When buying a bird, even as an inexperienced farmer, it is worth showing special care and caution. When choosing a rooster, it is worth looking first of all at the condition of the bird and its appearance, because this is the first indicator of whether the bird is healthy or not. The rooster should be vigorous, confidently stand on its feet and move around without any visible problems. As it may already be clear, when choosing a rooster, first of all, you should pay attention to his appearance. Here's how to choose a rooster yourself.

Further, when examining a rooster, it is worth checking its head well. It is very important to listen to the breathing of the bird, if she has any difficulties during this process, if she has any damage in her nostrils, and the air emitted by the rooster should not be with any smell. After the breath is checked, it is worth examining the crest of the bird. A healthy rooster should have a bright red comb. If the bird is healthy, it’s not scary to cook any dish out of it, therefore, you can learn how to cook a rooster from books.

When choosing a rooster, farmers attach great importance to its plumage, because the feathers of a healthy rooster should be smooth and shiny, without any flaws. The paws of the rooster should be just as smooth, there should be the correct number of fingers on the paws and each of them should have a claw. When examining the paws of a rooster, you need to examine the spurs well. In young people, the spurs are almost invisible, in adults, on the contrary, they are large. During the preparation of the rooster, its paws are removed. Before cooking, you should ask how to cook a rooster correctly.

In addition, when buying a rooster, it is advisable to pay attention to the presence of a tongue in the bird and that its beak is healthy, without flaws, and closes correctly, without overlap. When choosing and buying a bird, you need to understand how to distinguish an old rooster from a young one. It is logical that young and adult roosters differ in their physique. A young rooster is smaller and more puny, while an adult, on the contrary, has a more massive body.

It is very important to remember that if there are roosters with five fingers on the paw, and not four, then this is not a deviation, it is just a completely different breed. The plumage of roosters differs by age. In adult roosters, it is denser and has a glossy sheen; in young roosters, the plumage is duller. A chicken can be distinguished from a rooster by the crest on its head. Here's how to determine whether a given bird is a chicken or a rooster.

When buying a rooster, you should pay attention to his back and posture. The ridge of the rooster should be even, without visible humps. The tail and plumage near it must also be free of flaws and in good condition. It is worth choosing a healthy and well-groomed rooster, which will subsequently be eaten. Those who have not cooked poultry meat before may ask experienced housewives how to cook a homemade rooster. Actually, it's not difficult.

Man on skis, and without them.

On loose snow, a person walks with great difficulty, sinking deeply at every step. But, having put on skis, he can walk, almost without falling into it. Why? On skis or without skis, a person acts on the snow with the same force equal to his own weight. However, the effect of this force in both cases is different, because the surface area on which the person presses is different, with and without skis. The surface area of ​​the ski is almost 20 times the area of ​​the sole. Therefore, standing on skis, a person acts on every square centimeter of the snow surface area with a force 20 times less than standing on snow without skis.

The student, pinning a newspaper to the board with buttons, acts on each button with the same force. However, a button with a sharper end is easier to enter into the tree.

This means that the result of the action of a force depends not only on its modulus, direction and point of application, but also on the area of ​​the surface to which it is applied (perpendicular to which it acts).

This conclusion is confirmed by physical experiments.

Experience. The result of this force depends on what force acts per unit area of ​​the surface.

Nails must be driven into the corners of a small board. First, we set the nails driven into the board on the sand with their points up and put a weight on the board. In this case, the nail heads are only slightly pressed into the sand. Then turn the board over and put the nails on the tip. In this case, the area of ​​support is smaller, and under the action of the same force, the nails go deep into the sand.

Experience. Second illustration.

The result of the action of this force depends on what force acts on each unit of surface area.

In the considered examples, the forces acted perpendicular to the surface of the body. The person's weight was perpendicular to the surface of the snow; the force acting on the button is perpendicular to the surface of the board.

The value equal to the ratio of the force acting perpendicular to the surface to the area of ​​\u200b\u200bthis surface is called pressure.

To determine the pressure, it is necessary to divide the force acting perpendicular to the surface by the surface area:

pressure = force / area.

Let us denote the quantities included in this expression: pressure - p, the force acting on the surface, - F and the surface area S.

Then we get the formula:

p = F/S

It is clear that a larger force acting on the same area will produce more pressure.

The pressure unit is taken as the pressure that produces a force of 1 N acting on a surface of 1 m 2 perpendicular to this surface.

Unit of pressure - newton per square meter(1 N / m 2). In honor of the French scientist Blaise Pascal it's called pascal Pa). Thus,

1 Pa = 1 N / m 2.

Other pressure units are also used: hectopascal (hPa) and kilopascal (kPa).

1 kPa = 1000 Pa;

1 hPa = 100 Pa;

1 Pa = 0.001 kPa;

1 Pa = 0.01 hPa.

Let's write down the condition of the problem and solve it.

Given : m = 45 kg, S = 300 cm 2; p = ?

In SI units: S = 0.03 m 2

Decision:

p = F/S,

F = P,

P = g m,

P= 9.8 N 45 kg ≈ 450 N,

p\u003d 450 / 0.03 N / m 2 \u003d 15000 Pa \u003d 15 kPa

"Answer": p = 15000 Pa = 15 kPa

Ways to reduce and increase pressure.

A heavy caterpillar tractor produces a pressure on the soil equal to 40-50 kPa, that is, only 2-3 times more than the pressure of a boy weighing 45 kg. This is because the weight of the tractor is distributed over a larger area due to the caterpillar drive. And we have established that the larger the area of ​​​​the support, the less pressure produced by the same force on this support .

Depending on whether you need to get a small or a large pressure, the area of ​​\u200b\u200bsupport increases or decreases. For example, in order for the soil to withstand the pressure of a building being erected, the area of ​​\u200b\u200bthe lower part of the foundation is increased.

Truck tires and aircraft chassis are made much wider than passenger cars. Particularly wide tires are made for cars designed to travel in deserts.

Heavy machines, like a tractor, a tank or a swamp, having a large bearing area of ​​​​the tracks, pass through swampy terrain that a person cannot pass through.

On the other hand, with a small surface area, a large pressure can be generated with a small force. For example, pressing a button into a board, we act on it with a force of about 50 N. Since the area of ​​the button tip is approximately 1 mm 2, the pressure produced by it is equal to:

p \u003d 50 N / 0.000001 m 2 \u003d 50,000,000 Pa \u003d 50,000 kPa.

For comparison, this pressure is 1000 times more than the pressure exerted by a caterpillar tractor on the soil. Many more such examples can be found.

The blade of cutting and piercing tools (knives, scissors, cutters, saws, needles, etc.) is specially sharpened. The sharpened edge of a sharp blade has a small area, so even a small force creates a lot of pressure, and it is easy to work with such a tool.

Cutting and piercing devices are also found in wildlife: these are teeth, claws, beaks, spikes, etc. - they are all made of hard material, smooth and very sharp.

Pressure

It is known that gas molecules move randomly.

We already know that gases, unlike solids and liquids, fill the entire vessel in which they are located. For example, a steel cylinder for storing gases, a car tire tube or a volleyball. In this case, the gas exerts pressure on the walls, bottom and lid of the cylinder, chamber or any other body in which it is located. Gas pressure is due to other reasons than the pressure of a solid body on a support.

It is known that gas molecules move randomly. During their movement, they collide with each other, as well as with the walls of the vessel in which the gas is located. There are many molecules in the gas, and therefore the number of their impacts is very large. For example, the number of hits of air molecules in a room on a surface of 1 cm 2 in 1 s is expressed as a twenty-three-digit number. Although the impact force of an individual molecule is small, the action of all molecules on the walls of the vessel is significant - it creates gas pressure.

So, gas pressure on the walls of the vessel (and on the body placed in the gas) is caused by impacts of gas molecules .

Consider the following experience. Place a rubber ball under the air pump bell. It contains a small amount of air and has an irregular shape. Then we pump out the air from under the bell with a pump. The shell of the ball, around which the air becomes more and more rarefied, gradually swells and takes the form of a regular ball.

How to explain this experience?

Special durable steel cylinders are used for storage and transportation of compressed gas.

In our experiment, moving gas molecules continuously hit the walls of the ball inside and out. When air is pumped out, the number of molecules in the bell around the shell of the ball decreases. But inside the ball their number does not change. Therefore, the number of impacts of molecules on the outer walls of the shell becomes less than the number of impacts on the inner walls. The balloon is inflated until the force of elasticity of its rubber shell becomes equal to the pressure force of the gas. The shell of the ball takes the shape of a ball. This shows that gas presses on its walls equally in all directions. In other words, the number of molecular impacts per square centimeter of surface area is the same in all directions. The same pressure in all directions is characteristic of a gas and is a consequence of the random movement of a huge number of molecules.

