Relative humidity of air through pressure. Air humidity. Methods for determining air humidity. The role of evaporation processes for animal organisms

Humidity is the amount of water vapor in the atmosphere. This characteristic largely determines the well-being of many living beings, and also affects the weather and climatic conditions on our planet. For normal operation human body it must be within a certain range, regardless of the air temperature. There are two main characteristics of air humidity - absolute and relative:

• Absolute humidity is the mass of water vapor contained in one cubic meter of air. The absolute humidity unit is g/m3. Relative humidity is defined as the ratio of the current and maximum values ​​of absolute humidity at a certain air temperature.
• Relative humidity is usually measured in%. As the temperature increases, the absolute humidity of the air also increases from 0.3 at -30°C to 600 at +100°C. The relative humidity depends mainly on climatic zones Earth (middle, equatorial or polar latitudes) and seasons (autumn, winter, spring, summer).

There are auxiliary terms for determining humidity. For example, moisture content (g/kg), i.e. weight of water vapor per kilogram of air. Or the temperature of the "dew point", when the air is considered to be completely saturated, i.e. its relative humidity is 100%. In nature and refrigeration technology, this phenomenon can be observed on the surfaces of bodies whose temperature is lower than the dew point temperature in the form of water droplets (condensate), frost or frost.

Enthalpy

There is also such a thing as enthalpy. Enthalpy is a property of a body (substance) that determines the amount of energy stored in it. molecular structure, which is available for conversion into heat at a certain temperature and pressure. But not all energy can be converted into heat, because. part of the internal energy of the body remains in the substance to maintain its molecular structure.

Moisture calculation

Simple formulas are used to calculate humidity values. So, absolute humidity is usually denoted p and defined as

p = m aq. steam / V air

where m water. steam - mass of water vapor (g)
V air - the volume of air (m 3) in which it is contained.

The generally accepted notation for relative humidity is φ. Relative humidity is calculated using the formula:

φ \u003d (p / p n) * 100%

where p and p n are the current and maximum values ​​of absolute humidity. The value of relative humidity is most often used, since the state of the human body is largely affected not by the weight of moisture in the volume of air (absolute humidity), but rather by the relative water content.

Humidity is very important for the normal functioning of almost all living beings and, in particular, for humans. Its value (according to experimental data) should be in the range from 30 to 65%, regardless of temperature. For example, low humidity in winter (due to the small amount of water in the air) leads to drying out of all mucous membranes in a person, thereby increasing the risk of colds. High humidity, on the contrary, worsens the processes of thermoregulation and sweating through the skin. This creates a feeling of suffocation. In addition, maintaining air humidity is an important factor:

• for carrying out many technological processes in production;
• operation of mechanisms and devices;
• safety from destruction of building structures of buildings, interior elements made of wood (furniture, parquet, etc.), archaeological and museum artifacts.

Enthalpy calculation

Enthalpy is the potential energy contained in one kilogram of moist air. Moreover, in the equilibrium state of the gas, it is not absorbed and is not emitted into the external environment. The enthalpy of moist air is equal to the sum of the enthalpies of its constituent parts: absolutely dry air, as well as water vapor. Its value is calculated according to the following formula:

I = t + 0.001(2500 +1.93t)d

Where t is the air temperature (°С) and d is its moisture content (g/kg). Enthalpy (kJ/kg) is a specific quantity.

Wet bulb temperature

The wet bulb temperature is the value at which the process of adiabatic (constant enthalpy) saturation of air with water vapor takes place. To determine its specific value, an I - d diagram is used. First, a point corresponding to a given state of air is applied to it. Then an adiabatic ray is drawn through this point, crossing it with the saturation line (φ = 100%). And already from the point of their intersection, the projection is lowered in the form of a segment with a constant temperature (isotherm) and the temperature of the wet bulb is obtained.

The I-d diagram is the main tool for calculating / plotting various processes associated with a change in the state of air - heating, cooling, dehumidification and humidification. Its appearance greatly facilitated the understanding of the processes occurring in systems and units for air compression, ventilation and air conditioning. This diagram graphically shows the complete interdependence of the main parameters (temperature, relative humidity, moisture content, enthalpy and partial pressure of water vapor) that determine the heat-humidity balance. All values ​​are specified at a specific value atmospheric pressure. Usually it is 98 kPa.

