Goals:
- Educational: to form general ideas about pressure, pressure force, the formation of practical skills for calculating pressure;
- Developing: development of experimental skills, skills, logical thinking, substantiation of one's statements, development of teamwork skills, justify the need to increase or decrease pressure;
- Educational: the formation of independent work skills, the development of a desire for learning, the ability to work hard, the development of a sense of collectivism when working in pairs.
Type of lesson in question: learning new material.
Lesson form: combined lesson.
The place of the lesson in the curriculum. The topic “pressure and force of pressure” is discussed in the section “Pressure of solids, liquids and gases”. This topic is the first in the section and is the most interesting for students (because there is a great connection between the studied material and life, technology), so it takes 2 hours to study this topic. The main content of the studied material is set by the curriculum and the mandatory minimum content of education in physics.
Methods:verbal, visual, practical.
Equipment:
- stand-exhibition of cutting and piercing tools;
- presentation in Power Point, laboratory dynamometers, bars, rulers, buttons.
Lesson plan:
| 1. The stage of organizing the beginning of the lesson - | 1 min. |
| 2. The stage of preparation for the active and conscious assimilation of new material - | 7 min. |
| 3. The stage of assimilation of new knowledge (pressure force, pressure formula, pressure units) - | 20 minutes. |
| 4. Journey into biology - | 6 min. |
| 5. The world of technology - | 6 min. |
| 6. “Familiar letters” - | 2 minutes. |
| 7. Experimental tasks. - | 15 minutes. |
| 8. Test tasks. - | 13 min. |
| 9. Summing up - | 5 minutes. |
| 10. Homework. - | 5 minutes. |
Epigraph to the lesson: “Knowledge is only then knowledge when it is acquired by the efforts of thought, not memory” (AN Tolstoy).
During the classes
1. The stage of organizing the lesson.
2. The stage of preparation for active and conscious assimilation of the material.
The teacher draws the attention of the students to the illustration to the work of Mamin-Sibiryak “The Gray Neck” (see Slide No. 1 of the presentation) and reads out an excerpt from this work: “... The fox really came a few days later, sat down on the shore and spoke again:
I missed you, duck... Come out here; If you don't want it, I'll come to you myself. I'm not shy...
And the Fox began to crawl carefully over the ice to the very hole. Gray Sheika's heart sank…”.
Question. Why did the fox crawl carefully on the ice? (Listen to answers)
Teacher. To answer this question, you need to get acquainted with the topic “Pressure and the force of pressure”. The word "pressure" is well known to you. Do you understand the meaning of the following sentences:
- The pressure drops sharply, precipitation is possible.
- The defenders of the Dynamo team could not withstand the pressure of the attackers of Spartak.
- The patient's blood pressure suddenly increased.
- "Nautilus" glided into the bottomless depths, despite the enormous pressure of the external environment.
- “It was a woman,” said Commissar Maigret, “only the thin heels of women's shoes could produce so much pressure.
In all these sentences, the word “pressure” is used in different situations and has different meanings. We will consider pressure from the point of view of physics. To do this, invite an assistant to the lesson.
Children wanted honey - perish, blizzards and snowstorms,
For a good bee to visit the lesson.
Today the main character of our lesson will be a bee.
Teacher. Consider an example (a button on a petal): a boy rolls down the mountain on freshly fallen snow, suddenly falls, and the skis roll down. Having risen to his feet, the boy descends for skis, while his legs are deeply stuck in the snow.
Question: why does a boy on skis not fall into the snow, but fails without skis? The students conclude that in both cases the boy acts on the snow with the same force, but the result of the action of the force is different, therefore (the teacher leads to the thought) the result of the action also depends on some quantity.
Teacher: What has changed since the fall of the boy? Students conclude that the area of the boy's support on the snow has changed. When a boy is on skis, the area of support is greater than without skis.
Teacher: The effect of the force depends on:
1 – pressure force values;
2 - surface area, perpendicular to which the pressure force acts.
(Students work with OK.)
Teacher: The value that shows how much pressure force acts on each unit of surface area is called pressure.
