In the chapter of Solids, we study about the macroscopic view of forces and its effects on SOLIDS.
We know that the volume change in solids, on application of forces is very less. The reason for less change in volume is that the internally, the molecules are strongly attracted to near by molecules and hence the effect. In other words the intermolecular forces are very strong in case of solids and
Now in liquids the intermolecular forces are much less than compared to solids, obviously it is greater than in gases, so we may call in intermediate. The molecules of liquids can change its position relatively within the liquid easily and thus liquid can change its shape depending on the container.
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If liquid is kept at rest i.e. the liquid molecules are not moving. Obviously the net force on liquid molecules is zero. In other words the force on any particle by neighboring particles is zero. In other words the pressure applied on each particle is zero.
Here we define the Pressure. It is force per unit area. Important thing to understand is that to calculate the Pressure on any area we take the component of force perpendicular to that area.
SI Units of Pressure = Force/Area = Newton/Meter2 = N/m2, this is also called Pascal.
The air or atmosphere, which acts as fluid only, around us also exerts pressure on every body, This is termed as atmospheric pressure. The value was calculated experimentally by Torricelli. Let us understand his experiment. But before that be clear in mind that pressure due to fluid of density ‘ρ’ column of height ‘h’ is P = hρg
His experiment is shown in diagram. He took various tubes and after filling with mercury inverted them in liquid (Hg). You can notice even if the tube is tilted the height of Hg column remains same as 76 cm. So he concluded that this was due to pressure of atmosphere. If we calculate the pressure in terms of our usual units it comes out to be
P = 0.76 ×13600 ×9.8 = 1.01×105 N/m2 = 1 atm
Now 105 is termed as 1 bar so atmospheric pressure is 1.05 Bar but we take it 1 Bar
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