What causes gas pressure on the walls of a vessel? Quant. Gas pressure. Repetition of covered material

Behavior of atmospheric molecules The atmosphere consists of gases, but why don’t the molecules fly away into outer space? The atmosphere consists of gases, but why don’t the molecules fly away into outer space? Like all bodies, the gas molecules that make up the Earth's air envelope are attracted to the Earth. Like all bodies, the gas molecules that make up the Earth's air envelope are attracted to the Earth. To leave the Earth, they must have a speed of at least 11.2 km/s, this is the second cosmic speed. Most molecules have speeds less than 11.2 km/s. To leave the Earth, they must have a speed of at least 11.2 km/s, this is the second cosmic speed. Most molecules have speeds less than 11.2 km/s. Why doesn't the atmosphere settle to the surface of the Earth? Why doesn't the atmosphere settle to the surface of the Earth? The molecules of the gases that make up the atmosphere move continuously and randomly. The molecules of the gases that make up the atmosphere move continuously and randomly.

Under the influence of gravity, the upper layers of air in the atmosphere compress the lower ones. Under the influence of gravity, the upper layers of the atmosphere compress the lower ones. The layer adjacent to the Earth is compressed the most. The layer adjacent to the Earth is compressed the most. The earth's surface and the bodies on it experience pressure from the entire thickness of the air (according to Pascal's law) - atmospheric pressure. The earth's surface and the bodies on it experience the pressure of the entire thickness of air (according to Pascal's law) - atmospheric pressure.

Historical fact For the first time, the weight of air confused people in 1638, when the Duke of Tuscany’s idea to decorate the gardens of Florence with fountains failed - the water did not rise above 10.3 m. For the first time, the weight of air confused people in 1638, when the Duke of Tuscany’s idea to decorate the gardens of Florence with fountains failed - the water did not rise above 10.3 m. The search for the reasons for the stubbornness of water and experiments with a heavier liquid - mercury, undertaken in 1643. Torricelli, led to the discovery of atmospheric pressure. The search for the reasons for the stubbornness of water and experiments with a heavier liquid - mercury, undertaken in 1643. Torricelli, led to the discovery of atmospheric pressure.

Experience of Otto von Guericke In 1654, the Magdeburg burgomaster and physicist Otto von Guericke showed one experiment at the Reichstag in Regensburg, which is now called the Magdeburg hemispheres experiment throughout the world. In 1654, the Magdeburg burgomaster and physicist Otto von Guericke showed one experiment at the Reichstag in Regensburg, which is now called the Magdeburg hemisphere experiment throughout the world.

Atmospheric pressure and humans Atmospheric pressure is not felt by humans and animals. Atmospheric pressure is not felt by humans and animals. Tissues, blood vessels and the walls of other body cavities are exposed to external atmospheric pressure. Tissues, blood vessels and the walls of other body cavities are exposed to external atmospheric pressure. Blood and other liquids and gases filling these cavities exert the same pressure from the inside. Blood and other liquids and gases filling these cavities exert the same pressure from the inside.

Breathing The mechanism of inhalation is as follows: with muscular effort we increase the volume of the chest, while the air pressure inside the lungs becomes less than atmospheric pressure, and atmospheric pressure pushes a portion of air into an area of ​​​​lower pressure. The mechanism of inhalation is as follows: with muscular effort we increase the volume of the chest, while the air pressure inside the lungs becomes less than atmospheric pressure, and atmospheric pressure pushes a portion of air into an area of ​​​​lower pressure. How does exhalation occur? How does exhalation occur?

