Pressure is a physical quantity that plays a special role in human nature and life. This phenomenon, invisible to the eye, not only affects the state of the environment, but is also very well felt by everyone. Let's see 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 perpendicular pressure exerted on 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 equilibrium, according to Pascal's law, the pressure is the same for all points of the system.
In physics and chemistry, this is indicated by the letter "P", which is an abbreviation of the Latin name for the term - pressūra.
If we are talking about the osmotic pressure of the fluid (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 in question 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 is a unit of force) divided by one square meter (unit of area).
However, in practice, using Pascals is quite difficult, since this unit is very small. In this regard, in addition to the SI system standards, this value can be measured differently.
Below are its most famous analogues. Most of them are widely used in the open spaces of the former USSR.
- Bars One bar is 105 Pa.
- Torres, or millimeters of mercury. About one torr corresponds to 133, 3223684 Pa.
- Millimeters of water.
- Meters of water.
- Technical atmospheres.
- Physical atmospheres. One atm is 101 325 Pa and 1.033233 at.
- Kilogram-force per square centimeter. Ton-force and gram-force also stand out. In addition, there is an analogue pound-force per square inch.
General pressure formula (7th grade physics)
From the definition of this physical quantity, we can determine the method of finding it. It looks like in the photo below.
In it, F is the force, and S is the area. In other words, the formula for finding pressure is its strength 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.
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 strength) - the desired value decreases.
Hydrostatic pressure formula
Different aggregate states of substances provide for the presence of 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 is also applicable 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 acceleration of gravity, and h is the height. Based on this, the deeper the object or object is immersed, the higher the pressure exerted on it inside the liquid (gas).
The considered option is an adaptation of the classic 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. Moreover, 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 = pgh.
When considering pressure in liquids, it is worth remembering that, unlike solids, curving 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 = P 0 + 2QH. In this case, P 0 is the pressure of the undistorted layer, and Q is the surface of the fluid tension. H is the average curvature of the surface, which is determined by the Laplace's Law: H = ½ (1 / R 1 + 1 / R 2 ). 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 for both liquids and gases, the pressure in the latter is better calculated in a slightly different way.
The fact is that in nature, as a rule, absolutely pure substances are not very often found, because mixtures prevail 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.
Defining it is quite simple. It is equal to the sum of the pressure of each component of the mixture in question (ideal gas).
From this 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 frequent 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 into account the above P = P 1 + P 2 + P 3 ...
The most common pressure measuring instruments
Despite the fact that it is not difficult to calculate the considered thermodynamic quantity using the above formulas, sometimes there is simply no time to carry out the calculation. After all, you must always take into account the numerous nuances. Therefore, for convenience, a number of devices have been developed over several centuries that do this instead of people.
In fact, almost all devices of this kind are types of pressure gauge (helps to determine the pressure in gases and liquids). Moreover, they differ in design, accuracy and scope.
- Atmospheric pressure is measured using a manometer called a barometer. If it is necessary to determine the discharge (that is, the pressure is below atmospheric), another type of it, a vacuum gauge, is used.
- In order to know the blood pressure in a person, a sphygmomanometer is used. For most, it is better known under the name of non-invasive tonometer. There are many types of such devices: from mechanical mercury to fully automatic digital. Their accuracy depends on the materials from which they are made and the place of measurement.
- Differential pressure drops in the environment (in English - pressure drop) are determined using differential pressure gauges or differential sensors (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 worth learning about the varieties of this quantity. There are five of them.
- The absolute.
- Barometric
- Excessive.
- Vacuum gauge.
- Differential
The 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 the excess and atmospheric pressures. It is also the difference between the barometric and vacuum types.
It is calculated by the formula P = P 2 + P 3 or P = P 2 - P 4 .
As the reference point for absolute pressure in the conditions of planet Earth, the pressure inside the container is taken from which air is removed (that is, a classic 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 that are in it, including the Earth’s surface itself. Most of it is also known as atmospheric.
It is considered to be a thermodynamic parameter, and its value varies relative to the place and time of measurement, as well as weather conditions and being above / below sea level.
The magnitude of the barometric pressure is equal to the absolute value of the atmospheric force per unit area 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 unity.
The norm of barometric pressure is 101 325 Pa (760 mmHg at 0 degrees Celsius). Moreover, the higher the object is from the surface of the Earth, the lower the air pressure on it becomes. After every 8 km it decreases by 100 Pa.
Due to this property in the mountains, water in teapots wakes much faster than houses 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 in the vessels of higher temperatures than in conventional pots on the stove.
The formula for barometric altitude is used to calculate atmospheric pressure. It looks like in 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 of 8.3144598 J⁄ ( mol x K), and e is the Euler number equal to 2.71828.
Often in the above atmospheric pressure formula, instead of R, K is used - the Boltzmann constant. Through her product on the Avogadro number, the universal gas constant is often expressed. It is more convenient for calculations when the number of particles is given in moles.
When carrying out calculations, it is always worth taking into account the possibility of changing air temperature due to a change in the meteorological situation or when climbing above sea level, as well as geographical latitude.
Excessive and vacuum gauge
The difference between atmospheric and measured ambient pressures is called overpressure. Depending on the result, the name of the quantity changes.
If it is positive, it is called gauge pressure.
If the result obtained with a minus sign is called a vacuum gauge. It is worth remembering that it can not be more 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 the 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.