Let's try to reduce the volume of gas, but so that its mass remains unchanged. This means that in each cubic centimeter of gas there will be more molecules, the density of the gas will increase. Then the number of impacts of molecules on the walls will increase, i.e., the gas pressure will increase. This can be confirmed by experience.

On the image a A glass tube is shown, one end of which is covered with a thin rubber film. A piston is inserted into the tube. When the piston is pushed in, the volume of air in the tube decreases, i.e., the gas is compressed. The rubber film bulges outward, indicating that the air pressure in the tube has increased.

On the contrary, with an increase in the volume of the same mass of gas, the number of molecules in each cubic centimeter decreases. This will reduce the number of impacts on the walls of the vessel - the pressure of the gas will become less. Indeed, when the piston is pulled out of the tube, the volume of air increases, the film bends inside the vessel. This indicates a decrease in air pressure in the tube. The same phenomena would be observed if instead of air in the tube there would be any other gas.

So, when the volume of a gas decreases, its pressure increases, and when the volume increases, the pressure decreases, provided that the mass and temperature of the gas remain unchanged.

How does the pressure of a gas change when it is heated at a constant volume? It is known that the speed of movement of gas molecules increases when heated. Moving faster, the molecules will hit the walls of the vessel more often. In addition, each impact of the molecule on the wall will be stronger. As a result, the walls of the vessel will experience more pressure.

Hence, The pressure of a gas in a closed vessel is greater the higher the temperature of the gas, provided that the mass of the gas and the volume do not change.

From these experiments it can be concluded that the pressure of the gas is greater, the more often and stronger the molecules hit the walls of the vessel .

For storage and transportation of gases, they are highly compressed. At the same time, their pressure increases, gases must be enclosed in special, very durable cylinders. Such cylinders, for example, contain compressed air in submarines, oxygen used in metal welding. Of course, we must always remember that gas cylinders cannot be heated, especially when they are filled with gas. Because, as we already understand, an explosion can occur with very unpleasant consequences.

Pascal's law.

Pressure is transmitted to each point of the liquid or gas.

The pressure of the piston is transmitted to each point of the liquid filling the ball.

Now gas.

Unlike solids, individual layers and small particles of liquid and gas can move freely relative to each other in all directions. It is enough, for example, to lightly blow on the surface of the water in a glass to cause the water to move. Ripples appear on a river or lake at the slightest breeze.

The mobility of gas and liquid particles explains that the pressure produced on them is transmitted not only in the direction of the force, but at every point. Let's consider this phenomenon in more detail.

On the image, a a vessel containing a gas (or liquid) is depicted. The particles are evenly distributed throughout the vessel. The vessel is closed by a piston that can move up and down.

By applying some force, let's make the piston move a little inward and compress the gas (liquid) directly below it. Then the particles (molecules) will be located in this place more densely than before (Fig., b). Due to the mobility of the gas particles will move in all directions. As a result, their arrangement will again become uniform, but more dense than before (Fig. c). Therefore, the pressure of the gas will increase everywhere. This means that additional pressure is transferred to all particles of a gas or liquid. So, if the pressure on the gas (liquid) near the piston itself increases by 1 Pa, then at all points inside gas or liquid pressure will be greater than before by the same amount. The pressure on the walls of the vessel, and on the bottom, and on the piston will increase by 1 Pa.

The pressure exerted on a liquid or gas is transmitted to any point equally in all directions .

This statement is called Pascal's law.

Based on Pascal's law, it is easy to explain the following experiments.

The figure shows a hollow sphere with small holes in various places. A tube is attached to the ball, into which a piston is inserted. If you draw water into the ball and push the piston into the tube, then water will flow from all the holes in the ball. In this experiment, the piston presses on the surface of the water in the tube. The particles of water under the piston, condensing, transfer its pressure to other layers lying deeper. Thus, the pressure of the piston is transmitted to each point of the liquid filling the ball. As a result, part of the water is pushed out of the ball in the form of identical streams flowing from all holes.

If the ball is filled with smoke, then when the piston is pushed into the tube, identical streams of smoke will begin to come out of all the holes of the ball. This confirms that and gases transmit the pressure produced on them equally in all directions.

Pressure in liquid and gas.

Under the weight of the liquid, the rubber bottom in the tube will sag.

Liquids, like all bodies on Earth, are affected by the force of gravity. Therefore, each layer of liquid poured into a vessel creates pressure with its weight, which, according to Pascal's law, is transmitted in all directions. Therefore, there is pressure inside the liquid. This can be verified by experience.

Pour water into a glass tube, the bottom hole of which is closed with a thin rubber film. Under the weight of the liquid, the bottom of the tube will bend.

Experience shows that the higher the column of water above the rubber film, the more it sags. But every time after the rubber bottom sags, the water in the tube comes to equilibrium (stops), because, in addition to gravity, the elastic force of the stretched rubber film acts on the water.

Illustration.

The bottom moves away from the cylinder due to the pressure on it due to gravity.

Let's lower a tube with a rubber bottom, into which water is poured, into another, wider vessel with water. We will see that as the tube is lowered, the rubber film gradually straightens out. Full straightening of the film shows that the forces acting on it from above and below are equal. Full straightening of the film occurs when the water levels in the tube and vessel coincide.

The same experiment can be carried out with a tube in which a rubber film closes the side opening, as shown in figure a. Immerse this tube of water into another vessel of water, as shown in the figure, b. We will notice that the film straightens again as soon as the water levels in the tube and vessel are equal. This means that the forces acting on the rubber film are the same from all sides.

Take a vessel whose bottom can fall off. Let's put it in a jar of water. In this case, the bottom will be tightly pressed to the edge of the vessel and will not fall off. It is pressed by the force of water pressure, directed from the bottom up.

We will carefully pour water into the vessel and watch its bottom. As soon as the level of water in the vessel coincides with the level of water in the jar, it will fall away from the vessel.

At the moment of separation, a column of liquid in the vessel presses down on the bottom, and pressure is transmitted from bottom to top to the bottom of the same column of liquid in height, but located in the jar. Both of these pressures are the same, but the bottom moves away from the cylinder due to the action of its own gravity on it.

Experiments with water were described above, but if we take any other liquid instead of water, the results of the experiment will be the same.

So, experiments show that inside the liquid there is pressure, and at the same level it is the same in all directions. Pressure increases with depth.

Gases do not differ in this respect from liquids, because they also have weight. But we must remember that the density of a gas is hundreds of times less than the density of a liquid. The weight of the gas in the vessel is small, and in many cases its "weight" pressure can be ignored.

Calculation of liquid pressure on the bottom and walls of the vessel.

Calculation of liquid pressure on the bottom and walls of the vessel.

Consider how you can calculate the pressure of a liquid on the bottom and walls of a vessel. Let us first solve the problem for a vessel having the shape of a rectangular parallelepiped.

Force F, with which the liquid poured into this vessel presses on its bottom, is equal to the weight P the liquid in the vessel. The weight of a liquid can be determined by knowing its mass. m. Mass, as you know, can be calculated by the formula: m = ρ V. The volume of liquid poured into the vessel we have chosen is easy to calculate. If the height of the liquid column in the vessel is denoted by the letter h, and the area of ​​the bottom of the vessel S, then V = S h.

Liquid mass m = ρ V, or m = ρ S h .

The weight of this liquid P = gm, or P = g ρ S h.

Since the weight of the liquid column is equal to the force with which the liquid presses on the bottom of the vessel, then, dividing the weight P To the square S, we get the fluid pressure p:

p = P/S , or p = g ρ S h/S,

We have obtained a formula for calculating the pressure of a liquid on the bottom of a vessel. From this formula it can be seen that the pressure of a liquid at the bottom of a vessel depends only on the density and height of the liquid column.

Therefore, according to the derived formula, it is possible to calculate the pressure of the liquid poured into the vessel any form(Strictly speaking, our calculation is only suitable for vessels having the shape of a straight prism and a cylinder. In physics courses for the institute, it was proved that the formula is also true for a vessel of arbitrary shape). In addition, it can be used to calculate the pressure on the walls of the vessel. The pressure inside the fluid, including pressure from bottom to top, is also calculated using this formula, since the pressure at the same depth is the same in all directions.

When calculating pressure using the formula p = gph need density ρ expressed in kilograms per cubic meter (kg / m 3), and the height of the liquid column h- in meters (m), g\u003d 9.8 N / kg, then the pressure will be expressed in pascals (Pa).

Example. Determine the oil pressure at the bottom of the tank if the height of the oil column is 10 m and its density is 800 kg/m 3 .

Let's write down the condition of the problem and write it down.

Given :

ρ \u003d 800 kg / m 3

Decision :

p = 9.8 N/kg 800 kg/m 3 10 m ≈ 80,000 Pa ≈ 80 kPa.

Answer : p ≈ 80 kPa.

Communicating vessels.

Communicating vessels.

The figure shows two vessels connected to each other by a rubber tube. Such vessels are called communicating. A watering can, a teapot, a coffee pot are examples of communicating vessels. We know from experience that water poured, for example, into a watering can, always stands at the same level in the spout and inside.

