Page 110 - Physics - XI
P. 110

Result

              The surface tension of the water at t °C by capillary rise tube method is ..... Nm .
                                                                                             –1
              Precautions

              1.  Capillary tubes should be clean and liquid should be free from dirt and grease.
              2.  Capillary tubes and needle should be set vertical.
              3.  Microscope screw should be moved in lower direction only to avoid backlash error.

              4.  Do not use distilled water.
              5.  Internal diameter of capillary tubes should be measured along two mutually perpendicular directions.

              6.  Capillary tubes should be of uniform bore.
              7.  Temperature of the water should be measured in the beginning and also at the end as surface tension is
                  very sensitive to the temperature.


              Sources of Error
              1.  Contamination of water surface cannot be ruled out.

              2.  Capillary tubes may not have uniform bore.


                                                           VIVA  VOCE


               Q1.  What is the reason of existence of a pressure diff erence on the two sides of a curved liquid
                     surface?
              Ans.  Curved free liquid surface has more area. It tends to become fl at to minimise the surface area which
                     is the property of surface tension. Hence, pressure is not the same above and below the free surface
                     of liquid.

               Q2.  Defi ne force of cohesion.
              Ans.  The force of attraction between the molecules of the same substance is called force of cohesion.

               Q3.  Defi ne force of adhesion.
              Ans.  The force of attraction between the molecules of diff erent substances is called force of adhesion.

               Q4.  What is surface tension?
              Ans.  It is the property of the liquid by virtue of which the free surface of the liquid at rest behaves like a
                     stretched elastic membrane which try to acquire the minimum surface area. It is defi ned as the force
                     acting per unit length on an imaginary line drawn on the surface of the liquid in equilibrium.

               Q5.  What is the SI unit of surface tension?
              Ans.  S.I. unit of surface tension is N m .
                                                       –1
               Q6.  What is the cause of surface tension?
              Ans.  It is due to the force of cohesion between the molecules of a liquid.

               Q7.  What is the dimensional formula of surface tension?
                                                   Force
              Ans.  Its dimensional formula is              = [M L T ] or [MT ].
                                                                   0
                                                                     –2
                                                                               –2
                                                                1
                                                   Length
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