What is air density and what is it equal to under normal conditions? How much does air weigh? Examples of problem solving

Although we cannot feel the air around us, air is not nothing. Air is a mixture of gases: nitrogen, oxygen and others. And gases, like other substances, consist of molecules, and therefore have weight, although small.

Experiments can be used to prove that air has weight. In the middle of a stick about sixty centimeters long, we will attach a rope, and we will tie two identical balloons to both ends. Let's hang the stick by a string and see that it hangs horizontally. If you now pierce one of the inflated balloons with a needle, the air will come out of it, and the end of the stick to which it was tied will rise up. If you pierce the second ball, the stick will again take a horizontal position.

This happens because there is air in the inflated balloon. tighter, and therefore heavier than the one around it.

How much air weighs depends on when and where it is weighed. The weight of air above a horizontal plane is Atmosphere pressure. Like all objects around us, air is also subject to gravity. It is this that gives the air a weight that is equal to 1 kg per square centimeter. The density of air is about 1.2 kg/m 3, that is, a cube with a side of 1 m filled with air weighs 1.2 kg.

A column of air rising vertically above the Earth stretches for several hundred kilometers. This means that a column of air weighing about 250 kg presses on a person standing upright, on his head and shoulders, the area of ​​which is approximately 250 cm 2!

We would not be able to withstand such a weight if it were not resisted by the same pressure inside our body. The following experience will help us understand this. If you stretch a sheet of paper with both hands and someone presses a finger on it on one side, the result will be the same - a hole in the paper. But if you press two index fingers nothing will happen to the same place, but from different sides. The pressure on both sides will be the same. The same thing happens with the pressure of the air column and the counter pressure inside our body: they are equal.

By the way...

In everyday life, when we weigh something, we do it in the air, and therefore we neglect its weight, since the weight of air in the air is zero. For example, if we weigh an empty glass flask, we will consider the result obtained to be the weight of the flask, neglecting the fact that it is filled with air. But if the flask is sealed and all the air is pumped out of it, we will get a completely different result...

The main physical properties air: air density, its dynamic and kinematic viscosity, specific heat capacity, thermal conductivity, thermal diffusivity, Prandtl number and entropy. The properties of air are given in tables depending on temperature at normal atmospheric pressure.

Air density depending on temperature

A detailed table of dry air density values ​​is presented at different temperatures and normal atmospheric pressure. What is the density of air? The density of air can be determined analytically by dividing its mass by the volume it occupies. at given conditions(pressure, temperature and humidity). You can also calculate its density using the formula of the ideal gas equation of state. To do this, you need to know the absolute pressure and temperature of the air, as well as its gas constant and molar volume. This equation allows you to calculate the dry density of air.

On practice, to find out what the density of air is at different temperatures, it is convenient to use ready-made tables. For example, the given table of density values atmospheric air depending on its temperature. Air density in the table is expressed in kilograms per cubic meter and is given in the temperature range from minus 50 to 1200 degrees Celsius at normal atmospheric pressure (101325 Pa).

At 25°C, air has a density of 1.185 kg/m3. When heated, the air density decreases - the air expands (its specific volume increases). As the temperature increases, for example to 1200°C, a very low air density is achieved, equal to 0.239 kg/m 3, which is 5 times less than its value at room temperature. In general, reduction during heating allows a process such as natural convection to take place and is used, for example, in aeronautics.

If we compare the density of air relative to , then air is three orders of magnitude lighter - at a temperature of 4°C, the density of water is 1000 kg/m3, and the density of air is 1.27 kg/m3. It is also necessary to note the air density at normal conditions. Normal conditions for gases are those at which their temperature is 0°C and the pressure is equal to normal atmospheric pressure. Thus, according to the table, air density under normal conditions (at NL) is 1.293 kg/m 3.

Dynamic and kinematic viscosity of air at different temperatures

When performing thermal calculations, it is necessary to know the value of air viscosity (viscosity coefficient) at different temperatures. This value is required to calculate the Reynolds, Grashof, and Rayleigh numbers, the values ​​of which determine the flow regime of this gas. The table shows the values ​​of the dynamic coefficients μ and kinematic ν air viscosity in the temperature range from -50 to 1200°C at atmospheric pressure.

The viscosity coefficient of air increases significantly with increasing temperature. For example, the kinematic viscosity of air is equal to 15.06 10 -6 m 2 /s at a temperature of 20°C, and with an increase in temperature to 1200°C, the viscosity of air becomes equal to 233.7 10 -6 m 2 /s, that is, it increases 15.5 times! The dynamic viscosity of air at a temperature of 20°C is 18.1·10 -6 Pa·s.