The diagram is made in the system of oblique coordinates, i.e. the angle between its axes is 135°. This contributes to an increase in the zone of unsaturated moist air (φ = 5 - 99%) and greatly facilitates the graphic drawing of the processes occurring with the air. The diagram shows the following lines:

• curvilinear - humidity (from 5 to 100%).
• straight lines - constant enthalpy, temperature, partial pressure and moisture content.

Below the curve φ \u003d 100%, the air is completely saturated with moisture, which is in it in the form of a liquid (water) or solid (hoarfrost, snow, ice) state. It is possible to determine the state of air at all points of the diagram, knowing any two of its parameters (out of four possible). Graphic construction of the process of changing the state of air is greatly facilitated with the help of an additionally plotted pie chart. It shows the values ​​of the heat-humidity ratio ε at different angles. This value is determined by the slope of the process beam and is calculated as:

where Q is the heat (kJ/kg) and W is the moisture (kg/h) absorbed or released from the air. The value of ε divides the entire diagram into four sectors:

• ε = +∞ … 0 (heating + humidification).
• ε = 0 … -∞ (cooling + humidification).
• ε = -∞ … 0 (cooling + dehumidification).
• ε = 0 … +∞ (heating + dehumidification).

Humidity measurement

Measuring instruments for determining relative humidity values ​​are called hygrometers. Several methods are used to measure air humidity. Let's consider three of them.

1. For relatively inaccurate measurements in everyday life, hair hygrometers are used. In them, the sensitive element is a horse or human hair, which is installed in a steel frame in a taut state. It turned out that this hair in a fat-free form is able to sensitively respond to the slightest changes in the relative humidity of the air, changing its length. As the humidity increases, the hair lengthens, and as it decreases, on the contrary, it shortens. The steel frame, on which the hair is fixed, is connected to the arrow of the device. The arrow perceives the change in the size of the hair from the frame and rotates around its axis. At the same time, it indicates the relative humidity on a graduated scale (in %).
2. With more accurate thermotechnical measurements during scientific research condensation-type hygrometers and psychrometers are used. They measure relative humidity indirectly. The condensation type hygrometer is made in the form of a closed cylindrical container. One of its flat covers is polished to a mirror finish. A thermometer is installed inside the container and some low-boiling liquid, such as ether, is poured. Then, with a manual rubber diaphragm pump, air is pumped into the container, which begins to circulate intensively there. Because of this, the ether boils, lowers the temperature (cools) the surface of the container and its mirror, respectively. Drops of water condensed from the air will appear on the mirror. At this point in time, it is necessary to record the readings of the thermometer, which will show the temperature of the "dew point". Then, using a special table, the corresponding density of saturated steam is determined. And according to them, the value of relative humidity.
3. A psychrometric hygrometer is a pair of thermometers mounted on a base with a common scale. One of them is called dry, it measures the actual air temperature. The second is called wet. The wet bulb temperature is the temperature that humid air takes when it reaches a saturated state and maintains a constant air enthalpy equal to the initial one, i.e. this is the limiting temperature of adiabatic cooling. At the wet bulb thermometer, the ball is wrapped in a batiste cloth, which is immersed in a container of water. On the fabric, water evaporates, which leads to a decrease in air temperature. This cooling process continues until the air around the balloon is completely saturated (i.e. 100% relative humidity). This thermometer will show the "dew point". On the scale of the device there is also a so-called. psychrometric table. With its help, according to the dry bulb and the temperature difference (dry minus wet), the current value of relative humidity is determined.

Humidity control

Humidifiers are used to increase humidity (humidify the air). Humidifiers are very diverse, which is determined by the method of humidification and design. According to the method of humidification, humidifiers are divided into: adiabatic (nozzle) and steam. In steam humidifiers, water vapor is formed when water is heated on the electrodes. As a rule, steam humidifiers are most often used in everyday life. In ventilation and central air-conditioning systems, humidifiers of both steam and nozzle types are used. In industrial ventilation systems, humidifiers can be placed both directly in the ventilation units themselves, and as a separate section in the ventilation duct.

Most effective method removal of moisture from the air is carried out using compressor-based refrigeration machines. They dehumidify the air by condensing water vapor on the cooled surface of the evaporator heat exchanger. Moreover, its temperature should be below the "dew point". The moisture collected in this way is removed by gravity or with the help of a pump to the outside through the drainage pipe. Exist various types and appointments. By type, dehumidifiers are divided into monoblock and with a remote condenser. According to their purpose, dryers are divided into:

• household mobile;
• professional;
• stationary for swimming pools.

