viscosity, elasticity and surface tension
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PHYSICSNEET and JEE Main 2020 : 45 Days Crash Course
Viscosity, Elasticity and Surface Tension
By,Ritesh Agarwal, B. Tech. IIT Bombay
Viscosity
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
The property of the liquid by virtue of which it opposes the relative motion between its adjacent layersis known as 'viscosity’.Viscous Force, Velocity Gradient and Coefficient of Viscosity
Units of Coefficient of Viscosity
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Example
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Sol.
Stoke’s Law and Terminal Velocity
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Stoke’s Law
Terminal Velocity
When a body is dropped in a viscous fluid, it is first accelerated and then its acceleration becomes zeroand it attains a constant velocity called terminal velocity.
Calculation of Terminal Velocity
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Example
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Sol.
Elasticity
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
The property of a material body by virtue of which it regains its original configuration (i.e. shape andsize) when the external deforming force is removed is called elasticity.Stress
Types of Stress
Example
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Sol.
Strain
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Types of Strain
Hooke’s Law and Modulus of Elasticity
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Types of Modulus of Elasticity
All modulus of elasticity have same units and dimensional formula i.e. Nm–2 or Pascal (Pa) and [ML-1T-2]
Note:
Example
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Sol.
Variation of Strain with Stress
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
A material for which the plastic range CD isrelatively high is called ductile material. Thesematerials get permanently deformed beforebreaking. The materials for which plastic rangeis relatively small are called brittle materials.These materials break as soon as elastic limit iscrossed.
Analogy of Rod as a spring
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Elastic potential energy stored in a stretched wire or in a rod
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Surface Tension
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
The tension generated in the surface is called surface tension force. Due to surface tension, the liquidsurface behaves like a stretched membrane (rubber sheet) and try to minimize its area.
Note:
Example
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Sol.
Surface energy
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Potential energy stored due to surface tension force is called surface energy.
So, Surface tension is surface energy per unit surface area.Numerically surface energy density is same as surface tension.
Example
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Sol.
1000 small water drops, each of radius r, combine and form a big drop. In this process, find decrease in surface energy.
Angle of Contact, Cohesive and Adhesive force
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Cohesive force : The force of attraction between the molecules of the same substanceis called cohesive force.Adhesive force : The force of attraction between different substances is called adhesiveforce.Angle of Contact : At point of contact, angle between the tangent to the liquid surfaceand solid surface submerged in liquid is called angle of contact (θ).Shape of liquid surface is called meniscus.
Excess Pressure inside Bubble and Drop
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
Capillary Action
Physics by Ritesh Agarwal (B. Tech. IIT Bombay)
A glass tube of very small diameter is called capillary. The phenomenon of rise or fall of liquid in acapillary tube is called capillarity.
rc = radius of curvature of the water surfaceR = radius of capillary tuberc = R secθ
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