For refrigeration equipment. See what "Pressure (physics)" is in other dictionaries

To understand what pressure is in physics, consider a simple and familiar example. Which?

In a situation where it is necessary to cut a sausage, we will use the most sharp object- with a knife, not with a spoon, comb or finger. The answer is obvious - the knife is sharper, and all the force we apply is distributed along the very thin edge of the knife, bringing maximum effect in the form of a separation of a part of an object, i.e. sausages. Another example - we are standing on loose snow. Legs fail, walking is extremely uncomfortable. Why, then, do skiers rush past us with ease and at high speed, without drowning and not getting entangled in the same loose snow? It is obvious that snow is the same for everyone, both for skiers and for walkers, but the effect on it is different.

With approximately the same pressure, that is, weight, the surface area pressing on the snow varies greatly. The area of ​​skis is much larger than the area of ​​the sole of the shoe, and, accordingly, the weight is distributed over a larger surface. What helps or, on the contrary, prevents us from effectively influencing the surface? Why does a sharp knife cut bread better, and flat wide skis hold better on the surface, reducing penetration into the snow? In the seventh grade physics course, the concept of pressure is studied for this.

pressure in physics

The force applied to a surface is called pressure force. And pressure is a physical quantity that is equal to the ratio of the pressure force applied to a specific surface to the area of ​​this surface. The formula for calculating pressure in physics is as follows:

where p is pressure,
F - pressure force,
s is the surface area.

We see how pressure is denoted in physics, and we also see that with the same force, the pressure is greater when the support area, or, in other words, the contact area of ​​interacting bodies, is smaller. Conversely, as the area of ​​support increases, the pressure decreases. That is why a sharper knife cuts any body better, and nails driven into a wall are made with sharp tips. And that is why skis hold on the snow much better than their absence.

Pressure units

The unit of pressure is 1 newton per square meter - these are quantities already known to us from the seventh grade course. We can also convert pressure units N / m2 to pascals, units of measurement named after the French scientist Blaise Pascal, who derived the so-called Pascal's Law. 1 N/m = 1 Pa. In practice, other units of pressure are also used - millimeters of mercury, bars, and so on.

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 the laws of motion of the objects of the material world around us. As a result of this generality, there are no natural phenomena that do not have physical. properties... Physical Encyclopedia

A science that studies the simplest and at the same time the most general patterns of natural phenomena, the principles and 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 general laws 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

Stellar physics is one of the branches of astrophysics that studies the physical side of stars (mass, density, ...). Contents 1 Dimensions, masses, density, luminosity of stars 1.1 Mass of stars ... Wikipedia

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 ... ...

In a broad sense, pressure in excess of 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

- (from ancient Greek physis nature). The ancients called physics any study of the surrounding world and natural phenomena. This understanding of the term physics was preserved until the end of the 17th century. Later, a number of special disciplines appeared: chemistry, which studies the properties of ... ... Collier Encyclopedia

Investigation of the effect exerted on matter by very high pressures, as well as the creation of methods for obtaining and measuring such pressures. History of the development of physics high pressures an amazing example of unusually rapid progress in science, ... ... Collier Encyclopedia

Solid state physics is a branch of condensed matter physics whose task is to describe physical properties solids in terms of their atomic structure. Intensively developed in the XX century after the discovery quantum mechanics.… … Wikipedia

Contents 1 Preparation methods 1.1 Evaporation of liquids ... Wikipedia

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• Physics. 7th grade. Workbook with USE test tasks. Vertical. Federal State Educational Standard, Hannanova Tatyana Andreevna, Khannanov Nail Kutdusovich. Benefit is integral part UMK A.V. Peryshkin`Physics. Grades 7-9, which has been revised in accordance with the requirements of the new Federal State Educational Standard. AT…
• Physics. 7th grade. Didactic materials for the textbook by A. V. Peryshkin. Vertical. GEF, Maron Abram Evseevich, Maron Evgeny Abramovich. This manual includes training tasks, tests for self-control, independent work, test papers and examples of solving typical problems. In total, in the proposed set of didactic ...

B. Blood pressure curve

The term " blood (arterial) pressure» SKD per se refers to arterial KD in big circle circulation. Maximum value CD is achieved in the aorta during the ejection period in systole; this is - systolic pressure(Ps); the minimum aortic pressure is reached during the phase of isovolumic contraction (at the time when aortic valves closed) and is called diastolic pressure (Pd) (A1). The difference between systolic and diastolic pressures [Ps-Pd] is called true pulse amplitude or pulse pressure (PP), and is a function of stroke volume (SV) and arterial elasticity 1C = dV/dP). When C decreases at a constant SV, systolic pressure Ps rises faster than diastolic pressure Pd, i.e. PP will increase (usually in old age, as described below). The same thing happens with an increase in SV at a constant value of C.

