The vessel is taken out and placed on the table away from heating appliances. For several minutes, they observe the walls of the glass, where they detect the appearance of condensate in the form of water droplets. The results of the experiment are expressed as follows:
- the glass dried quickly - the humidity is lowered;
- the walls remained foggy - indoor humidity standards are met;
- water began to flow down the glass - the humidity is increased.
Assmann table
The Assmann table is designed to determine humidity using a psychrometer. It consists of two thermometers - a conventional one and a humidifier. The indicators measured by the second device will be slightly lower. According to a special table, using the obtained values, determine the humidity of the air.
Using a fir cone
They take an ordinary spruce cone and put it away from heating appliances. In dry air, its scales will open, and in moist air, they will shrink tightly.
generally accepted norms
The norms of humidity in the room depend on its purpose and time of year. Compliance with the recommended parameters will ensure good health, and will not adversely affect human immunity.
Norms for an apartment
For an apartment, all norms regarding climatic parameters are specified in GOST 30494-96. According to this document, air humidity in the cold season should range from 30-45%, and in the warm season - 30-60%. Despite these values, an indicator of 30% may be poorly perceived by the human body. Therefore, doctors recommend maintaining parameters of 40-60%, which are considered optimal at any time of the year.
Norms for a children's room
The child's body is unable to function properly with low humidity. This leads to a rapid drying of the mucous membranes, which is fraught with a decrease in local immunity.
Workplace
The norm of humidity in the workplace depends on the specifics of work. For example, for office workers it is 40-60%.
How to normalize the microclimate in the room?
To make the indoor climate comfortable for living, you need to use the following tips:
- use of humidifiers. Indispensable during the heating season in any premises;
- regular ventilation;
- increase in the number of indoor plants;
- exhaust ventilation. The supply hood will supply the room fresh air and normalizes the amount of water vapor;
- in some cases it is recommended to use special dehumidifiers equipped with absorbent substances;
- in residential premises it is forbidden to dry clothes, which negatively affects their microclimate.
Video: How to measure air humidity
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I-17="">Saturated steam, air humidity
Today's lesson will be devoted to a discussion of such a thing as air humidity, and methods for measuring it. The main phenomenon affecting air humidity will be the process of water evaporation, which we have already discussed earlier, and the most important concept that we will use will be saturated and unsaturated steam.
If we single out different states of vapor, then they will be determined by the interaction of the vapor with its liquid. If we imagine that some liquid is in a closed vessel and the process of its evaporation takes place, then sooner or later this process will come to a state where evaporation at equal time intervals will be compensated by condensation and the so-called dynamic equilibrium of the liquid with its vapor will come (Fig. 1) .
Rice. 1. Saturated steam
Definition.Saturated steam A vapor is in thermodynamic equilibrium with its liquid. If the steam is not saturated, then there is no such thermodynamic equilibrium (Fig. 2).
Rice. 2. Unsaturated steam
With the help of these two concepts, we will describe such an important characteristic of air as humidity.
Definition.Air humidity- a value indicating the content of water vapor in the air.
The question arises: why is the concept of humidity important to consider and how does water vapor get into the air? It is known that most of the Earth's surface is occupied by water (the World Ocean), from the surface of which evaporation occurs continuously (Fig. 3). Certainly, in various climatic zones the intensity of this process is different, which depends on the average daily temperature, the presence of winds, etc. These factors determine the fact that in certain places the process of water vaporization is more intense than its condensation, and in some places it is vice versa. On average, it can be argued that the vapor that forms in the air is not saturated, and its properties must be able to be described.
Rice. 3. Liquid evaporation (Source)
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 is worth noting that if we compare the humidity at different times of the year in the usual for us 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.
Absolute air humidity
Main Features 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:img="">
mass of steam (water) in air, kg (in SI) or g;
I-19="">Relative air humidity
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 terms, 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:
Img="" i-20="">Condensation hygrometer
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 (Source)
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 drops 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.
Hair hygrometer
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.
Rice. 5. Hair hygrometer (Source)
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 makes it possible to measure 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.
Psychrometer
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.
Humidity tables
Let's turn to the tables that allow you to determine the characteristics of air humidity.
Temperature,
Pressure, mm rt. Art.
steam density,
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.
