Fluid Statics
Fluid Statics (Hydrostatic)
Fluid
Hydrostatics (Fluid Statics)
Density
Relative Density (Specific Gravity)
Relative density of a substance is the ratio of its density to the density of water at 4°C.
Pressure
Derivation of Pressure in Liquid
Let us consider a liquid of density ‘ρ’ contained in a vessel of cross-sectional area ‘A’ up to height ‘h’ as shown in figure.
Now, total mass of liquid:
Weight of liquid is given by:
So, the normal force on the bottom of the vessel is:
Finally, the pressure exerted by liquid at the bottom of vessel is:
This is the required relation for pressure due to liquid and this relation shows that pressure due to liquid is directly proportional to:
- density of liquid,
- height of liquid in vessel,
- acceleration due to gravity at that place.
Principle of Flotation
OR
When a body is immersed in a liquid there are three cases:
i) Weight of body is greater than upthrust (W > U)
In this case, body sinks and set up at bottom.
ii) Weight of body is equal to upthrust
In this case, the body lies inside liquid with upper surface on the surface of liquid.
iii) Weight of body is less than upthrust
In this case, a part of body lies inside the liquid and remaining part outside the liquid surface.
Upthrust, Archimedes’ Principle, Pascal’s Law & Centre of Buoyancy
Upthrust
Archimedes’ Principle
Pascal’s Law
Centre of Buoyancy
A floating body is said to be in equilibrium if centre of gravity and centre of buoyancy of the body lie in same vertical line.
Surface Tension
Cohesive and Adhesive Force
Surface Tension
Mathematically: Surface tension is the force per unit length of an imaginary line drawn in the plane of liquid surface acting at right angle of this line.
If ‘F’ be the force acting on the imaginary line of length ‘l’, then:
Note: Surface-tension expressions for different objects
| Case | Source condition | Expression |
|---|---|---|
| Object | Object of length l on surface of liquid | T = F/l |
| Object | Object of length l inside the liquid | T = F/(2l) |
| Ring | Ring of perimeter 2πr on surface | T = F/(2πr) |
| Ring | Ring of perimeter 2πr inside liquid | T = F/(4πr) |
| Rectangle | Rectangle of perimeter 2(l+b) on surface | T = F/[2(l+b)] |
| Rectangle | Rectangle of perimeter 2(l+b) inside liquid | T = F/[4(l+b)] |
Surface Energy
Let us consider a rectangular frame ABCD of wire in which BC can be slid horizontally. Suppose it is dipped in a soap solution such that a thin layer is deposited over an area. Due to surface tension (T), the layer tends to minimize surface area. Suppose force ‘F’ is applied perpendicularly to BC towards outwards. So, the membrane is extended by distance ‘x’ horizontally.
So, workdone by this force F:
Hence,
This surface energy is equal to surface tension.
Shape of Liquid Meniscus
i) Convex Meniscus
In convex meniscus, the cohesive force between mercury molecules is greater than the adhesive force between mercury molecules and glass molecules.
ii) Concave Meniscus
In concave meniscus, the adhesive force between mercury molecule and water molecule is greater than the cohesive force between water molecules.
Angle of Contact
Capillarity
For example:
- When a capillary tube with narrow bore is dipped in water, the level of water rises in capillary tube.
- When same capillary tube is dipped in mercury, the level of liquid falls in capillary tube.
Measurement of Surface Tension by Capillary Rise Tube
Let us consider a capillary tube of radius ‘r’ is dipped in a liquid of density ‘ρ’. Let ‘h’ be the height above which liquid rise in the capillary tube. The surface tension acting tangentially at concave meniscus is equal and opposite to the relation. If ‘θ’ be angle of contact, then T can be resolved into two components—T sinθ along horizontal and T cosθ along vertical. The horizontal components cancel each other and vertical components add up together which pull liquid upwards.
The total vertical force acting upward throughout the circumference is given by:
This force pulls the liquid upwards until weight of rise liquid equals to upward force.