When air is heated, the values ​​of both kinematic and dynamic viscosity increase. These two quantities are related to each other through the air density, the value of which decreases when this gas is heated. An increase in the kinematic and dynamic viscosity of air (as well as other gases) when heated is associated with a more intense vibration of air molecules around their equilibrium state (according to MKT).

Note: Be careful! Air viscosity is given to the power of 10 6 .

Specific heat capacity of air at temperatures from -50 to 1200°C

A table of the specific heat capacity of air at various temperatures is presented. The heat capacity in the table is given at constant pressure (isobaric heat capacity of air) in the temperature range from minus 50 to 1200°C for air in a dry state. What is the specific heat capacity of air? The specific heat capacity determines the amount of heat that must be supplied to one kilogram of air at constant pressure to increase its temperature by 1 degree. For example, at 20°C, to heat 1 kg of this gas by 1°C in an isobaric process, 1005 J of heat is required.

Specific heat air increases with increasing temperature. However, the dependence of the mass heat capacity of air on temperature is not linear. In the range from -50 to 120°C, its value practically does not change - under these conditions, the average heat capacity of air is 1010 J/(kg deg). According to the table, it can be seen that temperature begins to have a significant effect from a value of 130°C. However, air temperature affects its specific heat capacity much less than its viscosity. Thus, when heated from 0 to 1200°C, the heat capacity of air increases only 1.2 times - from 1005 to 1210 J/(kg deg).

It should be noted that the heat capacity of humid air is higher than that of dry air. If we compare air, it is obvious that water has a higher value and the water content in air leads to an increase in specific heat capacity.

Thermal conductivity, thermal diffusivity, Prandtl number of air

The table presents such physical properties of atmospheric air as thermal conductivity, thermal diffusivity and its Prandtl number depending on temperature. Thermophysical properties of air are given in the range from -50 to 1200°C for dry air. According to the table, it can be seen that the indicated properties of air depend significantly on temperature and the temperature dependence of the considered properties of this gas is different.

Air density is a physical quantity that characterizes the specific gravity of air under natural conditions or the mass of gas in the Earth's atmosphere per unit volume. The value of air density is a function of the height of the measurements taken, its humidity and temperature.

The standard air density is taken to be 1.29 kg/m3, which is calculated as the ratio of its molar mass (29 g/mol) to the molar volume, the same for all gases (22.413996 dm3), corresponding to the density of dry air at 0° C (273.15°K) and a pressure of 760 mmHg (101325 Pa) at sea level (that is, under normal conditions).

Not long ago, information about air density was obtained indirectly through observations of auroras, the propagation of radio waves, and meteors. Since its inception artificial satellites Earth's air density began to be calculated thanks to data obtained from their braking.

Another method is to observe the spreading of artificial sodium vapor clouds created by weather rockets. In Europe, the air density at the Earth's surface is 1.258 kg/m3, at an altitude of five km - 0.735, at an altitude of twenty km - 0.087, at an altitude of forty km - 0.004 kg/m3.

There are two types of air density: mass and weight (specific gravity).

Weight density determines the weight of 1 m3 of air and is calculated by the formula γ = G/V, where γ is weight density, kgf/m3; G is the weight of air, measured in kgf; V is the volume of air, measured in m3. Determined that 1 m3 of air under standard conditions(barometric pressure 760 mmHg, t=15°С) weighs 1.225 kgf, based on this, the weight density (specific gravity) of 1 m3 of air is γ = 1.225 kgf/m3.

It should be taken into account that air weight is a variable quantity and changes depending on various conditions, such as geographic latitude and the force of inertia that occurs when the Earth rotates around its axis. At the poles the weight of air is 5% greater than at the equator.

Air mass density is the mass of 1 m3 of air, denoted by the Greek letter ρ. As you know, body weight is a constant quantity. The unit of mass is considered to be the mass of a platinum iridide weight, which is located in the International Chamber of Weights and Measures in Paris.

Air mass density ρ is calculated using the following formula: ρ = m/v. Here m is the mass of air, measured in kg×s2/m; ρ is its mass density, measured in kgf×s2/m4.

The mass and weight densities of air depend on: ρ = γ / g, where g is the gravitational acceleration coefficient equal to 9.8 m/s². It follows that the mass density of air under standard conditions is 0.1250 kg × s2/m4.

As barometric pressure and temperature change, the density of the air changes. Based on the Boyle-Marriott law, the greater the pressure, the greater the air density. However, as pressure decreases with altitude, air density also decreases, which introduces its own adjustments, as a result of which the law of vertical pressure changes becomes more complex.

