“Pressure” MHT-CET Class 12 Physics Crash Course| Chapter Mechanical Properties of Fluids
In this session G.S. Khairnar discussed the theory and MCQ’s of “Pressure”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.
Pressure:-
Hello students now we are going to start 2nd lesson of syllabus i.e. fluid dynamics. Before proceeding with the lesson we should recall few things that are essential basics for the lesson.
Fluid: Substance which can flow easily is called fluid. Gases and liquids has less intermolecular force so that their molecules are not tightly bound to each other, hence they show the property of flow.
The branch of physics that deals with the study of fluids at the rest is called as hydrostatic. An ideal fluid should possess following characteristics,
- It should be incompressible
- It should be non viscous
- Flow should be steady and
- Ability to take shape of container
When fluid is filled in any container it exerts the force on the walls of container, which in results exerts the pressure on wall of container.
Push what we exerted is termed as thrust and the impact of it on surface is known as pressure.
Let’s discuss the concept of pressure and its derivation in details…!
The force/thrust acting perpendicular to given surface area is known as pressure.
Hence,
Pressure= thrust/ area Or
Pressure=force/ area
P= F/A
Si unit of pressure is newton per metre (N/m2) and CGS unit is dyne/cm2
N/m2 is also called as pascal (Pa)
Pressure is inversely proportional to Area.
Let’s derive the formula for pressure exerted by liquid column…….!
Consider the fluid of density ‘ρ’ is filled in a vessel. Consider an imaginary cylinder of same liquid inside the fluid of height ‘h’ and cross-section ‘A’ as shown in figure below.
The weight of imaginary cylinder is the only force acting on the bottom of cylinder then the force, F= mg acts on area ‘A’
Then by definition of pressure,
Some important points to remember…..!
- Atmospheric pressure: Earth surface consist of huge and thick layer of atmosphere which is nothing but the mixture of different gases. Due to gravitational force the weight of these gases is also directed towards the surface of earth. This exerts the additional thrust on free surface of fluid. This additional pressure due to earth’s atmosphere is known as Atmospheric pressure (P0).
- Absolute pressure: Total pressure exerted by liquid column on its surface is known as the absolute pressure. Absolute pressure is then the sum of atmospheric pressure (P0) and pressure exerted by the liquid column (ρhg) on the surface,
∴ Absolute Pressure, P=P0+ρhg
- Gauge pressure: Gauge pressure is then the difference between absolute pressure and atmospheric pressure (P0).
∴ Gauge pressure=Absolute Pressure-Atmospheric pressure
∴ P – P0 =ρhg
Pressure:
Let’s learn the following MCQ’s to understand the concept of Pressure in detail….!
Q.1) Which among the following properties should possessed by an ideal fluid?
a) It should incompressible
b) It should be nonviscous
c) Both ‘a’ and ‘b’
d) Neither ‘a’ nor ‘b’
Answer
c) Both ‘a’ and ‘b’
Q.2) Which among the following stress could not be opposed by fluid at rest?
a) tensile stress
b) shear stress
c) volume stress
d) none of these
Answer
b) shear stress
Q.3) Which among the following is not the correct unit of pressure?
Q.4) Which among the following is the correct dimension of pressure?
Q.5) Pressure exerted mercury column of height 76 cm on surface. Density of mercury is 13600 kg/m3
a) 1.013 × 104 Pa
b) 10.13 × 104 Pa
c) 101.3 × 104 Pa
d) 1013 ×1 04 Pa
Q.6) Absolute pressure exerted by water at depth of 250 m from the surface if the atmospheric pressure is of 4 atm. Density of mercury is 1000 kg/m3
a) 28.55× 102 Pa
b) 28.55× 103 Pa
c) 28.55×104 Pa
d) 28.55× 105 Pa
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