With communicating vessels, the following simple experiment can be done. At the beginning of the experiment, we clamp the rubber tube in the middle, and pour water into one of the tubes. Then we open the clamp, and the water instantly flows into the other tube until the water surfaces in both tubes are at the same level. You can fix one of the tubes in a tripod, and raise, lower or tilt the other in different directions. And in this case, as soon as the liquid calms down, its levels in both tubes will equalize.

In communicating vessels of any shape and section, the surfaces of a homogeneous liquid are set at the same level(provided that the air pressure over the liquid is the same) (Fig. 109).

This can be justified as follows. The liquid is at rest without moving from one vessel to another. This means that the pressures in both vessels are the same at any level. The liquid in both vessels is the same, that is, it has the same density. Therefore, its heights must also be the same. When we raise one vessel or add liquid to it, the pressure in it increases and the liquid moves into another vessel until the pressures are balanced.

If a liquid of one density is poured into one of the communicating vessels, and another density is poured into the second, then at equilibrium the levels of these liquids will not be the same. And this is understandable. We know that the pressure of a liquid on the bottom of a vessel is directly proportional to the height of the column and the density of the liquid. And in this case, the densities of the liquids will be different.

With equal pressures, the height of a liquid column with a higher density will be less than the height of a liquid column with a lower density (Fig.).

Experience. How to determine the mass of air.

Air weight. Atmosphere pressure.

existence of atmospheric pressure.

Atmospheric pressure is greater than the pressure of rarefied air in a vessel.

The force of gravity acts on the air, as well as on any body located on the Earth, and, therefore, the air has weight. The weight of air is easy to calculate, knowing its mass.

We will show by experience how to calculate the mass of air. To do this, take a strong glass ball with a cork and a rubber tube with a clamp. We pump air out of it with a pump, clamp the tube with a clamp and balance it on the scales. Then, opening the clamp on the rubber tube, let air into it. In this case, the balance of the scales will be disturbed. To restore it, you will have to put weights on the other pan of scales, the mass of which will be equal to the mass of air in the volume of the ball.

Experiments have established that at a temperature of 0 ° C and normal atmospheric pressure, the mass of air with a volume of 1 m 3 is 1.29 kg. The weight of this air is easy to calculate:

P = g m, P = 9.8 N/kg 1.29 kg ≈ 13 N.

The air envelope that surrounds the earth is called atmosphere (from Greek. atmosphere steam, air, and sphere- ball).

The atmosphere, as shown by observations of the flight of artificial Earth satellites, extends to a height of several thousand kilometers.

Due to the action of gravity, the upper layers of the atmosphere, like ocean water, compress the lower layers. The air layer adjacent directly to the Earth is compressed the most and, according to Pascal's law, transfers the pressure produced on it in all directions.

As a result of this, the earth's surface and the bodies located on it experience the pressure of the entire thickness of the air, or, as is usually said in such cases, experience Atmosphere pressure .

The existence of atmospheric pressure can be explained by many phenomena that we encounter in life. Let's consider some of them.

The figure shows a glass tube, inside which there is a piston that fits snugly against the walls of the tube. The end of the tube is dipped in water. If you raise the piston, then the water will rise behind it.

This phenomenon is used in water pumps and some other devices.

The figure shows a cylindrical vessel. It is closed with a cork into which a tube with a tap is inserted. Air is pumped out of the vessel by a pump. The end of the tube is then placed in water. If you now open the tap, then the water will splash into the inside of the vessel in a fountain. Water enters the vessel because the atmospheric pressure is greater than the pressure of rarefied air in the vessel.

Why does the air shell of the Earth exist.

Like all bodies, the molecules of gases that make up the air envelope of the Earth are attracted to the Earth.

But why, then, do they not all fall to the surface of the Earth? How is the air shell of the Earth, its atmosphere, preserved? To understand this, we must take into account that the molecules of gases are in continuous and random motion. But then another question arises: why these molecules do not fly away into the world space, that is, into space.

In order to completely leave the Earth, a molecule, like a spacecraft or a rocket, must have a very high speed (at least 11.2 km/s). This so-called second escape velocity. The speed of most molecules in the Earth's air envelope is much less than this cosmic speed. Therefore, most of them are tied to the Earth by gravity, only a negligible number of molecules fly beyond the Earth into space.

The random movement of molecules and the effect of gravity on them result in the fact that gas molecules "float" in space near the Earth, forming an air shell, or the atmosphere known to us.

Measurements show that air density decreases rapidly with height. So, at a height of 5.5 km above the Earth, the air density is 2 times less than its density at the Earth's surface, at a height of 11 km - 4 times less, etc. The higher, the rarer the air. And finally, in the uppermost layers (hundreds and thousands of kilometers above the Earth), the atmosphere gradually turns into airless space. The air shell of the Earth does not have a clear boundary.

Strictly speaking, due to the action of gravity, the density of the gas in any closed vessel is not the same throughout the entire volume of the vessel. At the bottom of the vessel, the density of the gas is greater than in its upper parts, and therefore the pressure in the vessel is not the same. It is larger at the bottom of the vessel than at the top. However, for the gas contained in the vessel, this difference in density and pressure is so small that in many cases it can be completely ignored, just be aware of it. But for an atmosphere extending over several thousand kilometers, the difference is significant.

Measurement of atmospheric pressure. The Torricelli experience.

It is impossible to calculate atmospheric pressure using the formula for calculating the pressure of a liquid column (§ 38). For such a calculation, you need to know the height of the atmosphere and the density of the air. But the atmosphere does not have a definite boundary, and the air density at different heights is different. However, atmospheric pressure can be measured using an experiment proposed in the 17th century by an Italian scientist. Evangelista Torricelli a student of Galileo.

Torricelli's experiment is as follows: a glass tube about 1 m long, sealed at one end, is filled with mercury. Then, tightly closing the second end of the tube, it is turned over and lowered into a cup with mercury, where this end of the tube is opened under the level of mercury. As in any liquid experiment, part of the mercury is poured into the cup, and part of it remains in the tube. The height of the mercury column remaining in the tube is approximately 760 mm. There is no air above the mercury inside the tube, there is an airless space, so no gas exerts pressure from above on the mercury column inside this tube and does not affect the measurements.

Torricelli, who proposed the experience described above, also gave his explanation. The atmosphere presses on the surface of the mercury in the cup. Mercury is in balance. This means that the pressure in the tube is aa 1 (see figure) is equal to atmospheric pressure. When atmospheric pressure changes, the height of the mercury column in the tube also changes. As the pressure increases, the column lengthens. As the pressure decreases, the mercury column decreases in height.

The pressure in the tube at the level aa1 is created by the weight of the mercury column in the tube, since there is no air above the mercury in the upper part of the tube. Hence it follows that atmospheric pressure is equal to the pressure of the mercury column in the tube , i.e.

p atm = p mercury.

The greater the atmospheric pressure, the higher the mercury column in Torricelli's experiment. Therefore, in practice, atmospheric pressure can be measured by the height of the mercury column (in millimeters or centimeters). If, for example, atmospheric pressure is 780 mm Hg. Art. (they say "millimeters of mercury"), this means that the air produces the same pressure as a vertical column of mercury 780 mm high produces.

Therefore, in this case, 1 millimeter of mercury (1 mm Hg) is taken as the unit of atmospheric pressure. Let's find the relationship between this unit and the unit known to us - pascal(Pa).

The pressure of a mercury column ρ of mercury with a height of 1 mm is:

p = g ρ h, p\u003d 9.8 N / kg 13,600 kg / m 3 0.001 m ≈ 133.3 Pa.

So, 1 mm Hg. Art. = 133.3 Pa.

Currently, atmospheric pressure is usually measured in hectopascals (1 hPa = 100 Pa). For example, weather reports may announce that the pressure is 1013 hPa, which is the same as 760 mmHg. Art.

Observing daily the height of the mercury column in the tube, Torricelli discovered that this height changes, that is, atmospheric pressure is not constant, it can increase and decrease. Torricelli also noticed that atmospheric pressure is related to changes in the weather.

If you attach a vertical scale to the mercury tube used in Torricelli's experiment, you get the simplest device - mercury barometer (from Greek. baros- heaviness, metreo- measure). It is used to measure atmospheric pressure.

Barometer - aneroid.

In practice, a metal barometer is used to measure atmospheric pressure, called aneroid (translated from Greek - aneroid). The barometer is called so because it does not contain mercury.

The appearance of the aneroid is shown in the figure. Its main part is a metal box 1 with a wavy (corrugated) surface (see other fig.). Air is pumped out of this box, and so that atmospheric pressure does not crush the box, its cover 2 is pulled up by a spring. As atmospheric pressure increases, the lid flexes downward and tensions the spring. When the pressure decreases, the spring straightens the cover. An arrow-pointer 4 is attached to the spring by means of a transmission mechanism 3, which moves to the right or left when the pressure changes. A scale is fixed under the arrow, the divisions of which are marked according to the indications of a mercury barometer. So, the number 750, against which the aneroid needle stands (see Fig.), shows that at the given moment in the mercury barometer the height of the mercury column is 750 mm.