P - pressure
F d - pressure force
S- footprint.
– in order to get pressure, we need to divide the force of pressure on the area!
Let's conduct a qualitative analysis of this formula.
Question 1. The force of pressure does not change, but the area of support increases. How will the pressure change? Why? ( The pressure will decrease, because the pressure is inversely proportional to the area).
Question 2. The area of support does not change, but the pressure force increases. How will the pressure change? Why? ( The pressure will increase as pressure is directly proportional to pressure).
Students conclude that for the same force, the pressure is greater when the area of support is smaller, and, conversely, the larger the area of support, the less pressure.
Teacher:
Your goal is to penetrate the body - reduce the support to zero.
Going for a walk in the forest in winter, you increase the support S.
(To master the meaning of the formula for the pressure of a solid body).
To create visual images, the teacher introduces students to the various pressures encountered in technology, nature and everyday life (table 6 p. 84 Physics textbook - 7 cells)
Students work with OK (work with a triangle).
Question 1. How can you find the force of pressure, knowing the pressure and the surface area to which the force is applied? (F d \u003d p * S)
Question 2. How to find the area of the surface to which the force is applied, knowing the force of pressure? (S=F d/p)
Teacher. Let's derive the unit of pressure. (The bee on the slide flies to the second petal with a mouse click).
| Given: | |
| S=1m 2 | |
| F d \u003d 1H |
|
| p-? |
1 Pa is the pressure produced by a pressure force of 1 N acting on a surface of 1 m 2 perpendicular to this surface.
1 hPa - 100 Pa
1 kPa - 1000 Pa
1 MPa - 1000 000 Pa
Question. What does the entry mean: p \u003d 15,000 Pa, p \u003d 5000 Pa? (15,000 Pa is the pressure produced by a pressure force of 15,000 N acting on a surface of 1m 2 perpendicular to this surface.)
Teacher.
Seas and deserts, Earth and Moon
The light of the sun and the snow of an avalanche...
Nature is complex, but Nature is one.
The laws of nature are one!
Let's take a journey into biology (the bee on the slide flies to the third petal with a mouse click).
There are piranhas in the Amazon
It looks like a fish.
If you put your finger in the water
Snack him instantly.
Question: Why can a piranha bite a person's finger?
Here is a camel, and on a camel
Carry luggage and people go.
He lives in the desert
Eats tasteless bushes
He works all year round...
Why do people carry luggage and ride on a camel?
(The surface area of the camel's limbs is large, and the pressure exerted on the sand is small, so the camel does not sink into the sand.)
Hedgehog angry, gray hedgehog,
Where are you going, tell me?
You are so prickly that you can’t take your hand!
Why is the hedgehog prickly?
(The surface area of the needles is small and the pressure is high.)
The bee is a well-known worker,
Gives people honey and wax,
And the enemies will show a sting,
Will be remembered for a whole year!
Why does a bee's sting put so much pressure on human skin? (The sting of a bee has a small surface area, and the pressure exerted on human skin is high.)
Once a rose was asked:
Why, enchanting the eye,
You are prickly thorns
Are you scratching us hard?
(The surface area of rose thorns is small, but the pressure is high.)
Let's return to the heroes of the "Grey Neck". Why did the fox crawl carefully on the ice? (The fox chose this mode of locomotion to increase the surface area and reduce the pressure exerted on the ice.)
Teacher: The cunning fox knew the formula for pressure! We are convinced of the validity of this formula in nature - needles, cranberries, claws, teeth, fangs, stings. But. “The soul of science is the practical application of its discoveries” (W. Thomson).
Let's take a tour of the world of technology.(The bee flies to the fourth petal with a mouse click.)
We know that the larger the area of support, the less pressure produced by a given force, and, conversely, with a decrease in the area of support (with a constant force), the pressure increases. Therefore, depending on whether you want to get a small or large pressure, the area of support is increased or decreased. (Students work with OK - ways to change pressure). 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 vehicles, such as a tractor, tank, or swamp vehicle, can drive through swampy terrain that is not always possible for a person to pass through. Why? (Heavy machines, with a large footprint, exert little pressure.)