The main task of dehumidification systems is to ensure the well-being of people inside and the safe operation of structural elements of buildings. It is especially important to maintain the level of humidity in rooms with increased moisture release, such as swimming pools, water parks, baths and SPA complexes. The air in the pool has high humidity due to the intensive processes of water evaporation from the surface of the bowl. Therefore, excess moisture is the determining factor for. Excess moisture, as well as the presence of aggressive media in the air, such as chlorine compounds, have a devastating effect on the elements of building structures and interior decoration. Moisture condenses on them, causing mold growth or corrosion damage to metal parts.

For these reasons, the recommended value of relative humidity inside the pool should be maintained in the range of 50 - 60%. Building structures, in particular walls and glazed surfaces of the pool room, should be additionally protected from moisture falling on them. This can be realized by supplying a stream of fresh air to them, and always in the direction from the bottom to the top. From the outside, the building must have a layer of highly effective thermal insulation. To achieve additional benefits, we strongly recommend the use of a variety of dehumidifiers, but only in combination with optimally calculated and selected

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• ensure assimilation concept of air humidity ;
• develop student independence; thinking; ability to draw conclusions; development of practical skills when working with physical equipment;
• show practical application and importance of this physical quantity.

Type of lesson: lesson learning new material .

Equipment:

• For front work: a glass of water, a thermometer, a piece of gauze; threads, psychrometric table.
• for demonstrations: psychrometer, hair and condensation hygrometers, pear, alcohol.

During the classes

I. Review and check homework

1. Formulate the definition of the processes of vaporization and condensation.

2. What types of vaporization do you know? How do they differ from each other?

3. Under what conditions does the liquid evaporate?

4. On what factors does the evaporation rate depend?

5. What is the specific heat of vaporization?

6. What is the amount of heat supplied during vaporization spent on?

7. Why is hello jar easier?

8. Is the internal energy of 1 kg of water and steam the same at a temperature of 100 ° C

9. Why does water in a bottle tightly closed with a cork not evaporate?

II. Learning new material

Water vapor in the air, despite the huge surface of rivers, lakes, oceans, is not saturated, the atmosphere is an open vessel. The movement of air masses leads to the fact that in some places in this moment evaporation of water prevails over condensation, and vice versa in others.

Atmospheric air is a mixture of various gases and water vapor.

The pressure that water vapor would produce if all other gases were absent is called partial pressure (or elasticity) water vapor.

The density of water vapor contained in the air can be taken as a characteristic of air humidity. This value is called absolute humidity [g/m 3 ].

Knowing the partial pressure of water vapor or absolute humidity does not say anything about how far water vapor is from saturation.

To do this, a value is introduced showing how close water vapor at a given temperature is to saturation - relative humidity.

Relative humidity called the ratio of absolute humidity to the density 0 of saturated water vapor at the same temperature, expressed as a percentage.

P - partial pressure at a given temperature;

P 0 - saturated steam pressure at the same temperature;

absolute humidity;

0 is the density of saturated water vapor at a given temperature.

The pressure and density of saturated vapor at various temperatures can be found using special tables.

When moist air is cooled at constant pressure, its relative humidity rises, the lower the temperature, the closer the partial vapor pressure in the air to the saturated vapor pressure.

Temperature t, to which the air must be cooled so that the vapor in it reaches a state of saturation (at a given humidity, air and constant pressure), is called dew point.

Saturated water vapor pressure at air temperature equal to dew point, is the partial pressure of water vapor in the atmosphere. As the air cools down to the dew point, the vapors start to condense. : fog appears, falls dew. The dew point also characterizes the humidity of the air.

Air humidity can be determined with special devices.

1. Condensation hygrometer

It is used to determine the dew point. This is the most accurate way to change relative humidity.

2. Hair hygrometer

Its action is based on the property of defatted human hair With and lengthen with increasing relative humidity.

It is used in cases where high accuracy is not required in determining the humidity of the air.

3. Psychrometer

Usually used in cases where a sufficiently accurate and fast determination of air humidity is required.

The value of air humidity for living organisms

At a temperature of 20-25°C, air with a relative humidity of 40% to 60% is considered the most favorable for human life. When the environment has a temperature higher than the temperature of the human body, there is increased sweating. Abundant sweating leads to cooling of the body. However, such sweating is a significant burden for a person.

