Pressure is defined as the physical force exerted on an object. The force applied is perpendicular to the surface of objects per unit area. The basic formula for pressure is F/A (Force per unit area). Unit of pressure is Pascals (Pa). Types of Pressures are Absolute, Atmospheric, Differential, and Gauge Pressure. While sipping beverages from a straw, have you noticed that you actually suck the air out of the straw? While you sip the beverage, you’re actually applying ‘Pressure’. But, what does it mean? Let’s study more about it.
What is Pressure?
The amount of force exerted (thrust) on a surface per unit area is defined as ‘Pressure’. It can also be defined as the ratio of the force to the area (over which the force is acting).
Formula and Unit of Pressure
Pressure (P) = Thrust/ Area
The SI unit is ‘pascals (Pa)’. 1 Pa = 1N/m2
Example: It is easier to hammer a sharp pin than to hammer a blunt pin. This is because the area at the end of the sharp pin is smaller than the area at the end of a blunt pin. This leads to an increase in pressure leading to hammer the sharp pin easily.
Types of Pressure
- Atmospheric Pressure
- Absolute Pressure
- Differential Pressure
- Gauge Pressure
The earth’s atmospheric air is surrounded by a layer of gases and so this air surrounding the earth exerts a pressure known as the ‘atmospheric pressure’. Its value at sea level is 101325 Pa.
It is measured using a mercury barometer (hence atmospheric pressure is also known as barometric pressure), indicating the height of a column of mercury which exactly balances the weight of the column of atmosphere over the barometer. It can be expressed in several different systems of units such as millimeters (or inches) of mercury, pounds per square inch (psi), dynes per square centimeter, millibars (mb), standard atmospheres, or kilopascals.
The atmospheric pressure decreases near Earth’s surface, with height at a rate of about 3.5 millibars for every 30 meters (100 feet).
Check out our detailed article on Units of Pressure here.
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Pressure on walls of the container
The liquid at the bottom of the container (in which it is filled) exerts some pressure which depends upon the height of water filled in the container. The value of pressure exerted by the liquid at the side walls of the container; at any point depends upon the amount of water above that point. Since in a container the height of liquid above a surface is same for all the points, therefore, liquid exerts same pressure at a particular level.
Gases too exert pressure on the wall of the container containing them. A gas consists of molecules and every molecule has some kinetic energy. These molecules when colliding with the walls of a container, apply pressure on it.
Let us do an experiment using the tin can to demonstrate this. Fill the tin can with water and boil it. Next, seal the tin can with a stopper so that tin can contains steam at a low pressure. Then, pour some cold water on the tin can from outside. When you do so, it’s seen that the tin can collapses inwards. The tin can collapses because when cold water is poured, steam inside it condenses and the air pressure inside reduces. The difference in this pressure inside and outside causes a force that is applied to the outer walls of the container. This force is what causes the tin can to crush inwards.
Note: The tin-can retains its shape usually because the force applied by the gas molecules on the outside of the container is balanced by the force inside the container.
Solved Example For You
Q. In terms of physics, which of the following are considered as fluids?
a. Solids and Gases
b. Gases and liquids
c. Only liquids
d. Liquids and solids
Sol: b. Gases and liquids
A fluid is defined as a substance that has no fixed shape and yields easily to external pressure. They are of two kinds- Gases and Liquids.