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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Properties of Fluids Formulae Page 1

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1. Surface Energy


Surface energy = Surface tension surface area


2. Rise of Liquid in a Capillary



2 T cos q

 , where T is the surface tension of the liquid, q

r r g


is the angle of contact, r is the radius of the capillary bore and r the density of the liquid.


3. Free Surface Energy


For a drop, E = 4pr2T

For a bubble, E = 2 4pr2T, since a bubble has two free


4. Capillary Rise


Difference in heights in the two limbs of a U tube capillary, which is vertical, is



given by, h =  

2 T COsq




, where r1 and r2 are the radii of the two limbs of the capillary.




r g



 For two different capillaries: (same liquid)
    h1r1 = h2r2 


5. Total Surface Area of a Soap Bubble


Total surface area of a soap bubble = 2 surface area


6. Angle of Contact for Water and Glass


For water, angle of contact = q = 0 \ COsq = 1


r = 1 gm/cc : T =





7. Work Done in Breaking a Liquid Drop


If a liquid drop breaks into number of spherical droplets,
Work done =
T increase in surface area


8. Energy Released During Formation of a Liquid Drop


If liquid drops coalesce to form a single drop,
Decrease in surface energy =
T decrease in surface area


9. Work Done in Increasing the Radius of a Soap Bubble


Work done in increasing the radius of a soap bubble
T 2 increase in surface area


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