Pressure for dummies: definition, explanation in simple words

Pressure is a very important physical quantity that plays a huge role both in the surrounding nature and in human life. Outwardly invisible to the human eye, pressure can be very clearly felt by each of us. This has been learned especially well by older people who often suffer from high blood pressure (or vice versa from low blood pressure). But in our article we will talk more about pressure in physics, how it is measured and calculated, what formulas there are for calculating the pressure of different substances: air, liquid or solid.

Definition

In physics, pressure is understood as a thermodynamic quantity expressed as the ratio of the perpendicular pressure force to the surface area on which it acts. Moreover, according to Pascal's law, if the system is in a state of equilibrium, then the pressure on it will be the same for all points of the system.

In physics, as well as in chemistry, pressure is denoted by the capital letter P, coming from the Latin word “pressura” - pressure. (In English, pressure has remained almost unchanged - pressure).

Mechanical lever, moment of force

Archimedes spoke about a mechanical lever when he promised to turn the Earth upside down if only a suitable fulcrum could be found. It is a simple mechanism that helps lift weights attached to one end by applying force to the other end. In this case, the weight of the load greatly exceeds the applied force. In grade 7, physical formulas describing this process are studied in the same section of dynamics.

A lever is in equilibrium if the product of the force on the arm on one side is equal to the product of the force on the arm on the other side.

General formula

From the classical definition of what pressure is, a general formula for calculating it can be derived. It will look like this:

P = F/S

Where F is the pressure force, and S is the surface area on which it acts. That is, in other words, the formula for finding pressure is the force acting on a certain surface, divided by the area of ​​this very surface.

As can be seen from the formula, when calculating pressure the following principle always applies: the smaller the space affected by the force, the greater the amount of pressing force on it and vice versa.

This can be illustrated with a simple real-life example: it is easiest to cut bread with a sharp knife, because a sharp knife has a sharpened blade, that is, the surface area S from the formula is minimal, which means that the pressure of the knife on the bread will be maximally equal to the applied force F of the person holding the knife . But cutting bread with a dull knife is more difficult, since its blade has a large surface area S, and the pressure of the knife on the bread will be less, which means that in order to cut yourself a piece of bread you need to apply a greater amount of force F.

The general formula for pressure, in fact, perfectly describes the formula for the pressure of a solid body.

Mechanical movement: formulas for grade 7

In order to describe motion, a reference body, a coordinate system, and an instrument for measuring time are required. These are the components of the reference system.

The study of mechanical motion in the 7th grade physics course includes the following terms:

• The displacement of a body is a vector drawn from the starting point to the ending point.
• The trajectory of movement is a mental line along which the body moves.
• Path is the length of the body’s trajectory from the start to the end point.
• Speed ​​is the speed at which a body moves or the ratio of the distance it travels to the time it travels.
• Acceleration is the rate of change in speed at which a body moves.

Formula for the speed of uniform rectilinear motion:

V = S / t, where S is the path of the body, t is the time during which this path is covered.

Formula for the speed of uniform curvilinear motion:

where S1 and S2 are sections of the path, and t1 and t2 are the time during which each of them was covered.

The SI unit of speed is meters per second (m/s).

Formula for speed of uniformly accelerated motion:

V = V0 + at, where V0 is the initial velocity and is the acceleration.

The SI unit of acceleration is m/s2.

Units

According to the standards of the International Metric System, pressure is measured in pascals. One pascal from the classical formula is equal to one Newton (As we know, Newton is our unit of measurement of force) divided by one square meter.

But alas, in practice the pascal turns out to be a very small unit and it is not always convenient to use it to measure pressure, so other units are often used to measure pressure:

Types of quantities

The pressure can be different (excessive, barometric). The absolute concept is characteristic of a substance or object that is not affected by other gases. The indicator is measured in Pascals. It is calculated using the following calculator: normal P = P2 + P3 or P = P2 - P4.

The starting point comes from the planet Earth, the force inside the vessel from which the air has been removed. The quantity is used in many thermodynamic formulas. To define gravity, the concept of barometric or atmospheric pressure is used. It changes taking into account the temperature of the atmosphere, time, altitude.