Now, volume of liquid in the tube above free surface of liquid is given by:
So, weight of liquid rise:
At equilibrium:
Thus, knowing r, h, θ, ρ and g value, T can be calculated.
For narrow tube: h ≫ r/3, so h + r/3 ≈ h.
Numericals — Surface Tension & Capillarity
Q.1 — Capillary rise
A capillary tube of 0.3 m diameter is placed vertically inside a liquid of density 800 kgm−3, surface tension 5×10−4 Nm−1 and angle of contact 30°. Calculate the height to which the liquid rises in the capillary tube.
Q.2 — Rectangular plate and surface tension
A rectangular plate of dimensions 6 cm by 4 cm and thickness 2 mm is placed with its largest face flat on the surface of water. Calculate the downward force on the plate due to surface tension assuming zero angle of contact. What is the downward force if the plate is placed vertical so that its longest side just touches the water?
Downward force on the plate due to surface tension:
Again, downward force when the plate is placed vertical and its largest side just touches:
Q.3 — Breaking one drop into eight drops
Find the workdone required to break up a drop of water of radius 5×10−3 m into eight drops of water assuming isothermal condition.
Workdone = T × change in area
Q.4 — Breaking one drop into one million droplets
Calculate the work done in breaking a drop of water of 2 mm diameter into million droplets of same size. T of water = 72×10−3 N/m.
Again, workdone is:
Viscosity
Generally: Viscosity means the frictional force acting between the layers of fluid that oppose the relative motion between them.
Newton’s Formula for Viscosity
Let us consider a liquid flowing over the fixed solid horizontal surface. The liquid flows in different layers parallel to the fixed surface. The layer in contact with the fixed surface is at rest while the velocity of other layer increases uniformly upward.
Let ‘A’ be the area of layers in contact and v & v+dv are the velocity of layer at distance x & x+dx from fixed surface respectively, then dv/dx is velocity gradient.
According to Newton’s law of viscosity, viscous force ‘F’ acting between two layers of liquid is:
Combining eqn (i) and (ii), then:
where ‘η’ is proportionality constant called coefficient of viscosity. The negative sign indicates that viscous force decreases the relative motion/velocity between two layers.
Thus, coefficient of viscosity is defined as the viscous force acting per unit area having unit velocity gradient.
Dimensional formula of coefficient of viscosity
Poiseuille’s Formula
Poiseuille concluded that the volume ‘V’ of the liquid flowing per second through a capillary tube is:
- directly proportional to the pressure difference between two ends: V ∝ P,
- directly proportional to the fourth power of radius of tube: V ∝ r4,
- inversely proportional to the coefficient of viscosity of liquid: V ∝ 1/η,
- inversely proportional to the length of capillary tube: V ∝ 1/l.
Combining equations (i), (ii), (iii) & (iv), we get:
This is called Poiseuille’s formula.
Derivation of Poiseuille’s Formula by Dimensional Method
Consider a liquid through a capillary tube of radius r with length ‘l’ such that pressure difference between two end of tube is P = P1 − P2.
According to Poiseuille’s, the volume per second ‘V’ depends upon:
- the pressure gradient (P/l),
- radius of capillary tube (r),
- coefficient of viscosity (η).
The dimensional equation of eqn (i) is:
Equating dimensions:
Putting a = 1, b = 4, c = −1 in eqn (i):
Which is Poiseuille’s formula.
Stoke’s Law
When a spherical body falls through liquid, the upthrust and viscous force act in upward direction and weight of the body acts in downward direction. With increase in velocity of the body, the viscous force also increases and at certain time when total upward forces equal to downward force, the body falls with constant velocity called terminal velocity.
Derivation of Stoke’s Law by Dimensional Method
Consider a spherical body of radius ‘r’ falling through a liquid of coefficient of viscosity (η) with terminal velocity ‘v’. According to Stoke’s, viscous force (F) depends upon:
- coefficient of viscosity of liquid (η),
- terminal velocity of the body (v),
- radius of spherical body (r).
