Pressure Important Point (General Knowledge)
Complete Notes and important point on the topics Pressure in Physics for your upcoming exams such as UPSC, IAS, Banking, SSC, Railway etc.
|Formula:||Pressure = Force/Area|
|Unit:||CGS: dyne/cm2 SI: Pascal or N/m2|
What is Pressure?
Before knowing pressure I hope you have some kind of knowledge about Force and Area. Both Force and area are vector quantity. We realise that force can act on a point or on an area. But have you ever wonder why a sharp knife is easier to cut a piece of onion rather than a blunt one? In this case you probably apply same amount of force on the onion. The answer is how the force act on the onion. If you apply some amount of force on larger area the effect will be smaller. Form this concept a new physical quantity is required to describe this effect of force on an area.
Definition of Pressure: Force that applied normally on unit area is known as Pressure.
Formula: Pressure (P) = Force (F)/Area(A) or P = F/A
What is the unit of Pressure?
The unit of Force and area is Newton (N) and square meter (m2 ) respectively. Thus from the formula of pressure we get the unit of pressure is N/m2 in SI system and dyne/cm2 in cgs unit. In SI system N/m2 is also known as Pascal (Pa).
1 Pa = 10 dyne/cm2.
Thus we get that pressure is force/area. Both force and area are vectors. In this case a vector quantity (force) is divided by another vector quantity (area). So here a question has arisen. Is pressure a scalar or vector quantity? The answer is that the Pressure is a scalar quantity. Remember: We can not simply divide a vector by a vector.
The air is surrounding the earth is called the atmosphere. It is extended about 100 km high from the earth surface. The air contain various type of gases such as Nitrogen, Oxygen, Carbon dioxide etc. So air has mass. And the earth attract masses around it. The force by which gravity attract masses is called weight. So air has weight. The weight of air per unit area of the earth’s surface is called atmospheric pressure.
How to measure atmospheric pressure?
Look at the picture. The shaded portion is filled with mercury. The upper side of the tube is airtighted so that no air can fill the tube from upper side. It is seen that at STP (Standard Temperature and Pressure) the mercury column gets up to a height about 76 cm. So the pressure of air is equal to the weight of mercury of 76 cm height per unit area (1 cm2). That is 76 × 13.6 × Gravitational acceleration (980) = 1.013×106 dyne cm-2. This is called 1 atmospheric pressure or 1 atm. Another unit of atmospheric pressure is torr and bar. 1 torr = height of 1 mm mercury = 133.18 Pa. 1 bar = 105 Pa. And 1 atm = 1.01 bar.
The atmospheric pressure decrease as increase the height from earth surface. So at the high altitude (on the mountain), at low pressure it is difficult to cook.
Atmospheric pressure is measured by Barometer. If the reading of barometer fall suddenly means the pressure of that region is low and there is a possibility of storm.
Slow fall of barometric reading is the indication of rainfall. Again if the reading of barometer rises slowly means the weather is clean and dry.
Pressure in liquid :
Pressure also exerted by liquid to its all directions. Look out the picture. The height of liquid level is h. and the area considered is A on which the liquid exerts pressure.
So the volume of liquid is = Ah. let density of liquid is = ρ. The mass of the liquid is = Ahρ. And the weight of the liquid is = Ahρg, where g = Gravitational acceleration. Again we know the formula of Pressure which is
Pressure P = Weight / Area
or, Pressure P = Ahρg / A
or P = hρg
This is how much of pressure exerts at depth h by a liquid. From the above formula we can see that the pressure is proportional to the depth of liquid. It is also proportional to the density. Thus the density of mercury is more than water. Therefore at the same depth, mercury exerts more pressure than water. Pressure is also proportionally depends on the gravitational acceleration.
In the case of static liquid the pressure is same for all point at same horizontal plane. That’s why the surface of water in a pot is flat. At a particular point the liquid exert same pressure in all direction.
How does pressure effect on melting point and boiling point? By the effect on pressure melting point may increases or decrease depending on the substance. If the substance expand on fusion, the melting point increases with increase in pressure. Example: Wax. Again if the substance contract on fusion, then the melting point decrease on increase in pressure.