Pressure Important Point (General Knowledge)

Complete Notes and important points on the topics Pressure in Physics for your upcoming exams such as UPSC, IAS, Banking, SSC, Railway, etc.

Contents:

 Formula: Pressure = Force/Area Unit: CGS: dyne/cm2 SI: Pascal or N/m2 Dimension: [ML-1T-2] Quantity: Scalar

What is Pressure?

Before knowing the pressure I hope you have some kind of knowledge about Force and Area. Both Force and area are a vector quantity. We realize 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 the 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 a 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 the SI system and dyne/cm2 in the 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 a vector quantity? The answer is that Pressure is a scalar quantity. Remember: We can not simply divide a vector by a vector.

Atmospheric Pressure:

The air is surrounding the earth is called the atmosphere. It is extended about 100 km high from the earth’s surface. The air contains various types of gases such as Nitrogen, Oxygen, Carbon dioxide, etc. So air has mass. And the earth attracts masses around it. The force by which gravity attracts masses is called weight. So the 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 airtight so that no air can fill the tube from the upper side. It is seen that at STP (Standard Temperature and Pressure) the mercury column gets up to a height of 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 it increases the height from the earth’s 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 the barometer falls suddenly means the pressure of that region is low and there is a possibility of a storm.

The slow fall of barometric reading is an indication of rainfall. Again if the reading of the 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 the liquid level is h. and the area considered is A on which the liquid exerts pressure.

So the volume of liquid is = Ah. let the density of the 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 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. The pressure also proportionally depends on the gravitational acceleration.

In the case of static liquid, the pressure is the same for all points on the same horizontal plane. That’s why the surface of the water in a pot is flat. At a particular point, the liquid exerts the same pressure in all directions.

`Read Also: Motion – Its Equations > Important Physics GK [PDF]`

How do pressure effect on melting point and boiling point? By the effect of pressure melting point may increases or decrease depending on the substance. If the substance expands on fusion, the melting point increases with an increase in pressure. Example: Wax. Again if the substance contract on fusion, then the melting point decrease on an increase in pressure.