Relative humidity below 40% at normal air temperature is also harmful, as it leads to an increased loss of moisture in organisms, which leads to dehydration. Particularly low indoor air humidity in winter time; it is 10-20%. At low air humidity, rapid evaporation moisture from the surface and drying of the mucous membrane of the nose, larynx, lungs, which can lead to a deterioration in well-being. Also, at low air humidity in the external environment, pathogenic microorganisms persist longer, and more static charge accumulates on the surface of objects. Therefore, in winter, humidification is carried out in residential premises using porous humidifiers. Plants are good moisturizers.

If the relative humidity is high, then we say that the air damp and suffocating. High humidity is depressing because evaporation is very slow. The concentration of water vapor in the air in this case is high, as a result of which molecules from the air return to the liquid almost as quickly as they evaporate. If sweat from the body evaporates slowly, then the body is cooled very weakly, and we feel not quite comfortable. At 100% relative humidity, evaporation cannot occur at all - under such conditions, wet clothes or damp skin will never dry.

From the biology course, you know about the various adaptations of plants in arid areas. But plants are adapted to high humidity. So, the birthplace of Monstera is wet equatorial forest Monstera at a relative humidity close to 100% "weeps", it removes excess moisture through holes in the leaves - hydathodes. In modern buildings, air conditioning is used to create and maintain indoor air environment that is most favorable for people's well-being. At the same time, temperature, humidity, air composition are automatically regulated.

Humidity plays an important role in frost formation. If the humidity is high and the air is close to vapor saturation, then when the temperature drops, the air may become saturated and dew will begin to fall. But when water vapor condenses, energy is released (the specific heat of vaporization at a temperature close to 0 ° C is 2490 kJ / kg), therefore, the air near the soil surface during the formation of dew will not cool below the dew point and the likelihood of frost will decrease. The probability of freezing depends, firstly, on the rapidity of the temperature decrease and,

Secondly, from the humidity of the air. It is enough to know one of these data to more or less accurately predict the likelihood of a freeze.

Review questions:

1. What is meant by air humidity?
2. What is the absolute humidity of the air? What formula expresses the meaning of this concept? In what units is it expressed?
3. What is water vapor pressure?
4. What is the relative humidity of the air? What formulas express the meaning of this concept in physics and meteorology? In what units is it expressed?
5. Relative humidity of 70%, what does this mean?
6. What is called dew point?

What instruments are used to measure air humidity? What are the subjective sensations of air humidity by a person? After drawing a picture, explain the structure and principle of operation of a hair and condensation hygrometer and a psychrometer.

Laboratory work No. 4 "Measuring the relative humidity of the air"

Purpose: to learn how to determine the relative humidity of the air, develop practical skills when working with physical equipment.

Equipment: thermometer, gauze bandage, water, psychometric table

During the classes

Before performing the work, it is necessary to draw the attention of students not only to the content and progress of the work, but also to the rules for handling thermometers and glass vessels. It must be recalled that all the time while the thermometer is not used for measurements, it must be in the case. When measuring temperature, the thermometer should be held by the upper edge. This will allow you to determine the temperature with the greatest accuracy.

The first temperature measurements should be made with a dry bulb thermometer. This temperature in the auditorium will not change during operation.

To measure the temperature with a wet bulb thermometer, it is better to take a piece of gauze as a cloth. The gauze absorbs very well and moves water from the wet end to the dry end.

Using a psychrometric table, it is easy to determine the relative humidity value.

Let t c = h= 22 °С, t m \u003d t 2= 19 °C. Then t = tc- 1 W = 3 °C.

Find the relative humidity from the table. In this case, it is equal to 76%.

For comparison, you can measure the relative humidity of the air outside. To do this, a group of two or three students who have successfully completed the main part of the work can be asked to take similar measurements on the street. This should take no more than 5 minutes. The obtained humidity value can be compared with the humidity in the classroom.

The results of the work are summed up in the conclusions. They should note not only the formal values ​​of the final results, but also indicate the reasons that lead to errors.

III. Problem solving

Since this laboratory work quite simple in content and small in volume, the rest of the lesson can be devoted to solving problems on the topic under study. To solve problems, it is not necessary that all students begin to solve them at the same time. As the work progresses, they can receive assignments individually.