Therefore, atmospheric pressure is 750 mm Hg. Art. or ≈ 1000 hPa.

The value of atmospheric pressure is very important for predicting the weather for the coming days, since changes in atmospheric pressure are associated with changes in the weather. A barometer is a necessary instrument for meteorological observations.

Atmospheric pressure at various altitudes.

In a liquid, the pressure, as we know, depends on the density of the liquid and the height of its column. Due to the low compressibility, the density of the liquid at different depths is almost the same. Therefore, when calculating the pressure, we consider its density to be constant and take into account only the change in height.

The situation is more complicated with gases. Gases are highly compressible. And the more the gas is compressed, the greater its density, and the greater the pressure it produces. After all, the pressure of a gas is created by the impact of its molecules on the surface of the body.

The layers of air near the surface of the Earth are compressed by all the overlying layers of air above them. But the higher the layer of air from the surface, the weaker it is compressed, the lower its density. Hence, the less pressure it produces. If, for example, a balloon rises above the surface of the Earth, then the air pressure on the balloon becomes less. This happens not only because the height of the air column above it decreases, but also because the air density decreases. It is smaller at the top than at the bottom. Therefore, the dependence of air pressure on altitude is more complicated than that of liquids.

Observations show that atmospheric pressure in areas lying at sea level is on average 760 mm Hg. Art.

Atmospheric pressure equal to the pressure of a mercury column 760 mm high at a temperature of 0 ° C is called normal atmospheric pressure..

normal atmospheric pressure equals 101 300 Pa = 1013 hPa.

The higher the altitude, the lower the pressure.

With small rises, on average, for every 12 m of rise, the pressure decreases by 1 mm Hg. Art. (or 1.33 hPa).

Knowing the dependence of pressure on altitude, it is possible to determine the height above sea level by changing the readings of the barometer. Aneroids having a scale on which you can directly measure the height above sea level are called altimeters . They are used in aviation and when climbing mountains.

Pressure gauges.

We already know that barometers are used to measure atmospheric pressure. To measure pressures greater or less than atmospheric pressure, the pressure gauges (from Greek. manos- rare, inconspicuous metreo- measure). Pressure gauges are liquid and metal.

Consider first the device and action open liquid manometer. It consists of a two-legged glass tube into which some liquid is poured. The liquid is installed in both knees at the same level, since only atmospheric pressure acts on its surface in the knees of the vessel.

To understand how such a pressure gauge works, it can be connected with a rubber tube to a round flat box, one side of which is covered with a rubber film. If you press your finger on the film, then the liquid level in the manometer knee connected in the box will decrease, and in the other knee it will increase. What explains this?

Pressing on the film increases the air pressure in the box. According to Pascal's law, this increase in pressure is transferred to the liquid in that knee of the pressure gauge, which is attached to the box. Therefore, the pressure on the liquid in this knee will be greater than in the other, where only atmospheric pressure acts on the liquid. Under the force of this excess pressure, the liquid will begin to move. In the knee with compressed air, the liquid will fall, in the other it will rise. The liquid will come to equilibrium (stop) when the excess pressure of the compressed air is balanced by the pressure that the excess liquid column produces in the other leg of the pressure gauge.

The stronger the pressure on the film, the higher the excess liquid column, the greater its pressure. Hence, the change in pressure can be judged by the height of this excess column.

The figure shows how such a pressure gauge can measure the pressure inside a liquid. The deeper the tube is immersed in the liquid, the greater the difference in the heights of the liquid columns in the manometer knees becomes., so, therefore, and fluid produces more pressure.

If you install the device box at some depth inside the liquid and turn it with a film up, sideways and down, then the pressure gauge readings will not change. That's the way it should be, because at the same level inside a liquid, the pressure is the same in all directions.

The picture shows metal manometer . The main part of such a pressure gauge is a metal tube bent into a pipe 1 , one end of which is closed. The other end of the tube with a tap 4 communicates with the vessel in which the pressure is measured. As pressure increases, the tube flexes. Movement of its closed end with a lever 5 and gears 3 passed to the shooter 2 moving around the scale of the instrument. When the pressure decreases, the tube, due to its elasticity, returns to its previous position, and the arrow returns to zero division of the scale.

Piston liquid pump.

In the experiment we considered earlier (§ 40), it was found that water in a glass tube, under the action of atmospheric pressure, rose up behind the piston. This action is based piston pumps.

The pump is shown schematically in the figure. It consists of a cylinder, inside which goes up and down, tightly adhering to the walls of the vessel, the piston 1 . Valves are installed in the lower part of the cylinder and in the piston itself. 2 opening only upwards. When the piston moves upwards, water enters the pipe under the action of atmospheric pressure, lifts the bottom valve and moves behind the piston.

When the piston moves down, the water under the piston presses on the bottom valve, and it closes. At the same time, under pressure from the water, a valve inside the piston opens, and the water flows into the space above the piston. With the next movement of the piston upwards, the water above it also rises in the place with it, which pours out into the outlet pipe. At the same time, a new portion of water rises behind the piston, which, when the piston is subsequently lowered, will be above it, and this whole procedure is repeated again and again while the pump is running.

Hydraulic Press.

Pascal's law allows you to explain the action hydraulic machine (from Greek. hydraulicos- water). These are machines whose action is based on the laws of motion and equilibrium of liquids.

The main part of the hydraulic machine is two cylinders of different diameters, equipped with pistons and a connecting tube. The space under the pistons and the tube are filled with liquid (usually mineral oil). The heights of the liquid columns in both cylinders are the same as long as there are no forces acting on the pistons.

Let us now assume that the forces F 1 and F 2 - forces acting on the pistons, S 1 and S 2 - areas of pistons. The pressure under the first (small) piston is p 1 = F 1 / S 1 , and under the second (large) p 2 = F 2 / S 2. According to Pascal's law, the pressure of a fluid at rest is transmitted equally in all directions, i.e. p 1 = p 2 or F 1 / S 1 = F 2 / S 2 , from where:

F 2 / F 1 = S 2 / S 1 .

Therefore, the strength F 2 so much more power F 1 , How many times greater is the area of ​​the large piston than the area of ​​the small piston?. For example, if the area of ​​​​the large piston is 500 cm 2, and the small one is 5 cm 2, and a force of 100 N acts on the small piston, then a force 100 times greater will act on the larger piston, that is, 10,000 N.

Thus, with the help of a hydraulic machine, it is possible to balance a large force with a small force.

Attitude F 1 / F 2 shows the gain in strength. For example, in the example above, the gain in force is 10,000 N / 100 N = 100.

The hydraulic machine used for pressing (squeezing) is called hydraulic press .

Hydraulic presses are used where a lot of power is required. For example, for squeezing oil from seeds at oil mills, for pressing plywood, cardboard, hay. Steel mills use hydraulic presses to make steel machine shafts, railway wheels, and many other products. Modern hydraulic presses can develop a force of tens and hundreds of millions of newtons.

The device of the hydraulic press is shown schematically in the figure. The body to be pressed 1 (A) is placed on a platform connected to a large piston 2 (B). The small piston 3 (D) creates a large pressure on the liquid. This pressure is transmitted to every point of the fluid filling the cylinders. Therefore, the same pressure acts on the second, large piston. But since the area of ​​the 2nd (large) piston is larger than the area of ​​the small one, then the force acting on it will be greater than the force acting on piston 3 (D). Under this force, piston 2 (B) will rise. When piston 2 (B) rises, the body (A) rests against the fixed upper platform and is compressed. The pressure gauge 4 (M) measures the fluid pressure. Safety valve 5 (P) automatically opens when the fluid pressure exceeds the allowable value.

From a small cylinder to a large liquid is pumped by repeated movements of the small piston 3 (D). This is done in the following way. When the small piston (D) is lifted, valve 6 (K) opens and liquid is sucked into the space under the piston. When the small piston is lowered under the action of liquid pressure, valve 6 (K) closes, and valve 7 (K") opens, and the liquid passes into a large vessel.

The action of water and gas on a body immersed in them.

Under water, we can easily lift a stone that can hardly be lifted in the air. If you submerge the cork under water and release it from your hands, it will float. How can these phenomena be explained?

We know (§ 38) that the liquid presses on the bottom and walls of the vessel. And if some solid body is placed inside the liquid, then it will also be subjected to pressure, like the walls of the vessel.

