Pressure – it’s a very common term in various industries. There are various kinds of pressure we use in our practical life. Atmospheric pressure, absolute pressure, gauge pressure, fluid pressure, units of pressure, etc. will explore here.
Let us try to understand the Pressure. Take a wooden block, a bowling pin, a sharp pin, and a hammer. Now, if you hammer on the bowling pin keeping on the block, do you think it will enter.
No, it will not enter.
But about if you try to do the same thing on the sharp pin.
Don’t you think it will not enter? Yes, it will enter into the wall. Why this different behavior? Because of pressure!
If we analyze, we will see the following two things:
1) Bowling pin- not able to enter into the wood
2) Sharp pin – easily able to enter the wood.
In both cases, we apply force on the head of the pin by the hammer. If apply the same amount of force, both the pins act differently.
What do you think, what may be the reason for this different behavior even the magnitude of the force is the same?
It is nothing but an area of application. The concept of pressure comes related to force and the area.
Hence, the are two factors which determine, whether pins will move into the wood or not,
The pressure is related to force and area and it is expressed as the amount of force applied per area. For the sharp nail, the area of application is very less and it moves and we say, the pressure is more. For bowling pin, the area of a bowling pin is large, it will not be able to move and we say, the pressure is less.
Pressure can be defined as force per unit area which is applied perpendicular to the surface of the object.
The mathematical equation for pressure can be written as:
P= F/A = mg/A
Although the normal force is a vector quantity, the pressure is a scalar quantity (vector page).
P = F/A
Now, unit of P = Unit of Force (S.I Unit) / Unit of area (S.I Unit)
= N / m2 ((S.I. Unit)
This N/m2 is known as Pascal (pa). Hence, Pa is the unit of pressure in S.I Unit.
1 Pa means 1 N force is applied in 1 m2 area.
Pa is applied for very small, normally kPa is used.
1 kPa = 1000 Pa = 1000 N/m2.
Now, unit of P = Unit of Force (C.G.S Unit) / Unit of area (C.G.S Unit)
= dyne / cm2 ((C.G.S Unit)
This dyn/cm2 is known as bar. Hence, bar is the unit of pressure in C.G.S Unit.
1 bar means 1 dyn force is applied in 1 cm2 area.
Now, unit of P = Unit of Force (F.P.S Unit) / Unit of area (F.P.S Unit)
= lbf / inch2 (F.P.S Unit)
This lbf/inch2 is known as pounds per square inch (psi). Hence, PSI is the unit of pressure in F.P.S Unit.
1 PSI means 1 lbf force is applied in 1 inch2 area.
Bar to PSI
1 bar to PSI
2.2 bar in PSI
Pascal to PSI
PSI to Bar
Pa to PSI
kPa to PSI
kPa to Bar
PSI to kPa
Pressures are different kinds and well discuss here in details,
Air has pressure, and it will be understood if it can be kept in an enclosed area. Air exerted force on the wall of the surface. This atmospheric force per unit area is atmospheric pressure.
Vapor pressure is the pressure at a given temperature that is exerted by a gas in equilibrium with either a solid or liquid in a closed container.
Various units are used for vapor pressure:
Absolute pressure is the pressure, measured with respect to perfect vacuum. There is no negative pressure below absolute pressure. The gauge pressure and atmospheric pressure together make the absolute pressure.
Gauge pressure is the pressure, measured with respect to zero atmospheric pressure. The difference between absolute pressure and atmospheric pressure is simply known as gauge pressure. We measure pressure with the help of a pressure gauge. The reading which we measure in the pressure gauge is called gauge pressure.
Vacuum pressure is just like the gage pressure, as it is measured with respect to atmospheric pressure, but it is a measured below atmospheric pressure.
Differential pressure means the difference between two pressures.
Hydrostatic pressure is the pressure that is produced by the hydrostatic force per unit area. If a liquid is kept in a container, the hydrostatic force acts at the base of the container.
This pressure is caused due to the gravitational force, which naturally pulls the fluid into the center of gravity of the earth that means downside.
The pressure (P) at any depth (ρ) is given by the equation:
Bidirectional pressure is like gauge pressure with respect to atmospheric pressure.
We have already learned that the lowest possible pressure is absolute zero pressure. Then how the term negative pressure comes?
But in the case of gauge pressure, both positive and negative pressure is possible.
The Static pressure is the pressure which is measured when the fluid is at rest.
We know, from Bernoulli’s equation:
If there will not be any change in elevation and fluid is static that is there will not be any flow, then the pressure will be static pressure.
The pressure p is the static pressure, with the above conditions.
Dynamic pressure refers to the pressure related to the fluid flow. It means the kinetic energy of the flowing fluid.
Pressure can be expressed as,
p = 1/2 γ v2
p= dynamic pressure, N/m2 (Pa),
γ = density of fluid (kg/m3)
v = velocity (m/s)
What’s the difference between absolute pressure and gauge pressure?
Absolute pressure = gauge pressure + atmospheric pressure
The pressure is expressed in various units and there are certain standards for pressure units. These are as follows;
There is some little confusion between pressure and pressure head. So, how to differentiate between these two? It’s simple.
Pressure, on the other hand, depends on the liquid. If density changes, the pressure will be changed.
The force of a fixed height liquid column will change over a unitary area with different specific liquids means different densities or specific gravities.
Hence, we can say, same height can generate different pressure. That means, the pressure head is the same but the pressure is different.
Water is generally used in the column as the most common liquid. But to measure the pressure in the small U-tube, a heavier liquids needs to be provided.
Normally, liquid mercury (Hg) is used as the heavier liquid than the water. Mercury is 13.6 times heavier than water. When you use the heavier liquid you don’t need to have that long column to measure higher pressure, so you can make a smaller and more convenient size column.
Water pressure is one of the most familiar forms of pressure in day-to-day life.
The pressure is dependent on density, gravitational acceleration, and depth.
From Bernoulli’s equation, we can write,
P = ρgd
Let us solve a very small problem, to clear the doubt.
What will be the pressure?
P = ρgd
ρ = 1000 kg/m3
h = 30 m
g = 9.81 m/s2
P = ρgd
= 1000 x 9.81 x 30
= 294.3 kPa
We have learned the basic of pressure, along with its various types, like absolute pressure, atmospheric pressure, vacuum pressure, gauge pressure.
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