The following simple tasks can be suggested:

Cold autumn rain is falling outside. In which case will the laundry hung in the kitchen dry faster: when the window is open, or when it is closed? Why?

The humidity is 78% and the dry bulb reading is 12°C. What temperature does a wet bulb thermometer show? (Answer: 10 °C.)

The difference between dry and wet thermometer readings is 4°C. Relative air humidity 60%. What are the dry and wet bulb readings? (Answer: t c -l9°С, tm= 10 °C.)

Homework

• Repeat paragraph 17 of the textbook.
• Task number 3. p. 43.

Students' messages about the role of evaporation in the life of plants and animals.

Evaporation in plant life

For the normal existence of a plant cell, it must be saturated with water. For algae, it is a natural consequence of the conditions of their existence; for land plants, it is achieved as a result of two opposite processes: absorption of water by roots and evaporation. For successful photosynthesis, the chlorophyll-bearing cells of terrestrial plants must maintain the closest contact with the surrounding atmosphere, which supplies them with the carbon dioxide they need; however, this close contact inevitably leads to the fact that the water that saturates the cells continuously evaporates into the surrounding space, and the same solar energy that delivers the energy necessary for photosynthesis to the plant, being absorbed by chlorophyll, contributes to the heating of the leaf, and thereby to the intensification of the evaporation process.

Very few, and, moreover, low-organized plants, such as mosses and lichens, can withstand long interruptions in water supply and endure this time in a state of complete extinction. Of the higher plants, only some representatives of the rocky and desert flora are capable of this, for example, sedge, common in the sands of the Karakum. For the vast majority of large plants, such drying would be fatal, and therefore their water outflow is approximately equal to its inflow.

To imagine the scale of water evaporation by plants, let's give the following example: in one growing season, one flowering of sunflower or corn evaporates up to 200 kg or more of water, i.e., a barrel of solid size! With such an energetic consumption, no less energetic extraction of water is required. For this (the root system grows, the dimensions of which are huge, the number of roots and root hairs for winter rye gave the following amazing numbers: there were almost fourteen million roots, the total length of all roots is 600 km, and their total surface is about 225 m 2. On these roots had about 15 billion root hairs with a total area of ​​400 m 2 .

The amount of water used by a plant during its life depends to a large extent on the climate. In a hot dry climate, plants consume no less, and sometimes even more water than in a more humid climate, these plants have a more developed root system and less developed leaf surface. Plants of damp, shady tropical forests, shores of water bodies consume the least water: they have thin wide leaves, weak root and conducting systems. Plants in arid regions, where there is very little water in the soil, and the air is hot and dry, have various methods of adaptation to these harsh conditions. Desert plants are interesting. These are, for example, cacti plants with thick fleshy trunks, the leaves of which have turned into thorns. They have a small surface with a large volume, thick covers, little permeable to water and water vapor, with a few, almost always closed stomata. Therefore, even in extreme heat, cacti evaporate little water.

Other plants of the desert zone (camel thorn, steppe alfalfa, wormwood) have thin leaves with wide open stomata, which vigorously assimilate and evaporate, due to which the temperature of the leaves is significantly reduced. Often the leaves are covered with a thick layer of gray or white hairs, representing a kind of translucent screen that protects the plants from overheating and reduces the intensity of evaporation.

Many desert plants (feather grass, tumbleweed, heather) have tough, leathery leaves. Such plants are able to tolerate prolonged wilting. At this time, their leaves are twisted into a tube, and the stomata are inside it.

Evaporation conditions change dramatically in winter. From frozen soil, the roots cannot absorb water. Therefore, due to leaf fall, the evaporation of moisture by the plant decreases. In addition, in the absence of leaves, less snow lingers on the crown, which protects the plants from mechanical damage.

The role of evaporation processes for animal organisms

Evaporation is the most easily controlled way to reduce internal energy. Any conditions that impede mating violate the regulation of body heat transfer. So, leather, rubber, oilcloth, synthetic clothing makes it difficult to adjust body temperature.

Sweating plays an important role in the thermoregulation of the body, it ensures the constancy of the body temperature of a person or an animal. Due to the evaporation of sweat, internal energy decreases, thanks to which the body cools.

Air with a relative humidity of 40 to 60% is considered normal for human life. When the environment has a temperature higher than the human body, then there is an increase. Abundant sweating leads to cooling of the body, helps to work in conditions high temperature. However, such active sweating is a significant burden for a person! If, at the same time, the absolute humidity is high, then life and work become even more difficult (wet tropics, some workshops, for example, dyeing).