Consider the forces that act from the side of the liquid on the body immersed in it. To make it easier to reason, we choose a body that has the shape of a parallelepiped with bases parallel to the surface of the liquid (Fig.). The forces acting on the side faces of the body are equal in pairs and balance each other. Under the influence of these forces, the body is compressed. But the forces acting on the upper and lower faces of the body are not the same. On the upper face presses from above with force F 1 column of liquid tall h one . At the level of the lower face, the pressure produces a liquid column with a height h 2. This pressure, as we know (§ 37), is transmitted inside the liquid in all directions. Therefore, on the lower face of the body from the bottom up with a force F 2 presses a liquid column high h 2. But h 2 more h 1 , hence the modulus of force F 2 more power modules F one . Therefore, the body is pushed out of the liquid with a force F vyt, equal to the difference of forces F 2 - F 1 , i.e.

But S·h = V, where V is the volume of the parallelepiped, and ρ W ·V = m W is the mass of fluid in the volume of the parallelepiped. Hence, F vyt \u003d g m well \u003d P well, i.e. buoyant force is equal to the weight of the liquid in the volume of the body immersed in it(The buoyant force is equal to the weight of a liquid of the same volume as the volume of the body immersed in it). The existence of a force that pushes a body out of a liquid is easy to discover experimentally. On the image a shows a body suspended from a spring with an arrow pointer at the end. The arrow marks the tension of the spring on the tripod. When the body is released into the water, the spring contracts (Fig. b). The same contraction of the spring will be obtained if you act on the body from the bottom up with some force, for example, press it with your hand (raise it). Therefore, experience confirms that a force acting on a body in a fluid pushes the body out of the fluid. For gases, as we know, Pascal's law also applies. So bodies in the gas are subjected to a force pushing them out of the gas. Under the influence of this force, the balloons rise up. The existence of a force pushing a body out of a gas can also be observed experimentally. We hang a glass ball or a large flask closed with a cork to a shortened scale pan. The scales are balanced. Then a wide vessel is placed under the flask (or ball) so that it surrounds the entire flask. The vessel is filled with carbon dioxide, the density of which is greater than the density of air (therefore, carbon dioxide sinks down and fills the vessel, displacing air from it). In this case, the balance of the scales is disturbed. A cup with a suspended flask rises up (Fig.). A flask immersed in carbon dioxide experiences a greater buoyant force than that which acts on it in air. The force that pushes a body out of a liquid or gas is directed opposite to the force of gravity applied to this body. Therefore, prolcosmos). This explains why in the water we sometimes easily lift bodies that we can hardly keep in the air. A small bucket and a cylindrical body are suspended from the spring (Fig., a). The arrow on the tripod marks the extension of the spring. It shows the weight of the body in the air. Having lifted the body, a drain vessel is placed under it, filled with liquid to the level of the drain tube. After that, the body is completely immersed in the liquid (Fig., b). Wherein part of the liquid, the volume of which is equal to the volume of the body, is poured out from a pouring vessel into a glass. The spring contracts and the pointer of the spring rises to indicate the decrease in the weight of the body in the fluid. In this case, in addition to the force of gravity, another force acts on the body, pushing it out of the fluid. If the liquid from the glass is poured into the upper bucket (i.e., the one that was displaced by the body), then the spring pointer will return to its initial position (Fig., c). Based on this experience, it can be concluded that the force that pushes a body completely immersed in a liquid is equal to the weight of the liquid in the volume of this body . We reached the same conclusion in § 48. If a similar experiment were done with a body immersed in some gas, it would show that the force pushing the body out of the gas is also equal to the weight of the gas taken in the volume of the body . The force that pushes a body out of a liquid or gas is called Archimedean force, in honor of the scientist Archimedes who first pointed to its existence and calculated its significance. So, experience has confirmed that the Archimedean (or buoyant) force is equal to the weight of the fluid in the volume of the body, i.e. F A = P f = g m well. The mass of liquid m f , displaced by the body, can be expressed in terms of its density ρ w and the volume of the body V t immersed in the liquid (since V l - the volume of the liquid displaced by the body is equal to V t - the volume of the body immersed in the liquid), i.e. m W = ρ W V t. Then we get: F A= g ρ f · V t Therefore, the Archimedean force depends on the density of the liquid in which the body is immersed, and on the volume of this body. But it does not depend, for example, on the density of the substance of a body immersed in a liquid, since this quantity is not included in the resulting formula. Let us now determine the weight of a body immersed in a liquid (or gas). Since the two forces acting on the body in this case are directed in opposite directions (gravity is down, and the Archimedean force is up), then the weight of the body in fluid P 1 will be less than the weight of the body in vacuum P = gm to the Archimedean force F A = g m w (where m w is the mass of liquid or gas displaced by the body). Thus, if a body is immersed in a liquid or gas, then it loses in its weight as much as the liquid or gas displaced by it weighs. Example. Determine the buoyant force acting on a stone with a volume of 1.6 m 3 in sea water. Let's write down the condition of the problem and solve it. When the floating body reaches the surface of the liquid, then with its further upward movement, the Archimedean force will decrease. Why? But because the volume of the part of the body immersed in the liquid will decrease, and the Archimedean force is equal to the weight of the liquid in the volume of the part of the body immersed in it. When the Archimedean force becomes equal to the force of gravity, the body will stop and float on the surface of the liquid, partially immersed in it. The resulting conclusion is easy to verify experimentally. Pour water into the drain vessel up to the level of the drain pipe. After that, let's immerse the floating body into the vessel, having previously weighed it in the air. Having descended into the water, the body displaces a volume of water equal to the volume of the part of the body immersed in it. Having weighed this water, we find that its weight (Archimedean force) is equal to the force of gravity acting on a floating body, or the weight of this body in air. Having done the same experiments with any other bodies floating in different liquids - in water, alcohol, salt solution, you can make sure that if a body floats in a liquid, then the weight of the liquid displaced by it is equal to the weight of this body in air. It is easy to prove that if the density of a solid solid is greater than the density of a liquid, then the body sinks in such a liquid. A body with a lower density floats in this liquid. A piece of iron, for example, sinks in water but floats in mercury. The body, on the other hand, whose density is equal to the density of the liquid, remains in equilibrium inside the liquid. Ice floats on the surface of water because its density is less than that of water. The lower the density of the body compared to the density of the liquid, the smaller part of the body is immersed in the liquid . With equal densities of the body and liquid, the body floats inside the liquid at any depth. Two immiscible liquids, for example water and kerosene, are located in a vessel in accordance with their densities: in the lower part of the vessel - denser water (ρ = 1000 kg / m 3), on top - lighter kerosene (ρ = 800 kg / m 3) . The average density of living organisms inhabiting the aquatic environment differs little from the density of water, so their weight is almost completely balanced by the Archimedean force. Thanks to this, aquatic animals do not need such strong and massive skeletons as terrestrial ones. For the same reason, the trunks of aquatic plants are elastic. The swim bladder of a fish easily changes its volume. When the fish descends to a great depth with the help of muscles, and the water pressure on it increases, the bubble contracts, the volume of the fish's body decreases, and it does not push upwards, but swims in the depths. Thus, the fish can, within certain limits, regulate the depth of its dive. Whales regulate their diving depth by contracting and expanding their lung capacity. Sailing ships. Ships floating on rivers, lakes, seas and oceans are built from different materials with different densities. The hull of ships is usually made of steel sheets. All internal fasteners that give ships strength are also made of metals. For the construction of ships, various materials are used, which, compared with water, have both higher and lower densities. How do ships float, take on board and carry large loads? An experiment with a floating body (§ 50) showed that the body displaces so much water with its underwater part that this water is equal in weight to the weight of the body in air. This is also true for any ship. The weight of water displaced by the underwater part of the ship is equal to the weight of the ship with cargo in the air or the force of gravity acting on the ship with cargo. The depth to which a ship is submerged in water is called draft . The deepest allowable draft is marked on the ship's hull with a red line called waterline (from Dutch. water- water). The weight of water displaced by the ship when submerged to the waterline, equal to the force of gravity acting on the ship with cargo, is called the displacement of the ship. At present, ships with a displacement of 5,000,000 kN (5 10 6 kN) and more are being built for the transportation of oil, i.e., having a mass of 500,000 tons (5 10 5 t) and more together with the cargo. If we subtract the weight of the ship itself from the displacement, then we get the carrying capacity of this ship. Carrying capacity shows the weight of the cargo carried by the ship. Shipbuilding existed in Ancient Egypt, in Phoenicia (it is believed that the Phoenicians were one of the best shipbuilders), Ancient China. In Russia, shipbuilding originated at the turn of the 17th and 18th centuries. Mainly warships were built, but it was in Russia that the first icebreaker, ships with an internal combustion engine, and the nuclear icebreaker Arktika were built. Aeronautics. Drawing describing the balloon of the Montgolfier brothers in 1783: "View and exact dimensions of the Balloon Globe, which was the first." 