Pressure in physics. Definition and calculation formula

Pressure is a quantity that determines how a given force is concentrated on the surface. Pressure is a scalar quantity. It is calculated in continuous media such as liquids, gases and solids.

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1. Definition of pressure

Pressure is a scalar quantity defined as a given force concentrated on a surface. The definition is: pressure is the ratio of the force acting on a given surface to the value of that surface.

The basic unit of pressure is Pascal [Pa]. They can be entered in millimeters of water, mercury, bars or British units (pounds per inch).

Stress is a generalization of the concept of pressure. For pressure measurements, among others, a barometer and pressure gauge are used, but it can also be measured with other instruments. For example, when measuring the atmospheric pressure, we give it in hPa [Pascal ha], such a unit we usually hear when looking at the weather forecast.

1 hPa is 100 Pa. The physical reference is normal pressure of approximately 1013.25 hPa. We use a barometer or a barograph to measure the atmospheric pressure.

If the gas pressure is much lower than atmospheric pressure, it is a vacuum. We can distinguish a low, medium (from 10-2 Pa to 10-5 Pa) and high vacuum with a pressure above 10-5 Pa.

2. Interpretation of pressure

Pressure as a scalar quantity tells us how much the acting force is concentrated on the surface. An example would be a needle which, acting with its sharp point on a small area, generates a lot of pressure, thanks to which it easily punctures the surface.

Another example will be skis which, by distributing the weight of the human body over a large surface, prevent it from sinking into the snow. In this case, the pressure is specially reduced.

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3. Formula for calculating pressure

The pressure is given by the formula:

p = FS


p - pressure [Pa]; F - force acting perpendicularly on the surface S [N]; S - surface area affected by the force F [m²].

For steady-state gases at rest, the pressure that the gas exerts on the walls of the vessel is a function of the volume, temperature and mass. Hence, in thermodynamics it is treated as a state parameter.

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4. Relative and absolute pressure

If the pressure is relative to a vacuum, we call it absolute or absolute. If, however, it is defined in relation to the environment - hypertension (or otherwise relative, however, it is an ambiguous term).

Fluid pressure relative to atmospheric pressure is commonly measured and reported in the art. In this sense, overpressure is referred to as gauge pressure.

This can be understood from the example below: if the pressure in the container is 0.3 MPa (gauge), the absolute pressure is 0.3 MPa + 0.1 MPa (0.1 MPa is atmospheric pressure).

In the times before the dissemination of the SI system, gauge pressure was marked by adding the letter n after the symbol of the pressure dimension. The absolute pressure was marked by adding the letter a - in the previous example, the pressure would be given as 3 atm or 4 atm (approximately 0.1 MPa = 1 atm).

The unit found in the art is barg. It means the relative pressure bar.

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