Normally, the indicator is 760 mmHg. Art., and the temperature should correspond to zero Celsius. The higher an object is from the Earth, the lower the pressure on it. The value decreases by 100 Pa every eighth kilometer.

In the mountains, water boils faster than at home: pressure affects the boiling point. If it decreases, t decreases. The dependence also remains in reverse order. Some kitchen appliances operate on a similar property: pressure cooker, autoclave. As the nominal P rises, the temperature inside rises.

For calculations, a standard formula is used where variables are used:

• density of air masses near Earth level;
• height;
• acceleration;
• temperature;
• molar mass.

If the number of particles is specified in moles, a formula with a constant value of K is used. When carrying out calculations, the probability of temperature changes is taken into account, which is associated with changes in weather, elevation gain, and geographic latitude. If the measured value is subtracted from the atmospheric P, the excess force is obtained. Taking into account the result, the name of the indicator changes:

• positive - manometric;
• negative - vacuum.

The last value cannot exceed the barometric level. The difference in pressure at different points is called a differential phenomenon. It is used to determine P on certain equipment. This concept is used in the oil industry.

Hydrostatic Pressure Formula

As we know, different states of matter have different physical properties. Liquids differ in their properties from solids, and gases, in turn, differ from all of them. Therefore, it is logical that the methods for determining pressure for liquids, solids and gases will also be different. So, for example, the formula for water pressure (or hydrostatic pressure) will look like this:

P = p*g*h

Where small p is the density of the substance, g is the acceleration of gravity, h is the height.

In particular, this formula explains why when divers (or a bathyscaphe or a submarine) dive to a depth, the pressure of the surrounding water increases more and more. It is also clear from this formula why an object immersed in some kind of jelly will be subject to greater pressure than an object immersed simply in water, since the density of the jelly (p) is higher than that of water, and the higher the density of the liquid, the higher its hydrostatic pressure.

The formula we have given for hydrostatic pressure is valid not only for liquids, but also for gases. Therefore, when climbing high into the mountains (where the air is thinner, which means less pressure), as well as when descending into the underwater depths, a person, diver or climber must undergo special adaptation, get used to the fact that a different pressure will affect him.

A sudden change in pressure can lead to decompression sickness (in the case of divers) or “mountain sickness” (in the case of climbers). Both “queson” and “gornyashka”, as they are slangly called by divers and climbers, are caused by a sharp change in environmental pressure. That is, if an unprepared person suddenly begins to climb Everest, he will quickly catch a “miner,” and if the same person begins to descend to the bottom of the Mariinskaya Trench, he is guaranteed to receive a “queson.” In the first case, the reason will not be the body’s adaptation to low blood pressure, but in the second – to high blood pressure.

American divers in a decompression chamber designed to prepare them for deep-sea diving and adapt their bodies to the high pressure of the ocean depths.

Gravity, weight, mass, density

Formulas, concepts and definitions that describe these physical characteristics are studied in 7th grade as part of a section of physics called dynamics.

Body weight is measured in newtons, body mass in grams and kilograms.

Weight formula:

P = mg, where m is the mass of the body, g is the acceleration of gravity.

The acceleration of free fall occurs under the influence of gravity, to which all bodies on our planet are subject.

g = 9.806 65 m/s2 or 9.8 N/kg

If a body is at rest or in linear uniform motion, its weight is equal to the force of gravity.

Fstrand = mg

But these concepts cannot be identified: the force of gravity acts on a body due to the presence of gravity, while weight is the force with which the body itself acts on the surface.

Density Formula:

ρ = m / V, where m is the mass of a body or substance, V is the occupied volume.

SI unit of density: kg/m3.

Partial pressure and its formula

Although the hydrostatic pressure formula is applicable for gases, it is more convenient to calculate the pressure for them using another formula, the partial pressure formula.

The fact is that absolutely pure substances are rarely found in nature, and this applies to both liquids and gases. Usually, in practice, various mixtures predominate in the surrounding world, and it is logical that each of the components of such a mixture can exert different pressures; such different pressures are called partial. Determining the partial pressure is simple - it is equal to the sum of the pressures of each component of the mixture in question. Hence the partial pressure formula will have the following form:

P = P1+P2+P3

Where P1, P2 and P3 are the pressures of each of the components of the gas mixture, the so-called “ideal gas”.