1786 Since ancient times, people have dreamed of being able to fly above the clouds, to swim in the ocean of air, as they sailed on the sea. For aeronautics At first, balloons were used, which were filled either with heated air, or with hydrogen or helium. In order for a balloon to rise into the air, it is necessary that the Archimedean force (buoyancy) F A, acting on the ball, was more than gravity F heavy, i.e. F A > F heavy As the ball rises, the Archimedean force acting on it decreases ( F A = gρV), since the density of the upper atmosphere is less than that of the Earth's surface. To rise higher, a special ballast (weight) is dropped from the ball and this lightens the ball. Eventually the ball reaches its maximum lift height. To lower the ball, part of the gas is released from its shell using a special valve. In the horizontal direction, the balloon moves only under the influence of the wind, so it is called balloon (from Greek air- air, stato- standing). Not so long ago, huge balloons were used to study the upper layers of the atmosphere, the stratosphere - stratostats . Before they learned how to build large aircraft for transporting passengers and cargo by air, controlled balloons were used - airships. They have an elongated shape, a gondola with an engine is suspended under the body, which drives the propeller. The balloon not only rises by itself, but can also lift some cargo: a cabin, people, instruments. Therefore, in order to find out what kind of load a balloon can lift, it is necessary to determine it. lifting force. Let, for example, a balloon with a volume of 40 m 3 filled with helium be launched into the air. The mass of helium filling the shell of the ball will be equal to: m Ge \u003d ρ Ge V \u003d 0.1890 kg / m 3 40 m 3 \u003d 7.2 kg, and its weight is: P Ge = g m Ge; P Ge \u003d 9.8 N / kg 7.2 kg \u003d 71 N. The buoyant force (Archimedean) acting on this ball in the air is equal to the weight of air with a volume of 40 m 3, i.e. F A \u003d g ρ air V; F A \u003d 9.8 N / kg 1.3 kg / m 3 40 m 3 \u003d 520 N. This means that this ball can lift a load weighing 520 N - 71 N = 449 N. This is its lifting force. A balloon of the same volume, but filled with hydrogen, can lift a load of 479 N. This means that its lifting force is greater than that of a balloon filled with helium. But still, helium is used more often, since it does not burn and is therefore safer. Hydrogen is a combustible gas. It is much easier to raise and lower a balloon filled with hot air. For this, a burner is located under the hole located in the lower part of the ball. Using a gas burner, you can control the temperature of the air inside the ball, which means its density and buoyancy. In order for the ball to rise higher, it is enough to heat the air in it more strongly, increasing the flame of the burner. When the burner flame decreases, the temperature of the air in the ball decreases, and the ball goes down. It is possible to choose such a temperature of the ball at which the weight of the ball and the cabin will be equal to the buoyancy force. Then the ball will hang in the air, and it will be easy to make observations from it. As science developed, there were also significant changes in aeronautical technology. It became possible to use new shells for balloons, which became durable, frost-resistant and light. Achievements in the field of radio engineering, electronics, automation made it possible to design unmanned balloons. These balloons are used to study air currents, for geographical and biomedical research in the lower layers of the atmosphere. Question 1 The main provisions of the ICT and their experimental substantiation.? 1. All substances are composed of molecules, i.e. have a discrete structure, the molecules are separated by gaps. 2. Molecules are in continuous random (chaotic) motion. 3. Between the molecules of the body there are forces of interaction. Brownian motion?. Brownian motion is the continuous random motion of particles suspended in a gas. Forces of molecular interaction?. Both attraction and repulsion act simultaneously between molecules. The nature of the interaction of molecules is electromagnetic. Kinetic and potential energy of molecules?. Atoms and molecules interact and, therefore, have a potential energy E p. Potential energy is considered positive when molecules are repelled, negative when they are attracted. Question 2 Dimensions and masses of molecules and atoms Any substance consists of particles, therefore the amount of substance v (nu) is considered to be proportional to the number of particles, i.e. structural elements contained in the body. The unit of quantity of a substance is the mole. A mole is the amount of a substance that contains as many structural elements of any substance as there are atoms in 12 g of C12 carbon. The ratio of the number of molecules of a substance to the amount of a substance is called the Avogadro constant: N A =N/v(nu); N A \u003d 6.02 * 10 23 mol -1 The Avogadro constant shows how many atoms and molecules are contained in one mole of a substance. Molar mass - the mass of one mole of a substance, equal to the ratio of the mass of the substance to the amount of the substance: Molar mass is expressed in kg/mol. Knowing the molar mass, you can calculate the mass of one molecule: m 0 \u003d m / N \u003d m / v (nu) N A \u003d M / N A The average mass of molecules is usually determined by chemical methods, the Avogadro constant has been determined with high accuracy by several physical methods. The masses of molecules and atoms are determined with a considerable degree of accuracy using a mass spectrograph. The masses of molecules are very small. For example, the mass of a water molecule: m = 29.9 * 10 -27 The molar mass is related to the relative molecular mass Mg. Relative molecular weight is a value equal to the ratio of the mass of a molecule of a given substance to 1/12 of the mass of a C12 carbon atom. If the chemical formula of a substance is known, then its relative mass can be determined using the periodic table, which, when expressed in kilograms, shows the magnitude of the molar mass of this substance. Avogadro's number Avogadro's number, Avogadro's constant is a physical constant numerically equal to the number of specified structural units (atoms, molecules, ions, electrons or any other particles) in 1 mole of a substance. Defined as the number of atoms in 12 grams (exactly) of the pure carbon-12 isotope. It is usually designated as N A, less often as L N A = 6.022 140 78(18)×1023 mol −1 . Number of moles Mole (symbol: mol, international: mol) is a unit of measure for the amount of a substance. Corresponds to the amount of a substance that contains N A particles (molecules, atoms, ions, or any other identical structural particles). N A is Avogadro's constant, equal to the number of atoms in 12 grams of the carbon nuclide 12C. Thus, the number of particles in one mole of any substance is constant and equal to the Avogadro number N A . Molecule speed State of matter Aggregate state - a state of matter characterized by certain qualitative properties: the ability or inability to maintain volume and shape, the presence or absence of long-range and short-range order, and others. A change in the state of aggregation may be accompanied by a jump-like change in free energy, entropy, density, and other basic physical properties. There are three main states of aggregation: solid, liquid and gas. Sometimes it is not entirely correct to classify plasma as a state of aggregation. There are other states of aggregation, for example, liquid crystals or Bose-Einstein condensate. Question 3 Ideal gas, gas pressure An ideal gas is a gas in which there is no interaction force between molecules. The pressure of a gas is due to the impacts of molecules. The force of pressure for 1 second on a unit surface is called gas pressure. P – gas pressure [pa] 1 mmHg Art. =133 Pa P 0 (ro) \u003d 101325 Pa P= 1/3*m 0 *n*V 2- the basic equation of the MKT n - concentration of molecules [m -3] n=N/V- concentration of molecules V 2 - root mean square speed P= 2/3*n*E K basic equations P= n*k*T MKT E K - kinetic energy E K = 3/2kT(kT- kote) Choosing a system that distributes a gaseous substance, according to a criterion that evaluates pressure, the level of reduction and the principles of building systems that distribute gas pipelines (this can be a ring, dead end and mixed gas pipelines), is based on economic miscalculations and technical features. Considering the volume, structural nuances and density properties of the consuming gas level, the reliability and safe mode of the gas supply system, in addition, local buildings and operational features. Types of gas pipelines Gas pipeline systems are associated with pressure levels of a gaseous substance that moves through them, are divided into the following types: 1. Gas pipeline structure with the presence of high pressure of the first grade under the conditions of the working pressure of the gas substance within 0.71.3 MPa for natural substance and gas-air mixture and up to 1.7 MPa for LPG; 2. Gas pipeline with a high pressure level of the second category under pressure conditions within 0.40.7 MPa; 3. A gas pipeline structure with average pressure indicators has an operating pressure within 0.0060.4 MPa; 4. Low pressure gas channel pressure level up to 0.006Mpa. Types of gas supply systems The gas supply system can be of the following types: 1. Single-level, where gas is supplied to consumers only through a gas pipeline product of the same pressure indicators (either with low indicators or with average ones); 2. Two-level, where gas is supplied to the circle of consumers through a gas pipeline structure with two different types of pressure (indicators of medium-low or medium-high 1 or 2 level, or high indicators of category 2 low); 3. Three-level, where the passage of a gaseous substance is carried out through a gas pipeline with three pressures (high first or second level, medium and low); 4. Multilevel, in which gas moves along gas lines with four types of pressure: high 1 and 2 levels, medium and low. Gas pipeline systems with different pressures, which are included in the gas supply system, must be connected through hydraulic fracturing, KDD. For industrial heat installations and boiler equipment that are separate from gas pipelines, it is acceptable to use a gas substance with an available pressure within 1.3 MPa, provided that such pressure indicators are necessary for the specifics of the technical process. It is impossible to lay a gas pipeline system with a pressure index of more than 1.2 MPa for a multi-storey residential building in a populated area, in areas where public buildings are located, in places where a large number of people are located, for example, a market, a stadium, a shopping center, a theater building. The current distribution systems of the gas supply line consist of a complex complex composition of structures, which, in turn, take the form of basic elements such as gas ring, dead-end and mixed networks with low, medium and high pressure indicators. They are laid in urban areas, other settlements, in