B. Measurement of blood pressure according to the Riva-Rocci method

If the total peripheral resistance (TPR) increases, and the time of the release of the CR remains the same, Ps and Pd will increase by the same value (without changing the PD). However, an increase in TVR usually leads to a delay in the release of SV and a decrease in the ratio of arterial volume growth to the peripheral flask during the period of exile. Following this, Ps increases less sharply than Pf, and AP decreases.

normal range. Pd is usually between 60 and 80 mmHg. Art., Ps from 100 to 120 mm Hg. Art. at rest (sitting or reclining). If at rest Ps 120 -1 39 mm Hg. Art. and / or Rf 80-89 mm Hg. Art., then the condition is considered prehypertensive (according to the accepted classification) (B). Maintaining an optimal CD through regulation is necessary for tissue perfusion.

abnormal low value blood pressure (hypotension) can lead to shock, anoxia and tissue destruction. Chronically elevated BP (hypertension) also causes damage because important vessels (especially those of the heart, brain, kidneys, and retina) can be damaged.

AT. Blood pressure and age D. Blood pressure and blood flow

The average KD value (the average value of measurements taken at certain time intervals) is a decisive factor for peripheral perfusion.

Although the mean BP falls slightly as blood flows from the aorta to the arteries, at the most large arteries(e.g., in the femoral artery) Ps is usually higher than in the aorta (A1 vs. A2) because the elasticity of these large vessels lower than aortic elasticity (see pulse rate).

Direct invasive measurements KD show that the arterial pressure curve in the arteries distal to the heart is not synchronous with the aortic curve due to a delay in the time required for the pulse to pass (3-10 m/s); its shape is also different (A1, 2).

BP is usually measured (at the level of the heart) using the Riva-Rocci method using a sphygmomanometer (B). An inflatable cuff is comfortably wrapped around the arm near the crook of the elbow, and a stethoscope is placed over the brachial artery. The cuff is pressurized to an air pressure higher than the expected Ps (radial pulse disappears) and the manometer readings are observed while slowly releasing (2-4 mmHg/s) air from the cuff. The first sounds synchronous with the pulse (Korotkoff sounds) mean that the pressure in the cuff has dropped below Ps. This value is read from the manometer. These tones first become progressively louder, then quieter and more muffled, and finally disappear when the cuff pressure drops below Pd (second reading).

Causes of incorrect determination of blood pressure. When remeasuring blood pressure after 1-2 minutes, the air in the cuff should be completely deflated. Otherwise, venous deposition may mimic an increase in Pd. The sphygmomanometer cuff should be 20% wider than the diameter of the patient's forearm. High values Pd can be erroneously obtained if the cuff is too loose or too small compared to the circumference of the arm (i.e. in obese or muscular patients), or if the measurement is taken on the thigh.

blood pressure in pulmonary artery much lower than aortic pressure. Pulmonary vessels have thin walls, and their surroundings (filled with air) lung tissue) is very flexible. Therefore, with an increase in the minute cardiac output from the right ventricle there is an expansion of the pulmonary vessels and thereby a decrease in their resistance (G). It does not allow very strong pressure in the pulmonary artery during physical tension when cardiac output increases. The pulmonary vessels also compensate for short-term fluctuations in blood volume.

Mean BP can be determined by monitoring blood pressure using an arterial catheter, etc. (A). If the signal is deliberately weakened, then only the average pressure P can be measured. P - 1/3 (2Pf + Ps).

Why does a person standing on skis not fall into loose snow? Why does a car with wide tires have more flotation than a car with regular tires? Why does a tractor need caterpillars? We will find out the answer to these questions by getting acquainted with the physical quantity called pressure.

Solid body pressure

When a force is applied not to one point of the body, but to many points, then it acts on the surface of the body. In this case, one speaks of the pressure that this force creates on the surface of a solid body.

In physics, pressure is a physical quantity that is numerically equal to the ratio of the force acting on a surface perpendicular to it to the area of ​​this surface.

p = F/S ,

where R - pressure; F - force acting on the surface; S - surface area.