Homework Interesting information on the site Classroom Physics You can answer questions for a separate assessment Interesting information on the site Classroom Physics You can answer questions for a separate assessment §40 §40 Fill out the card Fill out the card Do and explain in writing one of the experiments Do and explain in writing one of the experiments

Why are airplane passengers advised to remove the ink from their fountain pens before boarding? Why are airplane passengers advised to remove the ink from their fountain pens before boarding? How to draw water from a glass straw? How to draw water from a glass straw? Why are not one, but two holes made in the lids of lubricating oil cans? Why are not one, but two holes made in the lids of lubricating oil cans? Why do they make a hole in the lid of a porcelain teapot? Why do they make a hole in the lid of a porcelain teapot? Why is it difficult to pull out feet stuck in soggy clay? Why is it difficult to pull out feet stuck in soggy clay? Who finds it easier to walk on mud? It is very difficult for a horse with a solid hoof to pull his foot out of deep mud. Under the leg, when she lifts it, a rarefied space is formed and atmospheric pressure prevents the leg from being pulled out. In this case, the leg works like a piston in a cylinder. It is very difficult for a horse with a solid hoof to pull his foot out of deep mud. Under the leg, when she lifts it, a rarefied space is formed and atmospheric pressure prevents the leg from being pulled out. In this case, the leg works like a piston in a cylinder. The external atmospheric pressure, which is enormous compared to what has arisen, does not allow one to raise the leg. In this case, the force of pressure on the leg can reach 1000 N. The external atmospheric pressure, which is enormous in comparison with the existing one, does not allow the leg to be raised. In this case, the force of pressure on the leg can reach 1000 N. It is much easier to move through such mud for ruminants, whose hooves consist of several parts and, when pulled out of the mud, their legs compress, allowing air into the resulting depression. It is much easier for ruminants to move through such mud, whose hooves consist of several parts and, when pulled out of the mud, their legs compress, allowing air into the resulting depression.

Atmospheric pressure and weather Atmospheric pressure helps predict the weather, which is necessary for people of different professions - pilots, agronomists, radio operators, polar explorers, doctors, scientists. If the atmospheric pressure rises, then the weather will be good: cold in winter, hot in summer; if it drops sharply, then you can expect cloudiness and saturation of the air with moisture. A decrease in pressure in summer portends cooling, and in winter – warming. Atmospheric pressure helps predict the weather, which is necessary for people of different professions - pilots, agronomists, radio operators, polar explorers, doctors, scientists. If the atmospheric pressure rises, then the weather will be good: cold in winter, hot in summer; if it drops sharply, then you can expect cloudiness and saturation of the air with moisture. A decrease in pressure in summer portends cooling, and in winter – warming. Atmospheric pressure increases if air masses move downwards (downdrafts). Falls from high altitudes dry air, so the weather will be good, without precipitation. Atmospheric pressure decreases with rising air currents. Air rises, abundantly saturated with water vapor. At the top it cools, which leads to cloudiness and precipitation - and the weather worsens. Atmospheric pressure increases if air masses move downwards (downdrafts). Dry air descends from high altitudes, so the weather will be good, without precipitation. Atmospheric pressure decreases with rising air currents. Air rises, abundantly saturated with water vapor. At the top it cools, which leads to cloudiness and precipitation - and the weather worsens.

What would happen on Earth if the air atmosphere suddenly disappeared? On Earth, the temperature would be approximately C. On Earth, the temperature would be approximately C. All water areas would freeze, and the land would be covered with an ice crust. All water areas would freeze, and the land would be covered with an ice crust. There would be complete silence, since sound does not travel in emptiness. there would be complete silence, since sound does not travel in emptiness; the sky would become black, since the color of the firmament depends on the air; there would be no twilight, dawn, white nights, the sky would become black, since the color of the firmament depends on the air; there would be no twilight, dawn, white nights, the twinkling of stars would stop, and the stars themselves would be visible not only at night, but also during the day (we don’t see them during the day due to scattering by air particles sunlight) the twinkling of stars would stop, and the stars themselves would be visible not only at night, but also during the day (we don’t see them during the day due to the scattering of sunlight by air particles), animals and plants would die, animals and plants would die

We have already said (§ 220) that gases always completely fill the volume limited by walls impenetrable to gas. So, for example, a steel cylinder used in technology for storing compressed gases (Fig. 375), or the inner tube of a car tire is completely and almost uniformly filled with gas.

