# Fluid Mechanics Questions and Answers – Surface Tension, Capillarity, Vapour Pressure and Cavitation

This set of Fluid Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Surface Tension, Capillarity, Vapour Pressure and Cavitation”.

1. Calculate the magnitude of capillary effect in millimeters in a glass tube of 7mm diameter, when immersed in mercury. The temperature of the liquid is 25℃ and the values of surface tension of mercury at 25℃ is 0.51 N/m. The angle of contact for mercury is 130°.
a) 140
b) 280
c) 170
d) 210

Explanation: Capillarity rise or fall
h=4*cosθ*σ/ρ*g*d
=4*cos130*0.51/13600*9.81*0.007
=140 mm.

2. Determine the minimum size of glass tube that can be used to measure water level if the capillary rise in the tube is restricted to 5mm. Consider surface tension of water in contact with air as 0.073 N/m
a) 5.95mm
b) 11.9mm
c) 2.97mm
d) 4.46mm

Explanation: d=4*cosθ*σ/ρ*g*h
=4*1*0.073/1000*9.81*0.005
=5.95mm.

3. An oil of vicosity 7 poise is used for lubrication between shaft and sleeve. The diameter of shaft is 0.6 m and it rotates is 360 rpm. Calculate the power lost in oil for a sleeve length of 160mm. The thickness of oil film is 1.0mm
a) 25.31 kW
b) 50.62 kW
c) 37.97 kW
d) 12.65 kW

Explanation: Power lost= torque * angular velocity
= shear stress * area* radius* angular velocity
Shear Stress = viscosity* velocity gradient
Power lost= 7916.8*3.142*0.3*0.3*0.3*2*3.142*60
= 25.31 kW.

4. Find the capillarity rise or fall if a capillary tube of diameter .03m is immersed in hypothetical fluid with specific gravity 6.5, surface tension 0.25 N/m and angle of contact 147°.
a) 0.44mm fall
b) 0.88mm fall
c) 0.44mm rise
d) 0.88mm rise

Explanation: h=4*cosθ*σ/ρ*g*d
=4*cos147*0.25/6.5*1000*9.81*0.03
=-0.44 mm i.e 0.44 mm fall.

5. Will capillary rise occur and if it occurs what will be capillary rise if glass capillarity tube is immersed in water and experiment is carried out by astronauts in space.
a) Capillarity rise will not occur
b) Capillarity rise will occur infinitely and will come out in form of fountain
c) Capillarity rise will occur finitely and will be the whole length of tube
d) None of the mentioned

Explanation: Capillary rise is given by
h=4*cosθ*σ/ρ*g*d
hence rise is inversely proportional to g
In space g is 0 m/s2
Hence, capillarity rise will occur finitely and will be the whole length of tube.

6. The surface tension of fluid in contact with air at 25℃ is 0.51N/m. The pressure inside a droplet is to be 0.05 N/cm2 greater than outside pressure. Determine the diameter of the droplet of water.
a) 4.08mm
b) 8.16mm
c) 2.04mm
d) None of the mentioned

Explanation: P=4*σ/d
d= 4*.51/500
=4.08 mm.

7. If a fluid of certain surface tension and diameter is used to create a soap bubble and a liquid jet. Which of the two, bubble or liquid jet, will have greater pressure difference on the inside and outside.
a) Liquid jet
b) Soap bubble
c) Both will have same pressure differrence
d) None of the mentioned

Explanation: For soap bubble,
P=8*σ/d
For liquid jet,
P=2*σ/d
Hence, soap bubble will be having more pressure difference.

8. Capillarity fall is reduced if we take the appartus (capillary tube immersed in fluid having acute angle of contact) considerable distance inside the earth( i.e below the earth crust).
a) True
b) False

Explanation: Capillary rise is given by
h=4*cosθ*σ/ρ*g*d
Inside the earth, g (acceleration due to gravity) decreases. Hence, capillary rise will increase compared to that on the earth’s surface.

9. For liquid fluids will capillarity rise (or fall) increase or decrease with rise in temperature.
a) Increase
b) Decrease
c) Remain constant
d) First decrease then increase

Explanation: Capillary rise is given by
h=4*cosθ*σ/ρ*g*d
As temperature increases, σ(surface tension) decreases. Therefore, correspondingly rise(or fall) will decrease as their is direct proportional relation between the two.

10. Cavitation is more pronounced in rough pipes than smooth surfaced pipes.
a) True
b) False

Explanation: Rough surfaced pipes have more friction with the fluid and hence possibility of cavitation is more pronounced.

Sanfoundry Global Education & Learning Series – Fluid Mechanics.

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