Relative humidity below 40% at normal air temperature is also harmful, as it leads to increased loss of moisture by the body, which leads to dehydration.

From the point of view of thermoregulation and the role of evaporation processes, some living beings are very interesting. It is known, for example, that a camel can not drink for two weeks. This is explained by the fact that it consumes water very economically. The camel hardly sweats even in forty-degree heat. His body is covered with thick and dense hair - the wool saves from overheating (on the back of a camel on a hot afternoon, it is heated to eighty degrees, and the skin under it is only up to forty!). Wool also prevents the evaporation of moisture from the body (in a sheared camel, perspiration increases by 50%). A camel never, even in the strongest heat, opens its mouth: after all, if you open your mouth wide, you evaporate a lot of water from the mucous membrane of the oral cavity! The respiratory rate of a camel is very low - 8 times a minute. Due to this, less water leaves the body with air. In the heat, however, his breathing rate increases to 16 times per minute. (Compare: a bull under the same conditions breathes 250, and a dog - 300-400 times per minute.) In addition, the camel's body temperature drops to 34 ° at night, and during the day, in the heat, rises to 40-41 °. This is very important for saving water. The camel also has a very curious device for storing water for the future. It is known that from fat, when it "burns" in the body, a lot of water is obtained - 107 g out of 100 g of fat. Thus, if necessary, a camel can extract up to half a centner of water from its humps.

From the point of view of economy in water consumption, the American jerboa jumpers (kangaroo rats) are even more amazing. They never drink at all. Kangaroo rats also live in the Arizona desert and gnaw on seeds and dry grasses. Almost all the water that is in their body is endogenous, i.e. produced in cells during the digestion of food. Experiments have shown that from 100 g of pearl barley, which was fed to kangaroo rats, they received, having digested and oxidized it, 54 g of water!

Air sacs play an important role in the thermoregulation of birds. In hot weather, moisture evaporates from the inner surface of the air sacs, which helps to cool the body. II connection with this bird in hot weather opens the beak. (Katz //./> Biophysics at the lessons of physics. - M .: Education, 1974).

n. Independent work

Which amount of heat released mri complete combustion of 20 kg of coal? (Answer: 418 MJ)

How much heat will be released during the complete combustion of 50 liters of methane? Take the density of methane equal to 0.7 kg / m 3. (Answer: -1.7 MJ)

On a glass of yogurt it is written: energy value 72 kcal. Express the energy value of the product in J.

The calorific value of a daily food ration for schoolchildren of your age is about 1.2 MJ.

1) Is it enough for you to consume for 100 g of fatty cottage cheese, 50 g of wheat bread, 50 g of beef and 200 g of potatoes. Required additional data:

• fat cottage cheese 9755;
• wheat bread 9261;
• beef 7524;
• potatoes 3776.

2) Is it enough for you to consume 100 g of perch, 50 g of fresh cucumbers, 200 g of grapes, 100 g of rye bread, 20 g of sunflower oil and 150 g of ice cream during the day.

Specific heat of combustion q x 10 3, J / kg:

• perch 3520;
• fresh cucumbers 572;
• grapes 2400;
• rye bread 8884;
• sunflower oil 38900;
• creamy ice cream 7498. ,

(Answer: 1) Approximately 2.2 MJ consumed - enough; 2) Consumed To 3.7 MJ is enough.)

When preparing for lessons for two hours, you spend about 800 kJ of energy. Will you restore energy if you drink 200 ml of skim milk and eat 50 g of wheat bread? The density of skimmed milk is 1036 kg/m 3 . (Answer: Approximately 1 MJ is consumed - enough.)

The water from the beaker was poured into a vessel heated by the flame of an alcohol lamp and evaporated. Calculate the mass of burned alcohol. Vessel heating and air heating losses can be neglected. (Answer: 1.26 g.)

• How much heat will be released during the complete combustion of 1 ton of anthracite? (Answer: 26.8. 109 J.)
• What mass of biogas must be burned to release 50 MJ of heat? (Answer: 2 kg.)
• What is the amount of heat released during the combustion of 5 liters of fuel oil. Raft ness take fuel oil equal to 890 kg / m 3. (Answer: approximately 173 MJ.)