Rice. 375. Steel cylinder for storing highly compressed gases

Trying to expand, the gas puts pressure on the walls of the cylinder, tire tubes or any other body, solid or liquid, with which it comes into contact. If we do not take into account the effect of the Earth's gravity field, which with the usual sizes of vessels only changes the pressure insignificantly, then at equilibrium the gas pressure in the vessel seems to us to be completely uniform. This remark applies to the macrocosm. If we imagine what happens in the microcosm of the molecules that make up the gas in the vessel, then there can be no talk of any uniform distribution of pressure. In some places on the surface of the walls, gas molecules hit them, while in other places there are no impacts; this picture changes all the time in a disorderly manner.

Let us assume for simplicity that all molecules before hitting the wall fly with the same speed directed normal to the wall. We will also assume that the impact is absolutely elastic. Under these conditions, the speed of the molecule upon impact will change direction to the opposite direction, remaining unchanged in magnitude. Therefore, the speed of the molecule after the impact will be equal to . Accordingly, the momentum of the molecule before the impact is equal to , and after the impact it is equal to ( - the mass of the molecule). Subtracting its initial value from the final value of the momentum, we find the increment in the momentum of the molecule imparted by the wall. It is equal. According to Newton's third law, the wall receives an impulse equal to .

If there are impacts per unit time per unit area of ​​the wall, then during the time molecules strike a section of the wall surface. Molecules impart to the area in time a total impulse equal in modulus to . By virtue of Newton's second law, this impulse equal to the product force acting on a section for a time. Thus,

Where .

Dividing the force by the area of ​​the wall section, we obtain the gas pressure on the wall:

It is not difficult to understand that the number of blows per unit time depends on the speed of the molecules, because the faster they fly, the more often they hit the wall, and on the number of molecules per unit volume, because the more molecules, the greater the number of blows they cause. Therefore, we can assume that proportional to and , i.e. proportional to

To calculate using molecular theory gas pressure, we must know the following characteristics of the microcosm of molecules: mass, speed and number of molecules per unit volume. In order to find these microcharacteristics of molecules, we must establish on what characteristics of the macrocosm the gas pressure depends, that is, establish the laws of gas pressure experimentally. By comparing these experimental laws with the laws calculated using molecular theory, we will be able to determine the characteristics of the microcosm, for example, the speed of gas molecules.

So, let's establish what gas pressure depends on?

Firstly, pressure depends on the degree of gas compression, i.e., on how many gas molecules are in a given volume. For example, by pumping more and more air into a car tire or compressing (reducing the volume ) closed chamber, we force the gas to press increasingly harder on the walls of the chamber.

Secondly, pressure depends on gas temperature. It is known, for example, that a ball becomes more elastic if it is held near a heated oven.

Typically, a change in pressure is caused by both reasons at once: a change in volume and a change in temperature. But it is possible to carry out the process in such a way that when the volume changes, the temperature changes negligibly, or when the temperature changes, the volume remains practically unchanged. We will deal with these cases first, having first made the following remark. We will consider the gas in a state of equilibrium. This means that both mechanical and thermal equilibrium have been established in the gas.

Mechanical equilibrium means no movement occurs individual parts gas To do this, it is necessary that the gas pressure be the same in all its parts, if we neglect the slight difference in pressure in the upper and lower layers of the gas that occurs under the influence of gravity.

Thermal equilibrium means that there is no transfer of heat from one part of the gas to another. To do this, it is necessary that the temperature throughout the entire volume of gas be the same.

Wherever the gas is located: in a balloon, car tire, or metal cylinder, it fills the entire volume of the vessel in which it is located.

Gas pressure arises for a completely different reason than solid pressure. It is formed as a result of collisions of molecules with the walls of the vessel.

Gas pressure on the walls of the vessel

Moving chaotically in space, gas molecules collide with each other and with the walls of the vessel in which they are located. The impact force of one molecule is small. But since there are a lot of molecules, and they collide with high frequency, then, acting together on the walls of the vessel, they create significant pressure. If a solid body is placed in a gas, it is also subject to impacts from gas molecules.