So, pressure occurs when a force acts on a surface perpendicular to it. The magnitude of the pressure depends on the magnitude of this force, and is directly proportional to it. The greater the force, the greater the pressure it creates per unit area. The elephant is heavier than the tiger, so it exerts more pressure on the surface. The car pushes against the road with more force than the pedestrian.

The pressure of a solid body is inversely proportional to the surface area on which the force acts.

Everyone knows that walking in deep snow is difficult due to the fact that the legs constantly fall through. But skiing is pretty easy. The thing is that in both cases a person acts on the snow with the same force - the force of gravity. But this force is distributed over surfaces with different area. Since the surface area of ​​the skis is greater than the area of ​​the soles of the boots, the weight of a person in this case distributed over a larger area. And the force acting per unit area is several times smaller. Therefore, a person standing on skis puts less pressure on the snow and does not fall into it.

By changing the surface area, you can increase or decrease the amount of pressure.

When going on a hike, we choose a backpack with wide straps to reduce pressure on the shoulder.

To reduce the pressure of the building on the ground, increase the area of ​​\u200b\u200bthe foundation.

Truck tires are made wider than tires cars so that they exert less pressure on the ground. For the same reason, a tractor or tank is made on tracks, and not on wheels.

Knives, blades, scissors, needles are sharpened sharply so that they have the smallest possible area of ​​\u200b\u200bthe cutting or piercing part. And then even with the help of a small applied force, a lot of pressure is created.

For the same reason, nature has provided animals sharp teeth, fangs, claws.

Pressure is a scalar quantity. AT solids it is transmitted in the direction of the force.

The unit of force is newton. The area unit is m 2 . Therefore, the unit of pressure is N/m 2 . This value in the international system of units SI is called pascal (Pa or Ra). It got its name in honor of the French physicist Blaise Pascal. A pressure of 1 pascal causes a force of 1 newton acting on a surface of 1 m 2 .

1 Pa = 1N/m2 .

Other systems use units such as bar, atmosphere, mmHg. Art. (millimeters of mercury), etc.

Pressure in liquids

If in solid body pressure is transmitted in the direction of the force, then in liquids and gases, according to Pascal's law, " any pressure exerted on a liquid or gas is transmitted in all directions without change ».

Let's fill a ball with tiny holes connected to a narrow tube in the form of a cylinder with liquid. Let's fill the ball with liquid, insert the piston into the tube and start moving it. The piston presses on the surface of the liquid. This pressure is transmitted to every point of the fluid. Liquid begins to pour out of the holes in the ball.

Filling the balloon with smoke, we will see the same result. This means that in gases pressure is also transmitted in all directions.

The force of gravity acts on the liquid, as on any body on the surface of the Earth. Each layer of liquid in the container creates pressure with its own weight.

This is confirmed by the following experiment.

If water is poured into a glass vessel, instead of the bottom of which has a rubber film, then the film will sag under the weight of water. And the more water there is, the more the film will bend. If we gradually immerse this vessel with water into another container, also filled with water, then as it sinks, the film will straighten. And when the water levels in the vessel and container are equal, the film will straighten completely.

At the same level, the pressure in the liquid is the same. But with increasing depth, it increases, since the molecules upper layers put pressure on the molecules of the lower layers. And those, in turn, put pressure on the molecules of the layers located even lower. Therefore, at the lowest point of the tank, the pressure will be the highest.

The pressure at depth is determined by the formula:

p = ρ g h ,

where p - pressure (Pa);

ρ - liquid density (kg / m 3);

g - free fall acceleration (9.81 m/s);

h - height of the liquid column (m).

It can be seen from the formula that the pressure increases with depth. The lower the submersible descends in the ocean, the more pressure it will experience.

Atmosphere pressure

Evangelista Torricelli

Who knows, if in 1638 the Duke of Tuscany had not decided to decorate the gardens of Florence with beautiful fountains, Atmosphere pressure would have been discovered not in the 17th century, but much later. We can say that this discovery was made by accident.

In those days, it was believed that the water would rise behind the piston of the pump, because, as Aristotle said, "nature does not tolerate emptiness." However, the event was not successful. The water in the fountains really rose, filling the resulting "void", but at a height of 10.3 m it stopped.

They turned to Galileo Galilei for help. Since he could not find a logical explanation, he instructed his students - Evangelista Torricelli and Vincenzo Viviani conduct experiments.

Trying to find the cause of the failure, Galileo's students found out that different liquids rise behind the pump to different heights. The denser the liquid, the lower the height it can rise. Since the density of mercury is 13 times that of water, it can rise to a height 13 times less. Therefore, they used mercury in their experiment.