For example, to determine air pressure, the usual formula for hydrostatic pressure is done only with

oxygen is not enough, since air in reality is a mixture of different gases, where in addition to the main component oxygen, which we all breathe, there are others: nitrogen, argon, etc.

Such calculations must be done using the partial pressure formula.

Physics cheat sheets for 7th grade

It is difficult to cover the entire physics course in one article, but we covered the main topics for grade 7 and this is enough to refresh your memory. Download and print both cheat sheets - one of them (detailed) will be useful for thoughtful preparation for the Unified State Exam and Unified State Exam, and the second (short) will be used for solving problems.
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For those who are homeschooled or are forced to study the material on their own due to absences due to illness, we also recommend a textbook on physics by A. V. Peryshkin with formulas for grade 7 and easy, accessible explanations on all topics. It was written several decades ago, but is still very popular and in demand.

Pressure measuring instruments

Of course, humanity has invented many devices that allow you to quickly and conveniently measure pressure levels. To measure ambient pressure, also known as atmospheric pressure, a device such as a pressure gauge or barometer is used.

This is what a classic barometer for measuring atmospheric pressure looks like.

To find out a person’s blood pressure, which is often the cause of ailments, a device known to most people called a non-invasive tonometer is used. There are many varieties of such devices.

In their research, biologists also calculate osmotic pressure - this is the pressure inside and outside the cell. And meteorologists, in particular based on pressure changes in the environment, predict the weather for us.

Archimedes' Law

Archimedean force formula:

Fa = ρ × g × V, where ρ is the density of the liquid, V is the volume of the liquid, g is the acceleration 9.8 m/s2.

Archimedes' law helps to calculate how a body will behave when immersed in media of different densities. The following statements are true:

• if the density of the body is higher than the density of the medium, it will sink to the bottom;
• if the density of the body is lower, it will float to the surface.

In other words, the body will rise to the surface if the Archimedean force is greater than the force of gravity.

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Author: Pavel Chaika, editor-in-chief of Poznavaika magazine

When writing the article, I tried to make it as interesting, useful and high-quality as possible. I would be grateful for any feedback and constructive criticism in the form of comments on the article. You can also write your wish/question/suggestion to my email [email protected] or Facebook, with respect, the author.

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Lever equilibrium equation:

F1 × l1 = F2 × l2

It follows from this that the lever is balanced when the magnitudes of the forces applied to its ends are inversely proportional to the arms of these forces.

Formula for moment of force:

M = F × l, where F is the force modulus, l is the arm length.

The SI unit of torque is newton meter (Nm).

This formula is correct if the force is applied perpendicular to the axis of the lever. If it is applied at an angle, such a case goes beyond the scope of the physics course for the 7th grade and is studied in detail in the 9th grade.

We can say it differently: a lever is in equilibrium if the sum of the moments of all forces applied to it relative to any axis is equal to zero.

M1 + M2 + Mn + … = 0

State of the vascular system with increased blood pressure

Vessels are hollow elastic tubes through which blood is distributed throughout the body. There are several types of vessels:

• arteries;
• veins;
• arterioles;
• venules;
• capillaries;
• arteriolovenular shunts.

In addition to transporting blood, each of the vessels performs certain functions. It is determined by the features of their structure. The walls of arteries and arterioles are mainly represented by muscle and elastic fibers. This provides them with good distensibility, so the arteries conduct well the pulse wave propagated by the myocardium.

Thanks to the muscle layer, the vessel quickly reduces/increases the lumen of the vessel, which affects the current level of blood pressure. Veins and venules are characterized by the presence of special valves. In addition, their wall is somewhat thinner than that of arteries. The valves prevent blood from flowing back. Thinner vessel walls make it easier for blood to move, since the task of “pushing” blood is accomplished by contracting skeletal muscles.

