Definition
Pressure (P) is defined as a mean distribution of a force applied over a given area.
It can be measured in a unit of force (F) divided by unit of area (A).
Atmospheric pressure
Atmospheric pressure is the pressure exerted by the force that the layer of gases that form the terrestrial atmosphere exerts on the area of its surface.
It is a known fact that the earth is surrounded by a gaseous layer called the atmosphere.
The first to measure it was the Italian physicist Evangelista Torricelli, based on an experience at sea level. Torricelli used a glass tube, about a meter long, closed at one end. He filled the tube with mercury and capped the open end with his finger.
He then inverted the tube and plunged it into a container containing mercury. Only then did he withdraw his finger from the tube.
Torricelli found that the mercury contained in the tube came down to a stop 76 cm above the level of the mercury contained in the open container.
Why did not all the mercury in the tube go down into the container? Simply because the atmospheric pressure, acting on the free surface of the mercury contained in the container, balanced the pressure exerted by the column of mercury contained in the tube.
Torricelli concluded that the atmospheric pressure equals the pressure exerted by a column of mercury of 76cm of height at the level of the sea for that value gave the name of atmosphere, whose symbol is atm.
Thus, we write:
The device invented by Torricelli was called a barometer.
Subsequently, several experiments were carried out to measure atmospheric pressure at different altitudes and it was concluded that this varies with altitude. In high places, the pressure decreases; In the lower places, increases. In fact, every 100m of variation in altitude, the atmospheric pressure varies 1cm of column of mercury.
The unit mmHg is called Torricelli (Torr), then:
Pascal repeated Torricelli's experiment using water instead of mercury and found that atmospheric pressure balances a water column 10.33m high. Like this:
Where m.c.a means meters of water column.
Relative or Manometric Pressure
The Manometric Pressure is determined by taking zero at local atmospheric pressure.
Absolute Pressure
Absolute Pressure equals the sum of the relative pressure plus the atmospheric pressure.
Negative or Vacuum Pressure
In one system we say that the pressure is negative or that the system is in a vacuum when the value of the relative pressure in that system is less than the atmospheric pressure.
The atmospheric pressure equals 1 atm, 760mmHg, 760 Torr, 10.33mA.c., 14.696 psi, 1.0133 bar, 1.033 Kgf / cm2 or 29.92 inHg. Any value below these is considered a vacuum.