In 1644 the experiment was carried out. The glass tube was filled with mercury. Then it was thrown into a container, also filled with mercury. After some time, the column of mercury in the tube rose. But he did not fill the entire tube. There was an empty space above the mercury column. It was later called the "Torricellian void". But mercury did not pour out of the tube into the container either. Torricelli explained this by the fact that atmospheric air presses on mercury and keeps it in the tube. And the height of the mercury column in the tube shows the magnitude of this pressure. This was the first time atmospheric pressure was measured.

The atmosphere of the Earth is its air shell, held near it by gravitational attraction. The gas molecules that make up this shell are constantly and randomly moving. Under the influence of gravity, the upper layers of the atmosphere press on the lower layers, compressing them. The lowest layer near the Earth's surface is compressed the most. Therefore, the pressure in it is the greatest. According to Pascal's law, it transmits this pressure in all directions. It is experienced by everything that is on the surface of the Earth. This pressure is called atmospheric pressure .

Since atmospheric pressure is created by the overlying layers of air, it decreases with increasing altitude. It is known that high in the mountains it is less than at the foot of the mountains. And deep underground it is much higher than on the surface.

Normal atmospheric pressure is the pressure equal to pressure a column of mercury 760 mm high at a temperature of 0 o C.

Atmospheric pressure measurement

Because atmospheric air has different densities different height, then the value of atmospheric pressure cannot be determined by the formulap = ρ · g · h . Therefore, it is defined using special devices called barometers .

Distinguish between liquid barometers and aneroids (non-liquid). The operation of liquid barometers is based on the change in the column of liquid level under the pressure of the atmosphere.

The aneroid is a sealed container made of corrugated metal, inside which a vacuum is created. The container contracts when the atmospheric pressure rises and straightens when it is lowered. All these changes are transmitted to the arrow by means of a springy metal plate. The end of the arrow moves along the scale.

By changing the readings of the barometer, one can assume how the weather will change in the coming days. If the atmospheric pressure rises, then clear weather can be expected. And if it goes down, it will be cloudy.

If you feel headache, as if something is squeezing your head, or, conversely, tearing it from the inside, then most likely you have problems with blood pressure. What is pressure? What does it happen? Now let's consider this question.

Pressure is a physical quantity that characterizes the force of impact on an object. The pressure value depends on the action force (F) and the interaction area (S).

The pressure of the outside world

Maybe you have not thought about it, but a huge layer of air presses on us all the time. This is atmospheric pressure. It affects all bodies on Earth. There are no exceptions.

The higher you climb the mountain, the lower will be the value of atmospheric pressure, measured in pascals or millimeters of mercury.

It is difficult to imagine the force with which the air presses on us. This is very great strength. So why do we feel absolutely normal in such conditions? And this happens for two reasons: firstly, the pressure of the air column acts on us evenly from all sides, and secondly, there is also pressure inside us that is opposite to the atmospheric pressure vector.

The pressure inside of us

Blood flows through our veins, which is set in motion by the contractions of the heart. The pressure that the blood exerts at the time of contraction is called arterial. It is also measured in millimeters of mercury.

Blood pressure has two indicators: systolic pressure (upper, first number) and diastolic (lower, second number). To calculate systolic pressure, use the formula: 109 + (0.5 × age) + (0.1 × weight). To determine the diastolic pressure, there is another formula: 63 + (0.1 × age) + (0.15 × weight). The two numbers you get are your normal blood pressure.

About how to measure arterial pressure in the human body in this moment, read in

With the course of evolution, living beings with blood have adapted to the pressure of air masses. Thus, arterial pressure (BP) is, in theory, equal to atmospheric pressure - 1 kgf / cm2. However, there are times when the heart works in extreme mode, which leads to pressure surges.

At any given time, you are under pressure from within and without. Atmospheric pressure (external pressure) is the force with which air masses act on the area of ​​your body. The higher you are above sea level, the lower the atmospheric pressure. Normal- 760 millimeters of mercury.

Simultaneously with external pressure, you also experience internal pressure. If a we are talking about the pressure of blood on the walls of blood vessels, then this is blood pressure. It is also measured in millimeters of mercury, but consists of two parameters: the upper pressure (inside the arteries) and the lower pressure (inside the veins). It is very important to monitor this indicator between the ages of 12 and 19 ( active growth) and from 45 years (aging).

If you experience frequent headaches, then contact your local clinic. Perhaps something is wrong with your heart.