As already mentioned, the pressure literally surrounds us. It exists in any subject or organism. It is enough to say that a pressure of 100 kN / m2 presses on any object or living creature located on the surface of the Earth. This is the air column pressure. People somehow more perceive the visual picture of pressure as directed forces in a closed volume. This is true for isolated vessels. In open containers, this picture is a consequence of column pressure. Any pressure in the liquid and gas is the result of the "behavior" of the molecules of the material itself. The statement that pressure as a physical quantity is the total force of impacts of all molecules on the surface is absolutely true for a liquid, gas, and solid.
From the course of elementary physics it is known that the formula for the pressure of liquid and gas has the form:
P = Ο Γ g Γ h, [Pa]
Consider where such components come from. If you take any volume of liquid / gas, then you can determine its pressure on the area as follows. Pressure is the force applied to the area - F / S. This is clear to any student. Now we will write down what force is from Newtonβs second law - the mass of the body multiplied by the acceleration (in this case, when the working fluid is in a state of calm - the acceleration of gravity). In turn, mass is the product of density and the volume occupied by the body. And the volume can be expanded as the product of the area by height. In the formula form we get:
P = F / S = (m Γ g) / S = (V Γ Ο Γ g) / S = (S Γ h Γ Ο Γ g) / S = Ο Γ g Γ h
where P is the pressure, F is the force, S is the area, m is the mass, g is the acceleration of gravity, V is the volume, Ο is the density, h is the height
Pa = N / m2 = (kg Γ m / s2) / m2 = (m3 Γ kg / m3 Γ m / s2) / m2 = (m2 Γ m Γ kg / m3 Γ m / s2) / m2 = m Γ kg / m3 Γ m / s2 = kg / (m Γ s2)
The units shown show how Pascal is formed. Blaise Pascal was a famous scientist, including in the field of physics. He is the author of the basic law, which answers the question: "How to find the pressure of a gas or liquid, if it is at rest?" Based on his experience, one of the basic laws of physics and hydraulics was obtained - the pressure from the body (liquid or gas) at any selected point is the same over the entire surface with which the body interacts.
A more modern science - hydraulics also has a law with its authorship - "The law of constancy of pressure." It states that the pressure of the liquid will be constant at any point in the liquid, provided it is at rest. These foundations laid the foundation for the calculations of today's complex systems and made it possible to create unusual technical solutions. Nowadays, any system or mechanism that works with a liquid or gas is compulsorily calculated to not exceed the maximum permissible values ββ(which are known from experiments for various materials) that form pressure in a liquid and gas.
If we consider the pressure when the fluid is at rest, then we should talk about hydrostatic pressure. If we are talking about dynamics, then it is more relevant to talk about water hammer. Water hammer is a phenomenon that occurs in tanks and pipes, due to a sharp increase in pressure anywhere. All hydromechanisms and their elements are calculated for extreme static pressures and short-term casts - water hammer. For example, a common cause of failure of water taps is precisely a water hammer. A big jump on the line of the highway can reach the crane, and it will fail (fully or partially).
It is curious that even though the liquid is incompressible (practically), on the one hand it improves the transmission of momentum. On the other hand, if the path is long and there will be interference on it (for example, a household filter), then the water hammer can completely die out. Other factors can change the pressure in the liquid and gas. The first thing that comes from the formula is a change in the height of the container. This principle was actively used in ancient times by various civilizations, and can also be clearly seen in the villages. Water towers still remaining from the time of the USSR work on this principle. Water is pumped to a certain level in a large tank under the pressure of the discharge pump. This is regulated automatically - by a float, as in the drain tank of your toilet bowl (when the level is reached, the float floats and the lever blocks the water supply). Further, the liquid flows by self-propelled, because its level above the ground is large, and the hole is small.
In addition, you can change the pressure by changing the temperature. The speed of the movement of molecules depends on it, and hence the number of strokes. It would seem that the larger it is, the higher the pressure in the liquid and gas will be. But is it? Not really. It's about the properties. Water, for example, hardens and increases in volume, and air decreases. A simple example - those who have summer cottages know that for the winter they need to drain water from the trunk system so that pipes and taps do not burst. Another example about air is to inflate a balloon and place it in the refrigerator for 10 ... 15 minutes. When you pull it, it will decrease in volume. If you pour it with hot water from the kettle, it will quickly swell.
Pressure is a phenomenon with which we have not only to live, but also to reckon with. Remember diving to a depth or a sound wave with a strong thunder. Always be careful and remember that pressure is the movement of molecules, and molecules are small particles of life!