On the box of sweets it is written: calorie content of 100 g is 580 kcal. Express the nyl content of the product in J.

Read the labels of different food products. Write down the energy I, with what value (caloric content) of products, expressing it in joules or ka-Yuri (kilocalories).

When cycling for 1 hour, you spend approximately 2,260,000 J of energy. Will you restore your energy reserve if you eat 200 g of cherries?

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 tables of density and pressure of saturated steam and the psychrometric table.

Humidity is a very important parameter for humans. environment, because 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 that everyone knows 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:

1. density of water vapor in air;
2. 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 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 increasing 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 the 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.

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 to know important characteristic 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 an 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

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

1. Internet portal "dic.academic.ru" ()
2. Internet portal "baroma.ru" ()
3. Internet portal "femto.com.ua" ()
4. Internet portal "youtube.com" ()

Homework

The concept of air humidity is defined as the actual presence of water particles in a certain physical environment, including the atmosphere. In this case, one should distinguish between absolute and relative humidity: in the first case, we are talking about a pure percentage of moisture. In accordance with the law of thermodynamics, the maximum content of water molecules in the air is limited. Maximum allowable level determines the relative humidity and depends on a number of factors:

• Atmosphere pressure;
• air temperature;
• the presence of small particles (dust);
• the level of chemical pollution;

The generally accepted measure of measurement is interest, and the calculation is carried out according to a special formula, which will be discussed later.

Absolute humidity is measured in grams per cubic centimeter, which are also converted to percentages for convenience. With increasing altitude, the amount of moisture may increase depending on the region, but upon reaching a certain ceiling (about 6-7 kilometers above sea level), the humidity decreases to near zero values. Absolute humidity is considered one of the main macroparameters: on its basis, planetary climate maps and zones.

Determining the humidity level

(Psychometer device - it determines the humidity by the temperature difference between dry and wet thermometers)

Humidity by absolute ratio is determined using special instruments that determine the percentage of water molecules in the atmosphere. As a rule, daily fluctuations are negligible - this indicator can be considered static, and it does not reflect important climatic conditions. On the contrary, relative humidity is subject to strong diurnal fluctuations, and reflects the exact distribution of condensed moisture, its pressure and equilibrium saturation. It is this indicator that is considered the main one and is calculated at least once a day.

The determination of relative air humidity is carried out according to a complex formula that takes into account:

• current dew point;
• temperature;
• saturated steam pressure;
• various mathematical models;

In the practice of synoptic forecasts, a simplified approach is used, when the humidity is calculated approximately, taking into account the temperature difference and the dew point (marks when excess moisture falls in the form of precipitation). This approach allows you to determine the required indicators with an accuracy of 90-95%, which is more than enough for everyday needs.

Dependence on natural factors

The content of water molecules in the air depends on climatic features specific region, weather conditions, atmospheric pressure and some other conditions. Thus, the highest absolute humidity is observed in tropical and coastal zones. Relative humidity additionally depends on the fluctuations of a number of factors discussed earlier. During a rainy period with conditions of low atmospheric pressure, relative humidity can reach 85-95%. High pressure reduces the saturation of water vapor in the atmosphere, thus lowering their level.

An important feature of relative humidity is its dependence on the thermodynamic state. The natural equilibrium humidity is 100%, which, of course, is unattainable due to the extreme instability of the climate. Technogenic factors also affect fluctuations in atmospheric humidity. In the conditions of megacities, there is an increased evaporation of moisture from asphalt surfaces, simultaneously with the release of a large amount of suspended particles and carbon monoxide. This causes a strong decrease in humidity in most cities of the world.

Impact on the human body

Atmospheric humidity limits that are comfortable for humans range from 40 to 70%. Prolonged exposure to conditions of a strong deviation from this norm can cause a noticeable deterioration in well-being, up to the development of pathological conditions. It should be noted that a person is especially sensitive to excessively low humidity, experiencing a number of characteristic symptoms:

• irritation of mucous membranes;
• development of chronic rhinitis;
• fatigue;
• deterioration in the condition of the skin;
• decreased immunity;

Among the negative effects of high humidity, one can note the risk of developing fungal and colds.

General information

Humidity depends on the nature of the substance, and in solids, in addition, on the degree of fineness or porosity. The content of chemically bound, so-called constitutional water, for example, hydroxides, which is released only during chemical decomposition, as well as crystalline hydrated water, is not included in the concept of humidity.