Let's do a simple experiment. Place a tied balloon, not completely filled with air, under the bell of the air pump. Since there is little air in it, the ball has irregular shape. When we begin to pump out the air from under the bell, the ball will begin to inflate. After some time it will take the shape of a regular ball.

What happened to our ball? After all, it was tied, therefore, the amount of air in it remained the same.

Everything is explained quite simply. During movement, gas molecules collide with the shell of the ball outside and inside it. If the air is pumped out of the bell, there are fewer molecules. The density decreases, and therefore the frequency of impacts of molecules on the outer shell also decreases. Consequently, the pressure outside the shell drops. And since the number of molecules inside the shell remains the same, the internal pressure exceeds the external one. The gas presses from the inside onto the shell. And for this reason, it gradually swells and takes the shape of a ball.

Pascal's law for gases

Gas molecules are very mobile. Thanks to this, they transmit pressure not only in the direction of the force causing this pressure, but also evenly in all directions. The law on pressure transfer was formulated by the French scientist Blaise Pascal: “ The pressure exerted on a gas or liquid is transmitted unchanged to any point in all directions" This law is called the basic law of hydrostatics - the science of liquids and gases in a state of equilibrium.

Pascal's law is confirmed by experience with a device called Pascal's ball . This device is a ball of solid with tiny holes made in it, connected to a cylinder along which the piston moves. The ball fills with smoke. When compressed by the piston, the smoke is pushed out of the holes of the ball in equal streams.

Gas pressure is calculated using the formula:

Where e lin - average kinetic energy of translational motion of gas molecules;

n - concentration of molecules

Partial pressure. Dalton's law

In practice, most often we encounter not pure gases, but their mixtures. We breathe air, which is a mixture of gases. Car exhaust gases are also a mixture. Pure carbon dioxide has not been used in welding for a long time. Gas mixtures are also used instead.

A gas mixture is a mixture of gases that do not enter into chemical reactions among themselves.

The pressure of an individual component of a gas mixture is called partial pressure .

If we assume that all the gases in the mixture are ideal gases, then the pressure of the mixture is determined by Dalton’s law: “The pressure of a mixture of ideal gases that do not interact chemically is equal to the sum of the partial pressures.”

Its value is determined by the formula:

Each gas in the mixture creates a partial pressure. Its temperature is equal to the temperature of the mixture.

The pressure of a gas can be changed by changing its density. The more gas is pumped into a metal container, the more molecules it will have hitting the walls, and the higher its pressure will become. Accordingly, by pumping out the gas, we rarefy it, and the pressure decreases.

But the pressure of a gas can also be changed by changing its volume or temperature, that is, by compressing the gas. Compression is carried out by applying force to a gaseous body. As a result of this effect, the volume it occupies decreases, pressure and temperature increase.

The gas is compressed in the engine cylinder as the piston moves. In production, high gas pressure is created by compressing it using complex devices - compressors, which are capable of creating pressure of up to several thousand atmospheres.

DEFINITION

Pressure in a vessel with a gas is created by the collision of molecules against its wall.

Due to thermal motion, gas particles occasionally hit the walls of the vessel (Fig. 1a). With each impact, the molecules act on the wall of the vessel with some force. Adding to each other, the impact forces of individual particles form a certain pressure force that constantly acts on the wall of the vessel. When gas molecules collide with the walls of a vessel, they interact with them according to the laws of mechanics as elastic bodies and transfer their impulses to the walls of the vessel (Fig. 1, b).

Fig.1. Gas pressure on the wall of a vessel: a) the appearance of pressure due to impacts of chaotically moving particles on the wall; b) pressure force as a result of elastic impact of particles.

In practice, most often they deal not with pure gas, but with a mixture of gases. For example, atmospheric air is a mixture of nitrogen, oxygen, carbon dioxide, hydrogen and other gases. Each of the gases included in the mixture contributes to total pressure, which exerts a mixture of gases on the walls of the vessel.