the heart of neighborhoods or buildings. In addition, they can be placed on the routes of a gas distribution station, gas control point and installation, a communication system, a system of automatic installations and telemechanical equipment. The whole structure must ensure the supply of consumer gas without problems. The design must have a disconnecting device, which is directed to its individual elements and sections of the gas pipeline for repair and elimination of emergencies. Among other things, it ensures trouble-free transportation of gaseous substances to gas consuming persons, has a simple mechanism, safe, reliable and convenient operation. It is necessary to design the gas supply of the entire region, city or village on the basis of schematic drawings and the layout of the area, the general plan of the city, taking into account the long-term development. All elements, devices, mechanisms and key parts in the gas supply system should be used the same. It is worth choosing a distribution system and principles for constructing a gas pipeline (ring, dead-end, mixed) based on technical and economic settlement operations, taking into account the volume, structure and density of gas consumption. The selected system must be of the highest efficiency, from an economic point of view, and must include construction processes and be able to put the gas supply system into operation partially. Classification of gas pipelines The main parts of the gas supply system are gas pipeline structures, which have types depending on the gas pressure and purpose. Depending on the highest gas pressure indicators that are transported, gas pipeline structures are divided into the following: 1. Gas pipeline structure with high first-level pressures under conditions of gaseous substance pressures of more than 0.7 MPa, up to 1.7 MPa for SGU; 2. A gas pipeline product with high pressure marks of the second level at a regime of more than 0.4 MPa and up to 0.7 MPa; 3. Wire with an average level of pressure indicators above 0.005 MPa and vary up to 0.4 MPa; 4. Low performance design, namely up to 0.004MPa. A gas pipeline system with low pressure marks is used to move gas to residential buildings and public buildings, to catering establishments, as well as to boiler rooms and household enterprises. It is allowed to connect small consumer installations and boiler rooms to the low pressure gas pipeline system. But large utilities should not be connected to lines with low pressure indicators, since it does not make sense to move a large amount of gas through it, it does not have an economic benefit. The gas pipeline structure with medium and high pressure modes is designed as a power source for the city distribution network with low and medium pressure into the gas pipeline of industrial workshops and public utilities. The city's high-pressure gas line is considered the main line that feeds the huge city. It is made as a huge, semi-ring or has a radial appearance. Through it, the gas substance is supplied by hydraulic fracturing to the network with medium and high levels, in addition, to large industrial enterprises, the technological process of which assumes the presence of gas with an operating mode of more than 0.8 MPa. City gas supply system Indicators of gas pressure in the pipeline up to 0.003 MPa The gas supply system of the city is a serious mechanism, including facilities, technical devices and pipelines that ensure the passage of gas to the destination and distribute it among enterprises, utilities, consumers, based on demand. It includes the following structures: 1. Gas network with low, medium and high climate; 2. Gas control station; 3. Gas control point; 4. Gas control equipment; 5. Control device and automatic control system; 6. Dispatch devices; 7. Operating system. The supply of a gaseous substance comes through a gas pipeline through gas control stations directly to the city gas line. At the gas distribution station, the pressure readings fall with the help of automatic valves on the regulator, and remain unchanged at the required level for urban consumption throughout the entire time. Technical specialists include in the GDS scheme a system that automatically provides protection. In addition, it guarantees the maintenance of pressure indicators in the city line, and also ensures that they do not exceed the permissible level. From the gas control stations, the gas substance through the gas line reaches the consumers. Since the main element of urban gas supply systems are gas lines, consisting of gas pipeline pressure differences, they can be presented in the following types: 1. Line with low pressure marks up to 4 kPa; 2. Line with average pressure values ​​up to 0.4 MPa; 3. Network with a high pressure regime of the second level up to 0.7 MPa; 4. Networks with high readings of the first level up to 1.3 MPa. Through gas pipeline structures with low pressure indicators, the gas moves and is distributed to a residential and public building and various premises, as well as to the workshops of household enterprises. In a gas pipeline located in a residential area, pressure indicators up to 3 kPa are permissible, and in the premises of a household enterprise and public buildings up to 5 kPa. As a rule, low pressures are maintained in the line (up to 3 kPa), and they try to connect all structures to a gas line that does not have a gas pressure regulator. In gas pipelines with medium and high pressure (0.6 MPa), the gaseous product is supplied by hydraulic fracturing to lines with low and medium pressure. There is a safety device inside the hydraulic fracturing unit that operates automatically. It eliminates the chances of pressure drops from a low level of more than an acceptable value. Through similar communications through the GRU, the gaseous substance is also supplied to the premises of industrial enterprises and municipal institutions. According to current regulations, the highest pressure for industrial, municipal and agricultural enterprises, as well as for heating system installations, is allowed within 0.6 MPa, and for household enterprises and adjacent buildings within 0.3 MPa. Gas supply with a pressure index of not more than 0.3 MPa is allowed for installations that are located on the facades of a residential building or public building. Gas pipeline structures with medium and high regime are the distribution networks of the city. A gas pipeline structure with high pressure marks is used exclusively in metropolitan cities. Industrial premises can be connected to a medium and high pressure network without using regulators, of course, if this is based on technical and economic calculations. City systems are built according to a hierarchy, which, in turn, is divided depending on the pressure of the gas pipeline. The hierarchy has several levels: 1. Lines with high and medium pressure are the basis of urban gas pipelines. Reservation takes place with the help of ringing and duplication of individual places. A dead-end network can only be in small towns. The gaseous substance gradually moves through low pressure levels, it is produced by vibrations on the hydraulic fracturing regulator valve and is at a constant level. If there are several different gas consumers in one section, it is allowed to lay gas pipelines with different pressures in parallel. But the design with high and medium pressure creates one network in the city, which has hydraulic nuances. 2. Low pressure network. It supplies gas to a variety of consumers. The network design is created with mixed features, while only the main gas pipelines are looped, in other cases dead ends are created. A low pressure gas pipeline cannot separate a river, lake or ravine, as well as a railway, a highway. It cannot be laid along industrial zones, so it cannot be part of a single hydraulic network. A low performance network design is created as a local line that has multiple power sources through which gas is supplied. 3. Gas construction of a residential building or public building, industrial workshop or enterprise. They are not reserved. The pressure depends on the purpose of the network and the level required for the installation. Depending on the number of degrees, city systems are divided : 1. A two-level network consists of low and medium pressure lines or low and high pressure lines. 2. Three-level line includes low, medium and high pressure system. 3. The step-level network consists of gas pipeline structures of all levels. The city gas pipeline with high and medium pressure is created as a single line that supplies gas to the enterprise, boiler house, utilities and the hydraulic fracturing itself. It is much more profitable to create a single line, in contrast to a separating one for industrial premises and, in general, for a domestic gas section. Choose a city system based on such nuances: 1. What is the size of the city. 2. Plan of the urban area. 3. Buildings in it. 4. What is the population in the city. 5. Characteristics of all enterprises in the city. 6. Prospects for the development of the metropolis. Having chosen the necessary system, it must be taken into account that it must meet the requirements of economy, safety and reliability in use. It expresses simplicity and ease of use, suggesting the shutdown of its individual sections for repair work. In addition, all parts, devices and fixtures in the selected system must have the same type of parts. Gas is supplied to the city via a multi-level line through two mains through the station, which, in turn, increases the level of reliability. The station is connected to a high pressure area, which is located on the outskirts of city lines. From this section, gas is supplied to the rings with high or medium pressure. If it is not feasible and unacceptable to create a high-pressure gas pipeline network in the center of a metropolis, then they must be divided into two parts: a medium-pressure network in the center and a high-pressure network on the outskirts. In order to be able to turn off parts of the gas pipeline with high and medium pressure, individual sections with low pressure, structures in residential buildings, industrial workshops and premises mount devices that turn off or, simply say, special taps (see). The valve must be installed at the input and output, on the branches of the street gas pipeline, at the intersection of various obstacles, railway installations and roads. On external lines, a valve is installed in the well along with showing the values ​​\u200b\u200bof temperature and voltage. In addition, provides a comfortable installation and disassembly of the shut-off elements of the valve. The well must be placed, given the gap of two meters from buildings or fences. The number of barriers should be justified and be as minimal as possible. When entering the room, the valve is installed on the wall, while it is necessary to maintain a certain gap from doors and windows. If the reinforcement is located above 2 meters, it is necessary to provide a place with a ladder in order to be able to serve it. In cottages, in most cases, gas is supplied through networks with