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For a surface molecule,...

For a surface molecule,

A

the net force on it is non zero

B

the net force on it zero

C

ther is net downward force

D

there is net upward force

Text Solution

Verified by Experts

The correct Answer is:
C
A surface molecule experiences a net downward force.
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Surface tension arises from the cohesive force between the surface molecules. Interplay between cohesion and adhesion force make the surface inclined at acute or obtuse angle with the contacting solid surfaces. This causes a capillary rise (or fail) given as h=(2Tcostheta)/(rhogr) , where theta= angle of contact T= surface tension rho= density of the liquid, g= acceleration due to gravity and r= radius of the capillary tube. Q. In capillary action theta can be:

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The force of interaction between the molecules of same substance is called cohesive force hile force of interaction between molecules of different substances is called adhesive force . Both the forces are attracive and have a range of about 10 A . If we consider a molecule . A well inside a liquid we find that it is equally attracted by the surrounding molecules in all the dirctions ( with a sphere of radius 10 Å ) and so it does not experience any resultant force . On or near the surface of a liquid , the molecules do not exist on the free side and so a molecule ( say B ) experiences a resultant force vertically downwards into the liquid . For this reason , the molecules of the surface have a tendency to go into the liquid . Thus , the surface tends to contract and gives rise to surface tension . As surface molecules like B experience a net downward force of cohension and have tendency to go down , so in increasing the area of the surface more molecules are to be shifted to the surface from inside . Thus , more mechnical work will have to be done against the net downward force of cohension . This work is stored by the molecules in the surfaces as potential energy called free surface energy . Furthermore , molcules like P , in contact with the liquid will stick to the solid surface , i.e will meet it if the force of adhesion A is greater than net force of cohension C . The liquid will not the solid surface if C gt A Also , molecules like Q at the surface in contact with the solid will experience a net force of cohension C_(R) . If adhesion A is greater then cohension C_(R) , the resultant force will be F . As the liquid can't support tangential force , F must be perpendicular to the surface of the liquid . So , the menisus will be concave and the angle of contact will be acute ( gt 90^(@)) . However , if adhesion is lesser than cohension , the resultant force F will be directed into the liquid and so the menisus ( which is perpendicular to F as liquid support tangential force ) will be convex and the angle of contact will be obtuse ( gt 90^(@)) The free suface energy , stored in the surface is because of work done

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If the water molecules in 1.0 g of water were distributed uniformly over the surface of earth, how many such molecules would there be in 1.0 cm^2 of earth's surface ?

NCERT FINGERTIPS-MECHANICAL PROPERTIES OF FLUIDS-Surface Tension
  1. The rain are in spherical shape due to

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  2. Which of the following statements is not true about surface tension ?

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  3. For a surface molecule,

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  4. Mercury does not wet glass. This is the property of liquid known as

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  5. A thin liquid film formed between a U-shaped wire and a light slider s...

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  6. Which graph represents the variation of surface tension with temperatu...

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  7. Angle of contact of a liquid with a solid depends on

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  8. Which of the following statements is not true about angle of contact ?

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  9. Which of the following statements is correct ?

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  10. A liquid will not wet the surface of a solid if the angle of contact ...

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  11. The soap bubble formed at the end of the tube is blown very slowly. Dr...

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  12. When a drop splits up into number of drops

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  13. The excess pressure inside a soap bubble is thrice the excess pressur...

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  14. Work W is required to form a bubble of volume V from a given solution....

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  15. Find the work done in blowing a soap bubble of surface tension 0.06 Nm...

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  16. The surface tension of soap solution is 0.03 N//m . The work done in b...

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  17. Surface tension of mercury is 0.465 N m^(-1). The excess pressure insi...

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  18. The surface tension of soap solution at a temperature 20^(@)C is 2.5xx...

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  19. In question number 88, if an air bubble of the same dimensions were fo...

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  20. The surface tension and vapor pressure of water at 20^(@)C is 7.28 xx ...

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