- Books Name
- Class-8 Science Book
- PathSet Publications
- CBSE Class 8
The force acting per unit area of a surface is called pressure or we can say force per unit area is called pressure.
Pressure = Force / Area
From this formula, we can say Pressure is indirectly proportional to the area, thus pressure decreases with an increase in area and increases with a decrease in area.
- When we cut an apple, we need to use the sharp edge of the knife instead of a blunt knife because the sharp edge of the knife has a small surface area and we need to exert less force with high pressure to cut the apple.
- When we put a nail into a wooden board, the pointed end of the nail is kept at the front. The pointed end of the nail has a very small surface area and this enables us to apply greater pressure with the applied force.
Unit of Pressure
The SI unit of pressure is Pascal.
The SI unit of pressure is Pascal (Pa) or N/m2
Applications of Pressure in our Daily Life
- Broad Straps of shoulder bags:- The straps of shoulder bags are made broader which increases the area of contact and reduces the pressure on the shoulders. We know that pressure is inversely proportional to area, thus the wider the area, the less pressure is exerted, making it easier to carry a shoulder bag with a wide strap.
- Elephants have broader feet, which increases the area of contact with the ground and results in lower pressure, and enables the elephants to walk easily.
- It is easier to hammer a sharp iron nail than to hammer a blunt one because the area of a sharp iron nail is smaller than that area of a blunt nail. So, it becomes easier to hammer the nail into the wall.
- The use of skies helps to walk on snow as skies increase the area of contact between our feet and snow. This reduces pressure on the snow.
- Peeling vegetables with a blunt knife is tough. It is much easier to peel vegetables using a sharp knife that has sharp edges rather than a blunt knife. This is because the sharp edge of the knife has a small surface area and less force delivers the amount of pressure needed to cut or peel vegetables.
Pressure Exerted by Liquids and Gases
- It is the force exerted by liquids or gases per unit area.
- Liquids and gases exert pressure in all directions. The pressure inside them increases with depth.
- Liquid and gas can also exert pressure on the inner walls of the container in which they are kept.
- When air is filled, a rubber balloon gets inflated from all sides. This happens due to air or gas exerts pressure over the inner walls of the balloon.
- Water starts leaking if there is a pore in the bottle. This happens due to water exerts pressure over the walls of the bottle.
Pressure Exerted By Liquids
- Liquids exert pressure due to the weight that liquid possesses.
- The liquid posses pressure in the downward direction.
- The pressure exerted by liquids at the bottom depends on the height of the liquid column. The pressure exerted by liquids increases with depth.
- For example – The body of the submarine is thick and built by strong material because the pressure of water is high in deep-sea so to withstand that pressure submarine is thick.
- Liquids also exert pressure in all directions on the walls of the container.
- For example – Take a container and make a hole in it. Now close the holes with tape and fill the container with water. Then remove the tape. We will observe that water comes out of the container in all directions.
- This shows that liquids exert pressure in all directions.
Pressure Exerted by Gases
- Gases too exert pressure on the walls of containers like liquids. Air pressure arises due to constant collisions of tiny molecules of gases present in the air with the walls of the container it is enclosed.
- For example- When air is pumped into a bicycle tube by using a pump, the bicycle gets inflated due to air pressure exerted by the collisions of gas molecules in air with the inner walls of the rubber tube. For this reason, bicycle tyres feel hard.
The amount of atmospheric pressure upon us is quite large due to the large surface area of the atmosphere around the earth but we do not experience any of its effects.
Atmosphere – The blanket of air that surrounds the earth is called the atmosphere.
- Our Atmosphere is made of different types of gases. Since gases exert pressure, thus Pressure exerted by air (present in the atmosphere) is called atmospheric pressure.
- The weight of air in a column of the height of the atmosphere and area 10 cm x 10 cm is as large as 1000 kg. The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.
- A pressed rubber sucker on a plane surface does not come off because of All fluids (liquids and gases) exert pressure. So the air also exerts a pressure called atmospheric pressure.
- Atmospheric pressure is the pressure within the atmosphere of Earth. Atmospheric pressure is due to the weight of air present in the atmosphere above us. This pressure causes air molecules to be more tightly packed together than those that are high in the atmosphere. On the surface of the earth, the atmospheric pressure is maximum at sea level.
- Atmospheric pressure also exists in all directions. The instrument used for the measurement of atmospheric pressure is known as a Barometer.
There are two types of barometer-
- The mercury barometer is an ideal one as it gives the standard unit of atmospheric pressure.
- In a mercury barometer, a column of mercury in a glass tube rises or falls the weight of atmospheric pressure changes.
- The approximate value of atmospheric pressure on the earth’s surface is 1.013×105 pascal.
However sometimes at higher altitudes where the atmospheric pressure is low as compared to that at the Earth’s surface (low altitudes), nose bleeding occurs.
This is so because at that time the blood pressure in our body becomes higher than the atmospheric pressure outside us.