Valid for a gas mixture Dalton's law:

the pressure of the gas mixture is equal to the sum of the partial pressures of each component of the mixture:

DEFINITION

Partial pressure- the pressure that the gas included in the gas mixture would occupy if it alone occupied a volume equal to the volume of the mixture at a given temperature (Fig. 2).

Fig.2. Dalton's law for a gas mixture

From the point of view of molecular kinetic theory, Dalton's law is satisfied because the interaction between the molecules of an ideal gas is negligible. Therefore, each gas exerts pressure on the wall of the vessel, as if there were no other gases in the vessel.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise

A closed container contains a mixture of 1 mole of oxygen and 2 moles of hydrogen. Compare the partial pressures of both gases (oxygen pressure) and (hydrogen pressure):

Answer

Gas pressure is caused by the impacts of molecules on the walls of the container; it does not depend on the type of gas. Under conditions of thermal equilibrium, the temperature of the gases included in the gas mixture, in this case oxygen and hydrogen, is the same. This means that the partial pressures of gases depend on the number of molecules of the corresponding gas. One mole of any substance contains

Class: 7

Presentation for the lesson

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Textbook"Physics. 7th grade.” A.V. Peryshkin - M.: Bustard, 2011

Lesson type: combined based on research activities.

Goals:

• establish the reason for the existence of pressure in gases from the point of view molecular structure substances;
• find out:
• what does gas pressure depend on?
• how can you change it.

Tasks:

• develop knowledge about gas pressure and the nature of pressure on the walls of the vessel in which the gas is located;
• to develop the ability to explain gas pressure based on the doctrine of the movement of molecules, the dependence of pressure on volume at constant mass and temperature, as well as when temperature changes;
• develop general educational knowledge and skills: observe, draw conclusions;
• contribute to instilling interest in the subject, developing attention, scientific and logical thinking in students.

Equipment and materials for the lesson: computer, screen, multimedia projector, presentation for the lesson, flask with a stopper, tripod, alcohol lamp, syringe, balloon, plastic bottle with a stopper.

Lesson plan:

1. Checking homework.
2. Updating knowledge.
3. Explanation of new material.
4. Reinforcement of the material covered in the lesson.
5. Lesson summary. Homework.

PROGRESS OF THE LESSON

I prefer things that can be seen, heard and learned. (Heraclitus)(Slide 2)

- This is the motto of our lesson.

– In previous lessons, we learned about the pressure of solids and what physical quantities pressure depends on.

1. Repetition of the material covered

1. What is pressure?
2. What does pressure depend on? solid?
3. How does pressure depend on the force applied perpendicular to the support? What is the nature of this dependence?
4. How does pressure depend on the area of ​​support? What is the nature of this dependence?
5. What is the reason for the pressure of a solid body on the support?

Qualitative task.

Are the forces acting on the support and the pressure the same in both cases? Why?

Knowledge test. Testing (verification and mutual verification)

Test

1. Physical quantity, having the dimension pascal (Pa), is called:

a) strength; b) mass; c) pressure; d) density.

2. The pressure force was increased by 2 times. How will the pressure change?

a) will decrease by 2 times; b) will remain the same; c) will increase 4 times; d) will increase by 2 times.

4. What pressure does a carpet weighing 200 N and area 4 m2 exert on the floor?

a) 50 Pa; b) 5 Pa; c) 800Pa; d) 80 Pa.

5. Two bodies of equal weight are placed on a table. Do they produce the same pressure on the table?

2. Updating knowledge(in the form of a conversation)

– Why are balloons and soap bubbles round?
Students inflate balloons.
– What did we fill the balloons with? (By air) What else can you fill the balloons with? (Gas)
– I suggest squeezing the balls. What's stopping you from squeezing the balls? What acts on the shell of the ball?
– Take plastic bottles, cap them and try to squeeze them.
- About what we'll talk in class?