Units of measurement and features of the definition of the concept of humidity

• Moisture is usually characterized by the amount of water in a substance, expressed as a percentage (%) of the original mass of the wet substance ( mass humidity) or its volume ( bulk moisture).

• Humidity can also be characterized by moisture content, or absolute humidity- the amount of water per unit mass of the dry part of the material. This definition of moisture is widely used to assess the quality of wood.

This value cannot always be accurately measured, because in some cases it is impossible to remove all unconstitutional water and weigh the object before and after this operation.

• Relative humidity characterizes the moisture content relative to the maximum amount of moisture that can be contained in a substance in a state of thermodynamic equilibrium. Relative humidity is usually measured as a percentage of the maximum.

Methods of determination

Titrator Karl Fischer.

Establishing the degree of humidity of many products, materials, etc. has importance. Only at a certain humidity many bodies (grain, cement, etc.) are suitable for the purpose for which they are intended. The vital activity of animal and plant organisms is possible only at certain limits of humidity and relative humidity of the air. Humidity can introduce a significant error in the weight of the item. Kilograms of sugar or grains with 5% and 10% moisture content will contain different amounts of dry sugar or grains.

Moisture measurement is determined by drying the moisture and titrating the moisture according to Karl Fischer. These methods are primary. In addition to them, many others have been developed that are calibrated according to the results of moisture measurements by primary methods and according to standard moisture samples.

Air humidity

Air humidity is a value that characterizes the content of water vapor in various parts of the Earth's atmosphere.

Humidity - the content of water vapor in the air; one of the most significant characteristics of weather and climate.

Humidity in the earth's atmosphere varies widely. Yes, at earth's surface the content of water vapor in the air ranges on average from 0.2% by volume in high latitudes to 2.5% in the tropics. The vapor pressure in the polar latitudes is less than 1 mb in winter (sometimes only hundredths of a mb) and in summer below 5 mb; in the tropics, it increases to 30 mb, and sometimes more. In sub tropical deserts vapor pressure is reduced to 5-10 mb.

Absolute air humidity (f) is the amount of water vapor actually contained in 1m³ of air:

f = (mass of water vapor in the air)/(volume of moist air)

Commonly used absolute humidity unit: (f) = g/m³

Relative humidity (φ) is the ratio of its current absolute humidity to the maximum absolute humidity at a given temperature (see table)

φ = (absolute humidity)/(maximum humidity)

Relative humidity is usually expressed as a percentage. These quantities are related to each other by the following relation:

φ = (f×100)/fmax

Relative humidity is very high in equatorial zone(average annual up to 85% or more), as well as in polar latitudes and in winter inside the continents of middle latitudes. In summer, monsoon regions are characterized by high relative humidity. Low values ​​of relative humidity are observed in subtropical and tropical deserts and in winter in monsoon regions (up to 50% and below).

Humidity decreases rapidly with altitude. At a height of 1.5-2 km, the vapor pressure is on average half that at the earth's surface. The troposphere accounts for 99% of the atmospheric water vapor. On average, over every square meter of the earth's surface, the air contains about 28.5 kg of water vapor.

Literature

Usoltsev V. A. Measurement of air humidity, L., 1959.

Gas humidity measurement values

The following quantities are used to indicate the moisture content in the air:

Absolute air humidity is the mass of water vapor contained in a unit volume of air, i.e. density of water vapor contained in the air, [g/m³]; in the atmosphere ranges from 0.1-1.0 g/m³ (over the continents in winter) to 30 g/m³ or more (in the equatorial zone); maximum air humidity (saturation limit) the amount of water vapor that can be contained in the air at a certain temperature in thermodynamic equilibrium (maximum value of air humidity at a given temperature), [g/m³]. With an increase in air temperature, its maximum humidity increases; vapor pressure pressure exerted by water vapor contained in the air (water vapor pressure as part of atmospheric pressure), [Pa]; humidity deficit difference between saturated vapor pressure and vapor pressure [Pa], i.e. between maximum and absolute air humidity [g/m³]; relative humidity ratio of vapor pressure to saturated vapor pressure, i.e. absolute air humidity to maximum [% relative humidity]; dew point temperature of a gas at which the gas is saturated with water vapor °C . The relative humidity of the gas is 100%. With a further influx of water vapor or when air (gas) is cooled, condensate appears. Thus, although dew does not fall at −10 or −50°C, it does