The teacher draws the attention of students to the exhibition of cutting and piercing objects and tools.
Question: Why do cutting and stabbing tools exert so much pressure on the body? (The surface area of cutting and piercing tools is small and the pressure is large.)
Teacher. We are convinced of the validity of the pressure formula in nature and technology. (A bee flies to the fifth petal at the click of a mouse.)
Game "Familiar Letters".
Letters are written on the board - designations of physical quantities: p, m, F, l, V. Your task: after listening to the proverbs, put them in line with one of these values.
Proverbs:
- Murder will out.
- You can't pick up a hedgehog with your bare hands.
- Don't put your finger in your mouth.
(Pressure)
Teacher.“Knowledge not born of experience, the mother of all certainty, is fruitless and full of errors.” (A bee flies to the 6th petal at the click of a mouse.)
Experimental tasks.
1. Task. Pressing the button into the board, we act on it with a force of 50N, the area of the tip of the button is 0.000 001 m 2. Determine the pressure produced by the button.
| Given: | |
| F d \u003d 50N |
[p]=Pa. |
| S=0.000 001m 2 | |
| p=? |  (Pa) |
Answer: 50 MPa.
2. Calculate the pressure of a rigid body on a support. (Work in pairs.)
Equipment: dynamometer, measuring ruler, wooden block.
The order of the work.
- Measure the pressure force of the bar on the table (the weight of the bar).
- Measure the length, width and height of the bar.
- Using all the data obtained, calculate the areas of the largest and smallest faces of the bar.
- Calculate the pressure that the bar produces on the table with the smallest and largest faces.
- Write down the results in a notebook.
- Form a conclusion based on the results obtained.
Students write the results of the experiments on the board and draw a conclusion about the dependence of pressure on the surface area of the support.
Teacher.
To keep the bee on its way
Need to gain knowledge.
We open leaves
And we do the work.
(The bee flies to the 7th petal at the click of the mouse.) "Test tasks".
Lesson summary
- What physical quantity did you meet today in the lesson?
- What force is called pressure force?
- What is pressure?
- Units of pressure?
- Units of pressure in SI?
Lesson grades: Test results, tokens are taken into account.
The final grade for the lesson is displayed. The teacher draws the attention of the students to the epigraph to the lesson.
Homework:§32b33; p.85 (experimental task).
Additional task.“Why are pointed objects prickly. Like Leviathan”, Entertaining physics. Ya.I. Perelman.
List of used literature.
- Physics - 7 cells. S.V. Gromov, N.A. Rodina. Moscow. “Enlightenment”, 2000
- Physics lesson in modern school. Creative search for teachers. Compiled by E.M. Braverman, edited by V.G. Razumovsky. Moscow, Enlightenment, 1993
- Checking the knowledge of students in physics (6-7th grade) A.V. Postnikov, Moscow, “Enlightenment”, 1986
- Newspaper “Physics” No. 45, 2004
- Journal "Physics at School" No. 8, 2002
- Reader in Literature. 1-4 cells Rostov-on-Don. JSC "Kniga", 1997
DEFINITION
Pressure is a scalar physical quantity equal to the ratio of the modulus acting perpendicular to the surface to the area of this surface:
The force applied perpendicular to the surface of the body, under the action of which the body is deformed, is called the pressure force. Any force can act as a pressure force. This may be a force that presses one body against the surface of another, or the weight of a body acting on a support (Fig. 1).
Rice. 1. Determination of pressure
Pressure units
In the SI system, pressure is measured in pascals (Pa): 1 Pa \u003d 1 N / m 2
The pressure does not depend on the orientation of the surface.
Off-system units are often used: normal atmosphere (atm) and millimeter of mercury (mm Hg): 1 atm = 760 mm Hg = 101325 Pa
Obviously, depending on the surface area, the same pressure force can exert different pressure on this surface. This relationship is often used in technology to increase or, conversely, reduce pressure. The designs of tanks and tractors provide for reducing the pressure on the ground by increasing the area with the help of a caterpillar drive. The same principle underlies the design of skis: on skis, a person easily slides on the snow, however, having removed the skis, he immediately falls into the snow. The blade of cutting and piercing tools (knives, scissors, cutters, saws, needles, etc.) is specially sharpened: 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.
