Pressure is the force applied to the surface of an object per unit area over which that force is distributed. Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square meter (N/m2). Pressure may also be expressed in terms of standard atmospheric pressure.
In this article, we will learn about What is Pressure, Formula for Pressure, the Unit of Pressure, and others in detail.
What is Pressure?
When cutting an apple, we should use the sharp edge of the knife rather than the soft edge of the knife since the sharp edge of the knife has a smaller surface area and requires less power with high pressure to cut the apple. When we drive a nail into a wooden board, we keep the pointed end of the nail in front. The pointed end of the nail has a relatively tiny surface area, allowing us to apply more pressure with the same amount of effort.
Pressure is force per unit area applied in a direction perpendicular to the surface of an object.
Pressure acting on a body is the ratio of the perpendicular force to the surface area of the object. The formula that is used to calculate the pressure acting on an area is,
P = F / A
- P is Pressure
- F is Force Applied
- A is Surface Area on which force is applied
From the above expression, it is observed that pressure is indirectly proportional to the area therefore pressure decreases when the area increases and pressure increases when the area decreases. In the above formula, the area is in the denominator. As a result, for the same force, the smaller the area, the greater the pressure on a surface.
Unit of Pressure
SI unit of Pressure is Pascal (Pa). Pascal is defined as the force of one newton applied over a surface area of a one-meter square. The dimension formula of pressure is [ML-1T-2]
Types of Pressure
There are various types of pressure but it can be broadly categorized into four categories.
- Atmospheric Pressure
- Differential Pressure
- Gauge Pressure
- Absolute Pressure
Learn more about Types of Pressure.
Factors Affecting Pressure
Pressure depends on the surface area over which the force is applied, the larger the surface area the smaller the pressure applied and the smaller the surface area larger the pressure applied. Thus, we conclude that pressure is inversely proportional to the surface area over which the force is applied.
Pressure ∝ 1 / Surface Area
Pressure is also dependent on the force applied i.e. the more force we apply the more pressure is experienced. Thus, pressure is directly proportional to the force applied.
Pressure ∝ Force Applied
Thus, we can say that pressure depends upon the force applied and surface area. This can be concluded with the following examples, when we pick a backpack with our hand we experience more pressure, whereas when the same bag is on our shoulder the pressure experienced is far less.
It can also be experienced as the sharper knives cut easily as the surface area is very less whereas a rough knife does not cut very easily, i.e. the sharper the knife the less it’s surface area the more easily it cuts.
Pressure Exerted by Liquid and Gas
Liquids pressure is also called Fluid pressure. Liquid generates pressure on the walls of the container they are placed in. The pressure exerted by a liquid on the bottom of a container is proportional to the liquid’s height in the container. The liquid exerts identical pressure on various spots on the same-depth container walls. Similarly, gases put pressure on the container’s walls. The molecules of gas with higher kinetic energy smash with walls with great force, and as a result, these molecules exert pressure on the container’s walls.
Let’s consider the following illustration:
- Take a glass tube or a plastic pipe that is translucent. The pipe/tube is of sufficient length and diameter. Take a thin sheet of good quality rubber, such as a rubber balloon, as well. Over one end of the pipe, stretch the rubber sheet tightly. Maintain a vertical posture for the pipe by holding it in the middle and pouring some water into the pipe. The bulge in the rubber sheet appears as the water gets stored in the pipe and the size of the bulge get increasing as more water is poured into the pipe i.e. the pressure exerted by water at the bottom of the container depends on the height of the water.
- Take a cylindrical container or an empty plastic bottle. Drill four holes all the way around the bottle toward the bottom. Keep all the holes at the same height from the ground. Fill the bottle halfway with water. The water that comes out of the holes falls at the same distance from the bottle i.e. Liquids exert equal pressure at the same depth.
Atmosphere is the envelope of air that surrounds us. Atmospheric air rises hundreds of kilometers above the earth’s surface. Pressure exerted by this air is known as atmospheric pressure.
