In our daily life, we often use the word pressure, like a pressure cooker, blood pressure, atmospheric pressure, etc. But what is the actual meaning of ‘pressure’? Lets us discuss it in this topic.
Whenever a force acts on a surface it causes some effect on it. In order to explain the effect produced by a force on a surface the term ‘pressure’ is used. To explain the pressure not only the force but also the area on which it acts is considered.
The force working simply on unit area of a surface is pressure.
We measure force in Newton which is find by multiplying mass in kg and acceleration in m/s square.
Force(N) = mass(kg) × acceleration (m/s sq.)
The area of the surface on which the force is acting we measure it in metre square. Hence the unit of pressure is Newton per square meter (N/m square).
Pressure = Force/Area
Or, P = F/A
From this expression, the following conclusions can be drawn.
- The pressure is directly proportional to the force applied i.e, when more force is applied pressure will be more ( for a fix area).
- The pressure is inversely proportional to the area over which force is applied i.e, the area, the more the pressure exerted ( for the same amount of force).
Thus, the pressure depends upon the force and the area on which we have to apply the force.
• The Pressure increases with the increase of force
• The pressure increase with a decrease in area
A solid exerts pressure on the surface supporting it. This is because of the weight of solid. Liquid also has weight and exerts pressure on the vessel containing it.
The pressure applied by fluid/liquid is called liquid pressure.
We calculate it by using,
P = hdg
Where h, is the height of the vessel, d is the density of the liquid and g stands for gravity.
Around our earth, there is a layer of air which stretches up to 100 km. The air present in the atmosphere of the earth is atmospheric air. The pressure exerted by the atmospheric air is the atmospheric pressure.
Air has weight and in a normal room, it approximately weighs about 500 Newton which is almost equal to our weights. Because of this weight atmospheric air exerts pressure.
The atmospheric pressure decreases with the increase in Altitude because air becomes thinner at higher altitude.
Note- we don’t normally feel the atmospheric pressure because the pressure exerted by our bodies from inside is equal to that of atmospheric pressure.
Uses of air pressure
We use air pressure in different activities and phenomena.
Some of them are as follows,
while drinking with straw we suck air from it and pass to the lungs. Now the atmospheric pressure acting on the surface of the liquid becomes more than in the straw and the liquid is force up to the mouth.
The filling of ink in a pen, filling liquid inside a dropper/syringe, lifting water by using a lift pump, etc. All support action due to the existence of air pressure.
When the rubber sucker is moistened and pressed on a smooth flat surface the air inside of it pushed out. The sucker sticks on the surface firmly due to atmospheric pressure. We use these suckers as holders at home and a car. We also use it to paint tall buildings which help to hold the painters along with their accessories against the wall.
Measurement of pressure
You use different types of instruments to measure pressure. Some of the popular instruments are as follows,
ManometerA manometer is an instrument we use for measuring the pressure that exerts by human lungs. It consists of a U- shaped transparent tube with some water in it. We can measure how much pressure our lungs can exert against outside pressure with the help of manometer. We use a similar device sphygmomanometer to measure blood pressure (b.p).
Pressure gaugeWe see that there is air inside the tyres of vehicles. If there is too much air in the tyres it bursts and if there is too little air in the tyres then the vehicles cannot run properly. Thus, we have to maintain the air pressure in the tyres. The instrument we use for measuring air pressure in a system like tyres is a pressure gauge.
Mercury barometerWe use a mercury barometer instrument for measuring atmospheric pressure. We can make a simple Mercury barometer in the laboratory.