When you inhale, stretching of the lung tissue and the veins located in it occurs. Due to this, the blood “rises” from the lower sections. As a result, blood returns to the myocardium. The purpose of capillaries is to ensure gas exchange. They have very thin walls consisting of only one layer (endothelium). Arteriolo-venular shunts are necessary to transfer blood from arterioles to venules, bypassing capillaries.

As a result of physiological aging and the negative impact of various diseases, irreversible changes occur in the structure of blood vessels. As a result of this, the quality of the functions they perform also changes. Thus, in advanced forms of arterial hypertension, the walls of blood vessels are constantly under significant pressure from the blood flow.

Such prolonged stress causes exhaustion and wear of the elastic and muscle layers. Under the influence of negative changes, the vessels are transformed into thin-walled tubes, unable to give an adequate response to changes in the volume of circulating blood. In the future, this becomes the cause of the development of an aneurysm or stroke.

So, with high blood pressure, are the blood vessels dilated or narrowed? With any fluctuation in blood pressure - an increase or decrease - certain changes occur in the condition of all arteries. In the case of an increase, rapid compression of the muscle layer occurs, as a result of which the clearance is significantly reduced. Blood movement slows down.

If the self-regulation scheme is not disturbed, then the vascular lumen expands independently and the pressure stabilizes. Developed chronic diseases can disrupt the functioning of the regulatory circuit, which leads to the formation of arterial hypertension. Various pathologies, in particular pheochromocytoma, can be potential causes of increased blood pressure.

The tumor makes the regulatory circuit work incorrectly. The formation produces specific hormones that cause the development of tachycardia (increased heart rate). At the same time, a narrowing of the vessels located on the periphery occurs. The result is an increase in blood pressure.

The next dangerous pathology is stenosis (spasm) of the kidney vessels. The condition becomes a consequence of nephroptosis (downward displacement of the kidneys) or atherosclerosis. Against the background of narrowing of the renal arteries, a qualitative – at the reflex level – change in the volume of substances of the renin system occurs:

• an increase in the volume of aldosterone causes an increase in circulating blood;
• antiotensin promotes narrowing of the vascular lumens, increasing the symptoms of ischemia.

As a result, a vicious circle is formed. Components of the RAAS system increase vascular stenosis, which, in turn, only enhances the pathological reaction.

Related factors

Blood pressure indicators are constantly changing and largely depend on the influence of external factors. Increased blood pressure can be caused by:

• drinking alcoholic beverages;
• active and passive smoking;
• being overweight – severe obesity is especially dangerous in this regard;
• general intoxication of the body;
• excessive presence of salt and salty foods in the daily diet;
• maintaining a sedentary lifestyle;
• chronic pathologies;
• regular lack of sleep;
• quick change of position;
• excessive physical activity.

Low blood pressure

Hypotension (hypotension) is a condition when blood pressure drops below normal, it comes in two types: acute and chronic.

In the acute form of hypotension, the supply of oxygen to the brain (hypoxia) sharply decreases and the function of the main human organs deteriorates, which leads to the need for immediate medical attention. The seriousness of the situation in this situation is determined not so much by the level of blood pressure in the vessels, but by the speed and magnitude of its drop.

Arterial hypotension of this form is manifested by a severe lack of blood volume in the vessels. Blood pressure may also drop due to serious intoxication with nitroglycerin, alcohol, narcotic substances, and strong medications such as clonidine, nifedipine. And also with severe infection, sepsis, dehydration and large blood loss.

Therefore, acute hypotension worsens the course of any disease. And the cause of its occurrence should first of all be taken into account in emergency medical care.

People who occasionally experience a slight drop in blood pressure are not at greater risk for cardiovascular complications than people with high blood pressure. But they get little attention in vain. Along with this, in old age, hypotension increases the chances of ischemic stroke. And among young people, their productivity decreases, which also greatly affects their quality of life.

In some people, when changing body position from horizontal to vertical, there is a sharp decrease in blood pressure. Typically it lasts for a few seconds. Such hypotension usually appears in the morning, and can be characterized by a deterioration in blood supply to the brain, as well as tinnitus, dizziness, and darkening of the eyes. Sometimes it leads to loss of consciousness and therefore poses a risk of ischemic stroke, as well as injuries after a fall. Severe illnesses, previous surgeries and certain medications, and the patient's long-term lying state will always contribute to the appearance of orthostatic hypotension.