Examples of problem solving
EXAMPLE 1
| Exercise | A person presses on a shovel with a force of 400 N. What pressure does a shovel exert on the ground if its blade is 20 cm wide and its cutting edge is 0.5 mm thick? |
| Decision | The pressure that a shovel exerts on the ground is determined by the formula: Surface area of the shovel that is in contact with the ground: where is the width of the blade, is the thickness of the cutting edge. Therefore, the pressure of the shovel on the ground: Let's convert the units to the SI system: blade width: cm m; incisal thickness mm m. Calculate: Pa MPa |
| Answer | The pressure of the shovel on the ground is 4 MPa. |
EXAMPLE 2
| Exercise | Find the edge of an aluminum cube if it exerts a pressure of 70 Pa on the table. |
| Decision | Cube pressure on the table: The pressure force in this case is the weight of the cube, so we can write: Given that and the volume of the cube in turn: |
PHYSICS. 1. The subject and structure of physics F. the science that studies the simplest and at the same time the most. general properties and laws of motion of the objects of the material world surrounding us. As a result of this generality, there are no natural phenomena that do not have physical. properties... Physical Encyclopedia
PHYSICS- a science that studies the simplest and at the same time the most general laws of natural phenomena, the structure of matter and the laws of its motion. The concepts of F. and its laws underlie all natural science. F. belongs to the exact sciences and studies quantities ... Physical Encyclopedia
PHYSICS- PHYSICS, a science that studies, together with chemistry, the general laws of the transformation of energy and matter. Both sciences are based on two basic laws of natural science - the law of conservation of mass (the law of Lomonosov, Lavoisier) and the law of conservation of energy (R. Mayer, Jaul ... ... Big Medical Encyclopedia
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Physics- I. The subject and structure of physics Physics is a science that studies the simplest and at the same time the most general patterns of natural phenomena, the properties and structure of matter and the laws of its motion. Therefore, the concepts of F. and its laws underlie everything ... ...
High pressure- in a broad sense, pressure exceeding atmospheric pressure; in specific technical and scientific tasks, pressure exceeding the value characteristic of each task. Equally conventionally found in the literature is the subdivision of D. century. to high and ... ... Great Soviet Encyclopedia
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Books
- Physics. 7th grade. Workbook for A. V. Peryshkin's textbook. Vertical. Federal State Educational Standard, Hannanova Tatyana Andreevna, Khannanov Nail Kutdusovich, The manual is an integral part of A. V. Peryshkin's Teaching Methods "Physics. Grades 7-9", which has been revised in accordance with the requirements of the new Federal State Educational Standard. ... Category: Physics. Astronomy (grades 7-9) Series: Physics Publisher: Drofa, Buy for 228 rubles
- Physics Grade 7 Workbook for the textbook A. V. Peryshkin, Khannanova T., Khannanov N., The manual is an integral part of A. V. Peryshkin’s teaching materials “Physics. Grades 7-9”, which has been revised in accordance with the requirements of the new Federal State Educational Standard. In… Category:
Pressure is a physical quantity that plays a special role in nature and human life. This phenomenon, imperceptible to the eye, not only affects the state of the environment, but is also very well felt by everyone. Let's figure out what it is, what types of it exist and how to find the pressure (formula) in different environments.
What is called pressure in physics and chemistry
This term refers to an important thermodynamic quantity, which is expressed in the ratio of the perpendicularly exerted pressure force to the surface area on which it acts. This phenomenon does not depend on the size of the system in which it operates, and therefore refers to intensive quantities.
In a state of equilibrium, the pressure is the same for all points in the system.
In physics and chemistry, this is denoted by the letter "P", which is an abbreviation for the Latin name of the term - pressūra.
If we are talking about the osmotic pressure of a liquid (the balance between the pressure inside and outside the cell), the letter "P" is used.