– Lesson topic: Gas pressure

3. Explanation of new material

Gases, unlike solids and liquids, fill the entire container in which they are located.
Trying to expand, the gas exerts pressure on the walls, bottom and lid of any body with which it comes into contact.
(Slide 9) Pictures of steel cylinders containing gas; car tire tubes; ball
Gas pressure is caused by factors other than the pressure of a solid body on the support.

Conclusion: The pressure of the gas on the walls of the container (and on the body placed in the gas) is caused by the impacts of the gas molecules.
For example, the number of impacts of air molecules in a room on a surface with an area of ​​\u200b\u200b1 cm 2 in 1 s is expressed as a twenty-three-digit number. Although the impact force of an individual molecule is small, the effect of all molecules on the walls of the vessel is significant, and it creates gas pressure.
Students work independently with the textbook. Read the experiment with a rubber ball under a bell. How to explain this experience? (p.83 fig. 91)

Students explain the experience.

(Slide 11) Watch a video clip explaining the experience to reinforce the material.

(Slide 12) A minute of rest. Exercise for the eyes.

“The feeling of mystery is the most beautiful experience available to us. It is this feeling that stands at the cradle of real science.”

Albert Einstein

(Slide 14) DO GASES HAVE VOLUME? IS IT EASY TO CHANGE THE VOLUME OF GASES? DO GASES OCCUPY THE ENTIRE VOLUME PROVIDED TO THEM? WHY?WHY? DO GASES HAVE A CONSTANT VOLUME AND OWN SHAPE? WHY?

rice. 92 page 84

(Slide 15) Students made models from syringes. Performing the experiment.
Students conclude: 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.

(Slide 16) Experiment with a flask

– How will the pressure of a gas change if it is heated at a constant volume?
When heated, the gas pressure in the flask will gradually increase until the stopper flies out of the flask.
Students conclude: the higher the gas temperature, the higher the gas temperature in a closed vessel, the higher the gas pressure, provided that the gas mass and volume do not change. (Slide 17)

Gases contained in a container can be compressed or compressed, thereby reducing their volume. The compressed gas is distributed evenly in all directions. The more you compress the gas, the higher its pressure will be.
Students conclude: the gas pressure increases, the more often and harder the molecules hit the walls of the vessel.

4. Reinforcement of the material covered in the lesson.

(Slide 18) Think about it

– What happens to gas molecules when the volume of the container in which the gas is located decreases?

• molecules begin to move faster
• molecules begin to move slower
• the average distance between gas molecules decreases,
• the average distance between gas molecules increases.

(Slide 19) Compare your answers

1. What causes gas pressure?
2. Why does the pressure of a gas increase when it is compressed and decrease when it expands?
3. When is the gas pressure greater: cold or hot? Why?

Answer 1. Gas pressure is caused by impacts of gas molecules on the walls of the vessel or on a body placed in the gas
Answer 2. When compressed, the density of the gas increases, due to which the number of impacts of molecules on the walls of the vessel increases. Consequently, the pressure also increases. When expanding, the density of the gas decreases, which entails a decrease in the number of impacts of molecules on the walls of the vessel. Therefore, the gas pressure decreases
Answer 3. The gas pressure is greater when it is hot. This is due to the fact that gas molecules begin to move faster as the temperature increases, causing their impacts to become more frequent and stronger.

(Slide 20) Qualitative tasks. (Collection of problems in physics V.I. Lukashik, E.V. Ivanova, Moscow “Enlightenment” 2007 p. 64)

1. Why does it become more and more difficult to move the pump handle each time you pump air into a car tire?

2. The masses of the same gas, located in different closed vessels at the same temperature, are the same. Which vessel has the greatest gas pressure? Least? Explain your answer

3. Explain the dent on the ball

Ball at room temperature

Ball in the snow on a frosty day

You can solve riddles forever.
The universe is infinite.
Thanks to all of us for the lesson,
And the main thing is that it will be used for future use!

Reflection.

5. Lesson summary

Homework:§35