Chronic arterial hypotension may be accompanied by a nervous state, depression, fatigue, low ability to work, headaches, and a predisposition to loss of consciousness. Sometimes it hurts my heart. They are also characterized by poor tolerance to cold, heat, stuffy rooms and heavy physical exertion.

Why does this happen?

For some people, chronic hypotension is normal. And it occurs due to large sports loads on the body, with constant stay in a tropical climate, in high mountain regions, or beyond the Arctic Circle. In such situations, low blood pressure is not considered a disease and the person practically does not feel it.

Sometimes chronic hypotension becomes a disease or accompanies other diseases, and therefore requires treatment. Its occurrence is caused by poor vascular condition or a reduction in the output of blood volume by the heart.

Survey

Regular blood pressure measurements at different times of the day help identify low blood pressure.

The examination certainly includes a search for the cause that led to a decrease in blood pressure. For this, a medical specialist, in addition to a detailed examination of the patient, may prescribe an electrocardiogram or Doppler echocardiography.

Blood pressure is a serious indicator of human health; it requires periodic monitoring.

Increased blood pressure

An increase in blood pressure occurs when the heart pumps blood into the vessels with greater force or when resistance in the vessels increases. The kidneys play a significant role in normalizing blood pressure.

High blood pressure may be practically unnoticeable and may be asymptomatic. Or, if your blood pressure rises, you may experience the following symptoms:

• headache, possibly in the back of the head;
• pain in temples;
• pounding pulse in the ears;
• darkening of the eyes;
• nausea;
• feeling of lack of air.

There are two main categories of hypertension:

• Arterial hypertension or, as it is also called, hypertension. This is a disease caused by an increase in blood pressure in the arteries. And it is not a consequence of diseases of other human organs, such as the kidneys, endocrine system and heart.
• Secondary hypertension (arterial hypertension), indirect or symptomatic, that is, caused by other diseases. These are changes during which an increase in blood pressure is associated with certain diseases or defects of organs and systems involved in regulating pressure in the arteries.

  In such a situation, hypertension is renal - in case of inflammation of the kidneys, central - in case of brain damage. And also, pulmonogenic, with chronic diseases of the respiratory system and as a result of diseases of the adrenal glands or thyroid gland. Hemodynamic, in case of damage to the aortic valve or in case of disruption of the heart. What matters is that therapy for this type of hypertension is the treatment of the disease that caused it. Arterial hypertension usually ceases to be a concern as a result of the elimination of the underlying disease.

What causes high blood pressure in the arteries?

Repeated increases in pressure inside the arteries from time to time can form due to improper breathing during sleep. An increase in blood pressure also often occurs due to stressful situations or neurotic disorders. In addition, blood pressure can jump significantly, even to critical levels, due to the illiterate use of certain medications, excessive consumption of caffeinated drinks and other stimulants.

Survey

To diagnose the disease, continuous blood pressure measurements are used over several days at different times of the day and night, forming an accurate profile of blood pressure changes. These recordings can be compared with an ECG.

To examine the disease, they can also use a whole range of diagnostic techniques aimed at studying increased blood pressure. The cause of hypertension is diseases associated with the kidneys; for this reason, a study is done based on contrast X-rays of blood vessels and ultrasound of the kidneys. At the very beginning, to identify damaged vessels, they rely on Doppler ultrasound data.

It is mandatory to examine the work of the heart using an electrocardiogram in various conditions, such as Holter monitoring, resting ECG, tests using exercise on a treadmill, and echocardiography. Even in the diagnosis of hypertension, the patient’s fundus plays an important role; it, like a mirror, reflects the condition of all the veins and arteries of not only the eye, but also the brain. Therefore, in addition to consulting a cardiologist, it will be important to visit an ophthalmologist who specializes in these issues.

Does blood pressure depend on a person's weight?

Undoubtedly! Obesity is the main cause of improper blood circulation and pressure on veins and blood vessels. With every extra kilogram of body weight, blood pressure also increases.

It has been clinically proven that losing weight is one of the important methods of combating hypertension.