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 as 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, 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 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 pressureinside and outside the cage), the letter "P" is used.
Pressure units
According to the standards of the International SI system, the considered physical phenomenon is measured in pascals (Cyrillic - Pa, Latin - Ra).
Based on the pressure formula, it turns out that one Pa is equal to one N (newton - a unit of force) divided by one square meter (a unit of area).
However, in practice, it is rather difficult to apply pascals, since this unit is very small. In this regard, in addition to the SI standards, 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.
- Torrs, 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 formula for pressure (7th grade physics)
From the definition of a given physical quantity, you 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 itaffects.
It can also be written like this: 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. However, 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 an 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 remember 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 the area multiplied by the height (V=Sh).
If you insert this data, it turns out that the area in the numerator andthe denominator can be reduced and the output - the above formula: P=pgh.
Considering the pressure in liquids, it is worth remembering that, unlike solids, the surface layer can often be distorted in them. And this, in turn, contributes to the formation of additional pressure.
For such situations, a slightly different pressure formula is used: P=P0 + 2QH. In this case P0 is the pressure of the 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=½ (1/R1+ 1/R2). The components R1 and R2 are the radii of principal curvature.
Partial pressure and its formula
Although the P=pgh method is applicable for 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 spot. 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=P1+ P2+ P 3… and so on, according to the number of components.
There are often times 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=P1+ P2+ P3…
Most common pressure gauges
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 the centuries to do this instead of people.
In fact, almost all devices of this kind are varieties of a manometer (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, the pressure is below atmospheric pressure), another version of it, a vacuum gauge, is used.
- In order to find out the blood pressure of 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 where they are measured.
- Pressure drops in the environment (according toEnglish - pressure drop) are determined using differential pressure gauges 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.
- Vacuometric.
- 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=P2 + P3 or P=P2- R4.
For the reference point for absolute pressure in 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. It is also known to most as atmospheric.
It is classified as a thermodynamic parameter, and its value varies depending on the place and time of measurement, as well as weather conditions and being above / below sea level.
Barometric pressure valueequal to the modulus of the force of the atmosphere on an area of unity 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.
Norm barometric pressure - 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 pots 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, P0 is the air density near the surface, g is free fall acceleration, h is the height above the Earth, m is the molar mass of gas, t is the temperature of the system, r is the universal gas constant 8.3144598 J⁄(mol x K) and e is the Euclair number equal to 2.71828.
Often, in the above atmospheric pressure formula, instead of R, K is usedis the Boltzmann 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, you should always take 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.
Gage and vacuum gauge
The difference between atmospheric pressure 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 vacuum. It is worth remembering that it cannot be more than barometric.
Differential
This value is the pressure difference at different measurement 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 what formulas it is found, we can conclude that this phenomenon is very important, and therefore knowledge about it will never be superfluous.