Pressure units
According to the standards of the International SI system, the physical phenomenon under consideration is measured in pascals (in Cyrillic - Pa, in Latin - Ra).
Based on the pressure formula, it turns out that one Pa is equal to one N (newton - divided by one square meter (a unit of area).
However, in practice, it is rather difficult to use pascals, since this unit is very small. In this regard, in addition to the standards of the SI system, this value can be measured in a different way.
Below are its most famous analogues. Most of them are widely used in the former USSR.
- bars. One bar is equal to 105 Pa.
- Torres, or millimeters of mercury. Approximately one Torr corresponds to 133.3223684 Pa.
- millimeters of water column.
- Meters of water column.
- technical atmospheres.
- physical atmospheres. One atm is equal to 101,325 Pa and 1.033233 at.
- Kilogram-force per square centimeter. There are also ton-force and gram-force. In addition, there is an analog pound-force per square inch.
General pressure formula (7th grade physics)
From the definition of a given physical quantity, one can determine the method of finding it. It looks like the photo below.
In it, F is force, and S is area. In other words, the formula for finding pressure is its force divided by the surface area on which it acts.
It can also be written as follows: P = mg / S or P = pVg / S. Thus, this physical quantity is related to other thermodynamic variables: volume and mass.
For pressure, the following principle applies: the smaller the space affected by the force, the greater the amount of pressing force it has. If, however, the area increases (with the same force) - the desired value decreases.
Hydrostatic pressure formula
Different aggregate states of substances provide for the presence of their properties that are different from each other. Based on this, the methods for determining P in them will also be different.
For example, the formula for water pressure (hydrostatic) looks like this: P = pgh. It also applies to gases. At the same time, it cannot be used to calculate atmospheric pressure, due to the difference in altitudes and air densities.
In this formula, p is the density, g is the gravitational acceleration, and h is the height. Based on this, the deeper the object or object sinks, the higher the pressure exerted on it inside the liquid (gas).
The variant under consideration is an adaptation of the classical example P = F / S.
If we recall that the force is equal to the derivative of the mass by the free fall velocity (F = mg), and the mass of the liquid is the derivative of the volume by the density (m = pV), then the pressure formula can be written as P = pVg / S. In this case, the volume is area multiplied by height (V = Sh).
If you insert this data, it turns out that the area in the numerator and denominator can be reduced and the output is the above formula: P \u003d pgh.
Considering the pressure in liquids, it is worth remembering that, unlike solids, the curvature of the surface layer is often possible in them. And this, in turn, contributes to the formation of additional pressure.
For such situations, a slightly different pressure formula is used: P \u003d P 0 + 2QH. In this case, P 0 is the pressure of a non-curved layer, and Q is the liquid tension surface. H is the average curvature of the surface, which is determined by Laplace's Law: H \u003d ½ (1 / R 1 + 1 / R 2). The components R 1 and R 2 are the radii of the main curvature.
Partial pressure and its formula
Although the P = pgh method is applicable to both liquids and gases, it is better to calculate the pressure in the latter in a slightly different way.
The fact is that in nature, as a rule, absolutely pure substances are not very common, because mixtures predominate in it. And this applies not only to liquids, but also to gases. And as you know, each of these components exerts a different pressure, called partial pressure.
It's pretty easy to define. It is equal to the sum of the pressure of each component of the mixture under consideration (ideal gas).
From this it follows that the partial pressure formula looks like this: P \u003d P 1 + P 2 + P 3 ... and so on, according to the number of constituent components.
There are often cases when it is necessary to determine the air pressure. However, some mistakenly carry out calculations only with oxygen according to the scheme P = pgh. But air is a mixture of different gases. It contains nitrogen, argon, oxygen and other substances. Based on the current situation, the air pressure formula is the sum of the pressures of all its components. So, you should take the aforementioned P \u003d P 1 + P 2 + P 3 ...