Dimensional equation of eqn (i):
Equating dimensions of both sides:
Putting a = 1, b = 1, c = 1 in eqn (i):
This is Stoke’s law.
Measurement of Coefficient of Viscosity by Using Stoke’s Law
Let us consider a sphere of radius ‘r’ and density ‘ρ’ falling through a liquid of density ‘σ’ with terminal velocity ‘v’. In this case force acting on a body are:
i) Weight of sphere (W) = mg:
ii) Upthrust (U) = weight of liquid displaced:
iii) Viscous force:
When speed of the body in downward direction increases, the viscous force is also increased and at certain time the upward force acting on a body equals to downward force and body attains equilibrium.
At equilibrium:
This is required expression for coefficient of viscosity.
Bernoulli’s Theorem
Let us consider a pipe AB in which non-viscous and incompressible fluid is flowing. Let a1, P1, v1, h1 and a2, P2, v2, h2 be the area of cross-section, pressure, velocity and height at end A & B respectively. Also let in time Δt the fluid at end A reach to A′ and at end B′ reach to B.
Force acting on the fluid at end A:
Similarly, workdone by fluid at end B′ to B:
Thus, net workdone on the fluid from moving A to B:
From equation of continuity:
Let ‘m’ be the mass of liquid flowing through the pipe in Δt. Then change in K.E.:
Similarly, change in P.E.:
Total workdone = Total change in energy:
Equation of Continuity
Let us consider a liquid flowing through pipe of cross-sectional area ‘a1’ and ‘a2’ at left & right end respectively. Also let v1, ρ1 and v2, ρ2 are velocities and density of liquid at respective end.
Volume per second of liquid entering into the pipe at left end:
Again, mass of liquid entering per second at left end:
Similarly, mass of liquid per second at right end:
If there is no loss of liquid in tube, then:
Also, if liquid is incompressible, then ρ1 = ρ2:
This is called equation of continuity. This equation states that if the area of cross-section of the tube becomes larger then liquid’s speed becomes smaller and vice-versa.
Numericals — Viscosity, Stoke’s Law, Bernoulli & Continuity
Q.5 — Mass of an aeroplane
Calculate mass of an aeroplane with the wings of area 55 m2 flying horizontally. The velocity of air above & below the wings is 155 m/s and 140 m/s respectively.
Then, by Bernoulli’s equation:
Q.6 — Terminal velocity of an air bubble
Calculate the magnitude and direction of the terminal velocity of an 1 mm radius air bubble rising in an oil of viscosity 0.20 N m−2s and specific gravity of 0.9 & density of air 1.29 kg/m3.
Q.7 — Eight raindrops coalesce
Eight/three/two spherical raindrops of equal size are falling vertically through air with terminal velocity 0.15 m/s. If they coalesce what would be the terminal velocity?
By Stoke’s relation v ∝ r²:
Q.8 — Steel ball in castor oil
Castor oil at 20°C has a coefficient of viscosity 2.42 N m−2s and density 940 kg/m3. Calculate the terminal velocity of the steel ball of radius 2.00 mm falling under gravity in the oil, taking density of steel as 7800 kgm−3.
Q.9 — Water through non-uniform pipe
Water flows steadily through a horizontal pipe of non-uniform cross-section. If the pressure of the water is 4×104 N m−2 at a point where the velocity of flow is 2 m s−1 & cross-section is 0.02 m2, what is the pressure at the point where cross section reduces to 0.01 m2?
From equation of continuity:
Again, from Bernoulli’s principle (where h1 = h2):
Q.10 — Glass ball falling through glycerol
What is the terminal velocity of the glass ball falling through a tall jar containing glycerol? The densities of the glass ball and glycerol are 8.5 g/cc and 1.32 g/cc respectively & viscosity of the glycerol is 0.85 poise & radius of the ball is 2 mm.
Discussion
Share a helpful question, idea, or explanation with other students.