Formula for air, vapor, liquid or solid pressure. How to find the pressure (formula)?

Pressure is a physical quantity that plays a special role in nature and human life. This invisible phenomenon not only affects the state of the environment, but is also very well felt by all. Let's figure out what it is, what kinds 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 perpendicular force of the pressure applied to the surface area on which it acts. This phenomenon does not depend on the size of the system in which it operates, therefore it refers to intensive quantities.

In a state of equilibrium, according to Pascal's law, the pressure is the same for all points in the system.

In physics and chemistry, this is indicated 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 (balance between pressure inside and outside the cell), the letter "P" is used.

Units of pressure

According to the standards of the International SI system, the physical phenomenon under consideration is measured in pascals (Cyrillic - Pa, Latin - Ra).

Based on the pressure formula, it turns out that one Pa is equal to one H (a newton - a unit of force) divided by one square meter (area unit).

However, in practice it is rather difficult to use pascal, 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 the most famous analogs. Most of them are widely used in the expanses of the former USSR.

• Bars . One bar equals 105 Pa.
• Torr, or millimeters of mercury. Approximately one torr corresponds to 133, 3223684 Pa.
• Millimeters of water column.
• Meters of water column.
• Technical atmosphere.
• Physical atmospheres. One atm is 101 325 Pa and 1.033233 at.
• Kilogram-force per square centimeter. Also, there are ton-force and gram-force. Besides this, there is an analogue of pound-force per square inch.

The general formula of pressure (physics of the 7th grade)

From the definition of a given physical quantity, you can determine the way it is found. It looks like this in the photo below.

In it, F is the force, and S is the area. In other words, the formula for finding the pressure is its force divided by the surface area on which it acts.

It can also be written as P = mg / S or P = pVg / S. Thus, this physical quantity is related to other thermodynamic variables: volume and mass.

The following principle applies to pressure: the smaller the space influenced by the force, the greater the amount of pressing force it takes. If, however, the area increases (for the same strength), the desired value decreases.

The hydrostatic pressure formula

Different aggregate states of substances, provide for the presence of different properties from each other. Proceeding from this, the methods of determining P in them will also be different.

For example, the formula for water pressure (hydrostatic) looks like this: P = pgh. It is also applicable to gases. However, it can not be used to calculate atmospheric pressure, due to the difference in altitude and air densities.

In this formula, p is the density, g is the acceleration due to gravity, and h is the height. Proceeding from this, the deeper the object or object is submerged, the higher the pressure exerted on it inside the liquid (gas).

The considered variant 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 velocity of free fall (F = mg), and the mass of the liquid is the derivative of the volume by the density (m = pV), then the formula can be written as P = pVg / S. Area multiplied by height (V = Sh).

If you insert these data, it turns out that the area in the numerator and denominator can be shortened and the above formula is output at the output: P = pgh.

Considering the pressure in liquids, it is worth remembering that, in contrast to solids, it is often possible for them to curl the surface layer. And this, in turn, contributes to the formation of additional pressure.

For such situations, a slightly different pressure formula is used: P = P ₀ + 2QH. In this case, P ₀ is the pressure of the non-curved layer, and Q is the surface of the fluid tension. H is the average curvature of the surface, which is determined by the Laplace law: H = 1 (1 / R ₁ + 1 / R ₂ ). The components of R ₁ and R ₂ are the radii of the main curvature.

The partial pressure and its formula

Although the method P = pgh 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, not absolutely often there are absolutely pure substances, after all mixtures predominate in it. And this applies not only to liquids, but also to gases. As you know, each of these components carries out a different pressure, called partial.

To define it is quite simple. It is equal to the sum of the pressure of each component of the mixture under consideration (ideal gas).

It follows that the partial pressure formula looks like this: P = 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 above P = P ₁ + P ₂ + P ₃ ...