The equation that expresses this law of pressure change with height in an atmosphere at rest is called basic equation of statics.

It states that with increasing altitude the pressure changes downward and when rising to the same height, the decrease in pressure is greater, the more more power gravity and air density.

Changes in air density play an important role in this equation. As a result, we can say that the higher you rise, the less pressure will drop when rising to the same height. Air density depends on temperature as follows: in warm air the pressure decreases less intensely than in cold air, therefore, at the same height, the pressure in a warm air mass is higher than in a cold one.

With changing values ​​of temperature and pressure, the mass density of air is calculated by the formula: ρ = 0.0473xB / T. Here B is the barometric pressure, measured in mm of mercury, T is the air temperature, measured in Kelvin.

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Density is also determined by air humidity. The presence of water pores leads to a decrease in air density, which is explained by the low molar mass of water (18 g/mol) against the background of the molar mass of dry air (29 g/mol). Wet air can be considered as a mixture of ideal gases, in each of which the combination of densities allows us to obtain the required density value for their mixture.

This kind of interpretation makes it possible to determine density values ​​with an error level of less than 0.2% in the temperature range from −10 °C to 50 °C. Air density allows you to obtain the value of its moisture content, which is calculated by dividing the density of water vapor (in grams) contained in the air by the density of dry air in kilograms.

The basic equation of statics does not allow solving constantly arising practical problems in real conditions of a changing atmosphere. Therefore, it is solved under various simplified assumptions that correspond to actual real conditions by making a number of partial assumptions.

The basic equation of statics makes it possible to obtain the value of the vertical pressure gradient, which expresses the change in pressure during ascent or descent per unit height, i.e., the change in pressure per unit vertical distance.

Instead of a vertical gradient, they often use its inverse value - the pressure level in meters per millibar (sometimes there is also an outdated version of the term “pressure gradient” - barometric gradient).

Low air density determines little resistance to movement. Many terrestrial animals, in the course of evolution, took advantage of the environmental benefits of this property of the air environment, due to which they acquired the ability to fly. 75% of all species of land animals are capable of active flight. They are mostly insects and birds, but there are also mammals and reptiles.

Video on the topic “Determination of air density”

Compressed air is air under pressure greater than atmospheric pressure.

Compressed air is a unique energy carrier along with electricity, natural gas and water. In industrial settings, compressed air is mainly used to drive pneumatically driven devices and mechanisms (pneumatic drive).

In everyday, everyday life, we practically do not notice the Air around us. However, throughout human history, people have used the unique properties of air. The invention of the sail and the forge, the windmill and hot air balloon became the first steps in using air as an energy carrier.

With the invention of the compressor, the era of industrial use of compressed air began. And the question: “ What is Air and what properties does it have? - became far from idle.

When starting to design a new pneumatic system or modernize an existing one, it would be useful to remember about some properties of air, terms and units of measurement.

Air is a mixture of gases, mainly consisting of nitrogen and oxygen.

The average relative molar mass is -28.98. 10 -3 kg/mol

*Air composition may vary. Typically, in industrial areas the air contains

Air is an intangible quantity, it cannot be touched or smelled, it is everywhere, but for humans it is invisible, it is not easy to find out how much air weighs, but it is possible. If the surface of the Earth, as in a children's game, is drawn into small squares measuring 1x1 cm, then the weight of each of them will be equal to 1 kg, that is, 1 cm 2 of atmosphere contains 1 kg of air.

Can this be proven? Quite. If you build a scale from an ordinary pencil and two balloons, securing the structure to a thread, the pencil will be in balance, since the weight of the two inflated balloons is the same. Once one of the balloons is pierced, the advantage will be in the direction of the inflated balloon, because the air from the damaged balloon has escaped. Accordingly, simple physical experience proves that air has a certain weight. But, if you weigh the air on a flat surface and in the mountains, then its mass will turn out to be different - Mountain air much easier than the one we breathe near the sea. There are several reasons for the different weights:

The weight of 1 m 3 of air is 1.29 kg.

• the higher the air rises, the more rarefied it becomes, that is, high in the mountains, the air pressure will not be 1 kg per cm 2, but half as much, but the content of oxygen necessary for breathing also decreases by exactly half, which can cause dizziness, nausea and ear pain;
• water content in the air.

The air mixture includes:

1.Nitrogen – 75.5%;

2. Oxygen – 23.15%;

3. Argon – 1.292%;

4. Carbon dioxide – 0.046%;

5. Neon – 0.0014%;

6. Methane – 0.000084%;

7. Helium – 0.000073%;

8. Krypton – 0.003%;

9. Hydrogen – 0.00008%;

10. Xenon – 0.00004%.

The amount of ingredients in the air may change and, accordingly, the mass of air also undergoes changes in the direction of increase or decrease.