medium pressure, but not with low pressure. Firstly, it provides for an additional control device, since the pressure indicators are higher. Secondly, gas boilers have recently been gaining popularity, then only at medium pressure can gas be supplied in the required amount to consumers. By gasifying under low pressure conditions, the performance of the end device will drop. For example, if a pressure of about 300 is considered acceptable in winter, then if you move away from the hydraulic fracturing, the indicators for consumers will drop to 120. Before frost, gas pressure is sufficient. But if a severe frost comes and everyone starts heating with gas boilers, turning on full power, the pressure on the owners of the cottage on the periphery drops significantly. And when the pressure is below 120, troubles begin to appear for the owners of the boilers, for example, the boiler installation goes out or shows that the gas supply has been stopped. Under conditions of medium pressure supply, gas in a compressed state moves through the pipeline. Further, through the regulator, the pressure drops to low levels, and the boiler works without problems. As you know, many substances in nature can be in three states of aggregation: solid, liquid and gaseous. The doctrine of the properties of matter in various states of aggregation is based on ideas about the atomic and molecular structure of the material world. The molecular-kinetic theory of the structure of matter (MKT) is based on three main provisions: all substances consist of the smallest particles (molecules, atoms, elementary particles), between which there are gaps; the particles are in continuous thermal motion; between the particles of matter there are forces of interaction (attraction and repulsion); the nature of these forces is electromagnetic. This means that the state of aggregation of a substance depends on the relative position of the molecules, the distance between them, the forces of interaction between them and the nature of their movement. The interaction of particles of matter in the solid state is most pronounced. The distance between molecules is approximately equal to their own sizes. This leads to a sufficiently strong interaction, which practically deprives the particles of the opportunity to move: they oscillate around a certain equilibrium position. They retain their shape and volume. The properties of liquids are also explained by their structure. Particles of matter in liquids interact less intensively than in solids, and therefore they can change their location abruptly - liquids do not retain their shape - they are fluid. Liquids retain volume. A gas is a collection of molecules moving randomly in all directions independently of each other. Gases do not have their own shape, they occupy the entire volume provided to them and are easily compressed. There is another state of matter - plasma. Plasma is a partially or fully ionized gas in which the densities of positive and negative charges are almost the same. When heated sufficiently, any substance evaporates, turning into a gas. If the temperature is increased further, the process of thermal ionization will sharply increase, i.e., the gas molecules will begin to decompose into their constituent atoms, which then turn into ions. Ideal gas model. Relationship between pressure and average kinetic energy. To clarify the patterns that govern the behavior of a substance in a gaseous state, an idealized model of real gases, an ideal gas, is considered. This is a gas whose molecules are considered as material points that do not interact with each other at a distance, but interact with each other and with the walls of the vessel during collisions. Ideal gas – it is a gas, the interaction between the molecules of which is negligible. (Ec>>Er) An ideal gas is a model invented by scientists to understand the gases that we observe in nature in reality. It may not describe any gas. Not applicable when the gas is highly compressed when the gas becomes liquid. Real gases behave like ideal gases when the average distance between molecules is many times greater than their sizes, i.e. at sufficiently high pressures. Ideal gas properties: the distance between the molecules is much larger than the size of the molecules; gas molecules are very small and are elastic balls; the forces of attraction tend to zero; interactions between gas molecules occur only during collisions, and collisions are considered to be absolutely elastic; the molecules of this gas move randomly; the movement of molecules according to Newton's laws. The state of a certain mass of a gaseous substance is characterized by mutually dependent physical quantities called state parameters. These include volumeV, pressurepand temperatureT. Gas volume denoted V. Volume gas always coincides with the volume of the vessel that it occupies. SI unit of volume m 3. Pressure– physical quantity equal to the ratio of forceFacting on a surface element perpendicular to it, to the areaSthis element. p = F/ S Unit of pressure in SI pascal[Pa] Until now, off-system units of pressure have been used: technical atmosphere 1 at = 9.81-104 Pa; physical atmosphere 1 atm = 1.013-105 Pa; millimeters of mercury 1 mmHg article = 133 Pa; 1 atm = = 760 mmHg Art. = 1013 hPa. How is gas pressure generated? Each gas molecule, hitting the wall of the vessel in which it is located, acts on the wall with a certain force for a short period of time. As a result of random impacts on the wall, the force from all molecules per unit area of ​​the wall rapidly changes with time relative to some (average) value. Gas pressurearises as a result of chaotic impacts of molecules on the walls of the vessel in which the gas is located. Using the ideal gas model, one can calculate gas pressure on the vessel wall. In the process of interaction of a molecule with the vessel wall, forces arise between them that obey Newton's third law. As a result, the projection υ x velocity of the molecule perpendicular to the wall changes its sign to the opposite, and the projection υ y velocity parallel to the wall remains unchanged. Instruments that measure pressure are called manometers. Pressure gauges record the time-averaged pressure force per unit area of ​​its sensitive element (membrane) or other pressure receiver. Liquid manometers: open - for measuring small pressures above atmospheric closed - for measuring small pressures below atmospheric, i.e. small vacuum Metal pressure gauge- to measure high pressures. Its main part is a curved tube A, the open end of which is soldered to the tube B, through which gas flows, and the closed end is connected to the arrow. Gas enters through the cock and tube B into tube A and unbends it. The free end of the tube, moving, drives the transmission mechanism and the arrow. The scale is graduated in units of pressure. The basic equation of the molecular-kinetic theory of an ideal gas. The basic equation of the MKT: the pressure of an ideal gas is proportional to the product of the mass of the molecule, the concentration of the molecules, and the mean square of the speed of the molecules p= 1/3m 0· n v 2 m 0 is the mass of one gas molecule; n = N/V is the number of molecules per unit volume, or the concentration of molecules; v 2 - root mean square speed of molecules. Since the average kinetic energy of the translational motion of molecules is E \u003d m 0 * v 2 /2, then multiplying the basic MKT equation by 2, we get p \u003d 2/3 n (m 0 v 2) / 2 \u003d 2/3 E n p = 2/3 E n The gas pressure is equal to 2/3 of the average kinetic energy of the translational motion of molecules contained in a unit volume of gas. Since m 0 n = m 0 N/V = m/V = ρ, where ρ is the gas density, we have p= 1/3 ρv 2 United gas law. Macroscopic quantities that uniquely characterize the state of a gas are calledthermodynamic parameters of the gas. The most important thermodynamic parameters of a gas are itsvolumeV, pressure p and temperature T. Any change in the state of a gas is calledthermodynamic process. In any thermodynamic process, the gas parameters that determine its state change. The ratio between the values ​​of certain parameters at the beginning and end of the process is calledgas law. The gas law expressing the relationship between all three gas parameters is calledunified gas law. p = nkT Ratio p = nkT which relates the pressure of a gas to its temperature and concentration of molecules, was obtained for the model of an ideal gas, the molecules of which interact with each other and with the walls of the vessel only during elastic collisions. This ratio can be written in another form, establishing a relationship between the macroscopic parameters of the gas - the volume V, pressure p, temperature T and the amount of matter ν. To do this, you need to use the equalities where n is the concentration of molecules, N is the total number of molecules, V is the volume of gas Then we get either Since N remains unchanged at a constant mass of gas, Nk is a constant number, which means At a constant mass of gas, the product of volume and pressure, divided by the absolute temperature of the gas, is the same value for all states of this mass of gas. The equation establishing the relationship between pressure, volume and temperature of a gas was obtained in the middle of the 19th century by the French physicist B. Clapeyron and is often called Claiperon equation. The Claiperon equation can be written in another form. p = nkt, given that Here N is the number of molecules in the vessel, ν is the amount of substance, N A is the Avogadro constant, m is the mass of gas in the vessel, M is the molar mass of the gas. As a result, we get: The product of the Avogadro constant N A byBoltzmann's constantk is called universal (molar) gas constant and is marked with the letter R. Its numerical value in SI R= 8.31 J/mol K Ratio called ideal gas equation of state. In the form we received, it was first recorded by D. I. Mendeleev. Therefore, the equation of state of the gas is called the Clapeyron–Mendeleev equation.` For one mole of any gas, this ratio takes the form: pV=RT Let's install physical meaning of the molar gas constant. Suppose that in a certain cylinder under the piston at temperature E there is 1 mole of gas, the volume of which is V. If the gas is heated isobarically (at constant pressure) by 1 K, the piston will rise to a height Δh, and the gas volume will increase by ΔV. Let's write the equation pV=RT for heated gas: p (V + ΔV) = R (T + 1) and subtract from this equation the equation pV=RT corresponding to the state of the gas before heating. We get pΔV = R ΔV = SΔh, where S is the base area of ​​the cylinder. Substitute in the resulting equation: pS = F is the pressure force. We get FΔh = R, and the product of the force and the displacement of the piston FΔh = A is the work of displacement of the piston, performed by this force against external forces during the expansion of the gas. Thus, R = A. The universal (molar) gas constant is numerically equal to the work that 1 mole of gas does when it is heated isobarically by 1 K. RELATED ARTICLES Feedback site `s map About the site