The most common devices for measuring pressure

Despite the fact that it is not difficult to calculate the thermodynamic quantity under consideration by the above formulas, sometimes there is simply no time to calculate. After all, you must always consider the numerous nuances. Therefore, for convenience for several centuries, a number of instruments have been developed that make it in place of people.

In fact, almost all such devices are types of a manometer (it helps to determine the pressure in gases and liquids). At the same time, 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 discharge (i.e., the pressure below atmospheric pressure), another kind of vacuum gauge is used.
• In order to know the blood pressure in a person, a sphygmomanometer is used. Most he is better known as a non-invasive tonometer. Such devices are many varieties: from mercury mechanical to fully automatic digital. Their accuracy depends on the materials from which they are made and the place of measurement.
• Differential pressure in the environment (in English - pressure drop) are determined using differential pressure gauges or differential meters (not to be confused with dynamometers).

Types of pressure

Considering the pressure, the formula for its location and its variations for different substances, it is worthwhile to learn about the varieties of this magnitude. There are five.

• Absolute.
• Barometric
• Excessive.
• Vacuum measurement.
• Differential.

Absolute

This is the total pressure, under which the substance or object is located, without taking into account the influence of other gaseous constituents of the atmosphere.

It is measured in pascals and is the sum of excess and atmospheric pressures. It is also a difference in the barometric and vacuum metric types.

It is calculated according to the formula P = P ₂ + P ₃ or P = P ₂ - P ₄ .

For the reference point for absolute pressure in the conditions of the planet Earth, pressure is taken inside the tank, from which air is removed (that is, a classical vacuum).

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 directly the surface of the Earth. Most of it is also known as atmospheric.

It is regarded as a thermodynamic parameter, and its magnitude varies with respect to the place and time of measurement, as well as weather conditions and the location above / below sea level.

The magnitude of the barometric pressure is equal to the modulus of the force of the atmosphere at a unit area along the normal to it.

In a stable atmosphere, the magnitude of this physical phenomenon is equal to the weight of the air column on the base with an area equal to one.

The barometric pressure is 101 325 Pa (760 mm Hg at 0 degrees Celsius). In this case, the higher the object is from the surface of the Earth, the lower is the pressure on it of air. After every 8 km it decreases by 100 Pa.

Due to this property in the mountains the water in the kettles nods much faster than the houses on the stove. The fact is that the pressure affects the boiling point: with its decrease, the latter decreases. And vice versa. On this property, the work of such kitchen appliances as a pressure cooker and autoclave is constructed. Increasing the pressure inside them contributes to the formation of vessels in the vessels of higher temperatures than in ordinary pots on the stove.

The barometric altitude formula is used to calculate atmospheric pressure. It looks like this in the photo below.

P is the required value at altitude, P ₀ is the air density near the surface, g is the free fall acceleration, h is the height above the Earth, m is the molar mass of the gas, m is the system temperature, r is the universal gas constant 8,3144598 J / ( Mole x K), and e is Eicler's number equal to 2.71828.

Often in the above atmospheric pressure formula, instead of R, K is the Boltzmann constant. Through its product on the Avogadro number, a universal gas constant is often expressed. It is more convenient for calculations when the number of particles is given in moles.

When performing calculations, it is always worth taking into account the possibility of changing the air temperature due to a change in the meteorological situation or when climbing over the sea level, as well as geographical latitude.

Excessive and vacuum

The difference between atmospheric and measured ambient pressure is called overpressure. Depending on the result, the name of the variable changes.

If it is positive, it is called gauge pressure.

If the result obtained with the minus sign is called vacuum gauge. It is worth remembering that it can not be more barometric.

Differential

This value is the difference in pressure at various points in the measurement. Typically, it is used to determine the pressure drop on any equipment. This is especially true in the oil industry.

Having dealt with the fact that the thermodynamic quantity is called pressure and by what formulas it is found, we can conclude that this phenomenon is very important, and therefore knowledge of it will never be superfluous.