Suppose there is a unit area and a very long cylinder filled with air standing on it, the weight of the air in the cylinder equals the atmospheric pressure. The weight of air in a column with a diameter of 10 cm and a diameter of 10 cm can be as much as 1000 kg. Because the pressure inside our bodies is equal to the atmospheric pressure and cancels out the pressure from outside, we are not crushed under this weight.
Take a high-quality rubber sucker, and It has a structure like a little rubber cup. On a smooth horizontal surface, press it smoothly. When the sucker is pressed, most of the air trapped between its cup and the surface escapes. Because of the pressure of the atmosphere, the sucker sticks to the surface. The applied force must be great enough to overcome atmospheric pressure in order to lift the sucker off the surface. It would not be possible for any human being to pull the sucker off the surface if there were no air at all between the sucker and the surface.
Pressure on Walls of a Container
The container filled with liquid experience pressure which depends upon the height of the water filled in the container. Similarly, the value of pressure experienced by the side walls of the container depends upon the volume of liquid above it. Also, the pressure at any level is the same as the volume of the liquid above that level is always same. Pressure exerted by the liquid on various levels is shown in the image below,
Gases also exert pressure on the wall of the container which contains them. A gas comprises trillions of molecules and every molecule moves in a random direction the moving molecules of gas have some kinetic energy. When these molecules collide with the walls of the container they apply pressure on it.
Example on Pressure Formula
Example 1: If the force of 10 N acts on an area of 2 m2. Find the pressure acting on that area.
P = F/A
P = 10/2 = 5
Thus, the pressure acting on the surface is 5 N/m2
Example 2: What is the force acting on the body of the pressure acting is 25 N/m2 and its surface area is 5 m2?
P = F/A
25 = F/5
F = 25×5 = 125 N
Thus, the force acting on the surface is 125 N.
Example 3: If the force of 100 N acts on an area of 12 m2. Find the pressure acting on that area.
P = F/A
P = 100/12 = 8.33
Thus, the pressure acting on the surface is 8.33 N/m2
Example 4: What is the force acting on the body of the pressure acting is 220 N/m2 and its surface area is 11 m2?
P = F/A
220 = F/11
F = 220×11 = 2420 N
Thus, the force acting on the surface is 2420 N.
Practice Questions on Pressure Formula
Q1. Find is the force acting on the body of the pressure acting is 22 pascal and its surface area is 11 cm2?
Q2. What is the surface area of the object in the force applied is 200 n and the pressure at the surface is, 12 Pascal?
Q3. If the force of 1.5 dyne acts on an area of 0.03 cm2. Find the pressure acting on that area.
Q4. If the force of 90 N acts on an area of 5 cm2. Find the pressure acting on that area.
FAQs on Pressure
1. What is Blood Pressure?
Blood pressure is a measure of the force that your heart uses to pump blood around your body.
2. What is Normal Blood Pressure Range?
The normal blood pressure range is 120 mmHg to 80 mmHg.
3. What is Pressure Class 8? How is Pressure Calculated?
Pressure is defined as the force acting per unit area of a surface. Its SI unit is Pascal (Pa). Pressure can be calculated by using the formula,
Pressure = Force / Area
4. What is a Pascal?
If one-newton force is applied over a surface area of one-meter square area we say the force applied on the surface is one-pascal.
5. What is Atmospheric Pressure?
Atmospheric pressure at a point is defined as the force acting normally on a unit area around that point, due to the total height .
6. What is Osmotic Pressure?
Osmotic pressure is the pressure that would have to be applied to a pure solvent to prevent it from passing into a given solution by osmosis, often used to express the concentration of the solution.
7. What is Air Pressure?
Air pressure is the weight of air molecules pressing down on the Earth. The pressure of the air molecules changes as you move upward from sea level into the atmosphere. The highest pressure is at sea level where the density of the air molecules is the greatest.
8. What is Pressure SI Unit?
The SI unit of pressure is Pascal. It is represented as, Pa and 1 Pa is equal to one Newton per square metre, i.e.
1 Pa = 1 N/m2
9. What is Pressure at STP?
STP is the standard condition at which all physical experiments takes place. The pressure at STP is 1 atm.
10. What is Pressure Gradient?
The rate of rise and fall of the pressure with respect to the distance is called the pressure gradient.
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