• air always contains water vapor. The physical law is that the higher the air temperature, the more water it contains. This indicator is called air humidity and affects its weight.

What is the weight of air measured in? There are several indicators that determine its mass.

How much does a cube of air weigh?

At a temperature of 0° Celsius, the weight of 1 m 3 of air is 1.29 kg. That is, if you mentally allocate a space in a room with a height, width and length equal to 1 m, then this air cube will contain exactly this amount of air.

If air has weight and weight that is quite noticeable, why does a person not feel heaviness? This physical phenomenon, like atmospheric pressure, implies that every inhabitant of the planet is pressed by an air column weighing 250 kg. The average palm area of ​​an adult is 77 cm2. That is, in accordance with physical laws, each of us holds 77 kg of air in the palm of our hand! This is equivalent to the fact that we constantly carry 5 pound weights in each hand. IN real life Even a weightlifter cannot do this, however, each of us can handle such a load easily, because atmospheric pressure presses from both sides, both outside the human body and from the inside, that is, the difference is ultimately zero.

The properties of air are such that it affects the human body differently. High in the mountains, due to a lack of oxygen, people experience visual hallucinations, and at great depths, the combination of oxygen and nitrogen in a special mixture - “laughing gas” - can create a feeling of euphoria and a feeling of weightlessness.

Knowing these physical quantities, we can calculate the mass of the Earth’s atmosphere - the amount of air that is held in the near-Earth space by gravitational forces. Upper limit the atmosphere ends at an altitude of 118 km, that is, knowing the weight of m 3 of air, you can divide the entire borrowed surface into air columns, with a base of 1x1 m, and add up the resulting mass of such columns. Ultimately, it will be equal to 5.3 * 10 to the fifteenth power of tons. The weight of the planet's air armor is quite large, but it is only one millionth of total mass globe. The Earth's atmosphere serves as a kind of buffer that protects the Earth from unpleasant cosmic surprises. From solar storms alone that reach the surface of the planet, the atmosphere loses up to 100 thousand tons of its mass per year! Such an invisible and reliable shield is air.

How much does a liter of air weigh?

A person does not notice that he is constantly surrounded by transparent and almost invisible air. Is it possible to see this intangible element of the atmosphere? Visually, moving air masses broadcast daily on the television screen - a warm or cold front brings long-awaited warming or heavy snowfall.

What else do we know about air? Probably, the fact that it is vitally necessary for all living beings living on the planet. Every day a person inhales and exhales about 20 kg of air, a quarter of which is consumed by the brain.

The weight of air can be measured in different physical quantities, including in liters. The weight of one liter of air will be equal to 1.2930 grams, at a pressure of 760 mm Hg. column and a temperature of 0°C. In addition to the usual gaseous state, air can also be found in liquid form. For a substance to transition into this state of aggregation, it will require exposure to enormous pressure and very low temperatures. Astronomers suggest that there are planets whose surfaces are completely covered with liquid air.

The sources of oxygen necessary for human existence are the Amazon forests, which produce up to 20% of this important element all over the planet.

Forests are truly the “green” lungs of the planet, without which human existence is simply impossible. Therefore the living houseplants in an apartment are not just a piece of furniture, they purify the indoor air, the pollution of which is tens of times higher than outside.

Clean air has long become a shortage in megacities; air pollution is so great that people are ready to buy clean air. “Air sellers” first appeared in Japan. They produced and sold clean air in cans, and any resident of Tokyo could open a can of clean air for dinner and enjoy its freshest aroma.

Air purity has a significant impact not only on human health, but also on animal health. In polluted areas of equatorial waters, near human-populated areas, dozens of dolphins are dying. The cause of death for mammals is a polluted atmosphere; in autopsies of animals, the lungs of dolphins resemble the lungs of miners, clogged with coal dust. The inhabitants of Antarctica, penguins, are also very sensitive to air pollution if the air contains a large number of harmful impurities, they begin to breathe heavily and intermittently.

For a person, clean air is also very important, so after working in the office, doctors recommend taking daily hour-long walks in the park, forest, or outside the city. After such “air” therapy, the body’s vitality is restored and well-being significantly improves. The recipe for this free and effective medicine has been known since ancient times; many scientists and rulers considered daily walks in the fresh air a mandatory ritual.

For a modern city dweller, air treatment is very relevant: a small portion of life-giving air, weighing 1-2 kg, is a panacea for many modern ailments!