The most common instruments for measuring pressure
Despite the fact that it is not difficult to calculate the thermodynamic quantity under consideration using the above formulas, sometimes there is simply no time to carry out the calculation. After all, you must always take into account numerous nuances. Therefore, for convenience, a number of devices have been developed over several centuries to do this instead of people.
In fact, almost all devices of this kind are varieties of a pressure gauge (it helps to determine the pressure in gases and liquids). However, they differ in design, accuracy and scope.
- Atmospheric pressure is measured using a pressure gauge called a barometer. If it is necessary to determine the vacuum (that is, pressure below atmospheric pressure), another version of it, a vacuum gauge, is used.
- In order to find out the blood pressure in a person, a sphygmomanometer is used. To most, it is better known as a non-invasive tonometer. There are many varieties of such devices: from mercury mechanical to fully automatic digital. Their accuracy depends on the materials from which they are made and the place of measurement.
- Pressure drops in the environment (in English - pressure drop) are determined using or difnamometers (not to be confused with dynamometers).
Types of pressure
Considering the pressure, the formula for finding it and its variations for different substances, it is worth learning about the varieties of this quantity. There are five of them.
- Absolute.
- barometric
- Excess.
- Vacuum.
- Differential.
Absolute
This is the name of the total pressure under which a substance or object is located, without taking into account the influence of other gaseous components of the atmosphere.
It is measured in pascals and is the sum of excess and atmospheric pressure. It is also the difference between barometric and vacuum types.
It is calculated by the formula P = P 2 + P 3 or P = P 2 - P 4.
For the reference point for absolute pressure under the conditions of the planet Earth, the pressure inside the container from which air is removed (that is, classical vacuum) is taken.
Only this type of pressure is used in most thermodynamic formulas.
barometric
This term refers to the pressure of the atmosphere (gravity) on all objects and objects found in it, including the surface of the Earth itself. Most people also know it under the name atmospheric.
It is referred to and its value varies with the place and time of measurement, as well as weather conditions and being above / below sea level.
The value of barometric pressure is equal to the modulus of the force of the atmosphere per unit area along the normal to it.
In a stable atmosphere, the magnitude of this physical phenomenon is equal to the weight of a column of air on a base with an area equal to one.
The norm of barometric pressure is 101,325 Pa (760 mm Hg at 0 degrees Celsius). Moreover, the higher the object is from the surface of the Earth, the lower the air pressure on it becomes. Every 8 km it decreases by 100 Pa.
Thanks to this property, in the mountains, water in kettles boils much faster than at home on the stove. The fact is that pressure affects the boiling point: with its decrease, the latter decreases. And vice versa. The work of such kitchen appliances as a pressure cooker and an autoclave is built on this property. The increase in pressure inside them contributes to the formation of higher temperatures in the dishes than in ordinary pans on the stove.
The barometric altitude formula is used to calculate atmospheric pressure. It looks like the photo below.
P is the desired value at height, P 0 is the density of air near the surface, g is the free fall acceleration, h is the height above the Earth, m is the molar mass of the gas, t is the temperature of the system, r is the universal gas constant 8.3144598 J⁄ ( mol x K), and e is the Eclair number, equal to 2.71828.
Often in the above formula for atmospheric pressure, instead of R, K is used - Boltzmann's constant. The universal gas constant is often expressed in terms of its product by the Avogadro number. It is more convenient for calculations when the number of particles is given in moles.
When making calculations, it is always worth taking into account the possibility of changes in air temperature due to a change in the meteorological situation or when climbing above sea level, as well as geographical latitude.
Gauge and vacuum
The difference between atmospheric and measured ambient pressure is called overpressure. Depending on the result, the name of the value changes.
If it is positive, it is called gauge pressure.
If the result obtained is with a minus sign, it is called a vacuum gauge. It is worth remembering that it cannot be more than barometric.
differential
This value is the pressure difference at different measuring points. As a rule, it is used to determine the pressure drop on any equipment. This is especially true in the oil industry.
Having figured out what kind of thermodynamic quantity is called pressure and with the help of what formulas it is found, we can conclude that this phenomenon is very important, and therefore knowledge about it will never be superfluous.
>>Pressure and force of pressure
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