# Solved Examples  3.    A steel wire of length 2.0 m is stretched through 2.0 mm. The cross-sectional area of the wire is 4.0 mm2. Calculate the elastic potential energy stored in the wire

in the stretched condition. Young’s modulus of steel = 2.0 × 1011 N/m2. 4.    Water is kept in a beaker of radius 5.0 cm. Consider a diameter of the beaker on the surface of the water. Find the force by which the surface on one side of the

diameter pulls the surface on the other side. Surface tension of water = 0.075 N/m. 5.    A water drop of radius 10–2 m is broken into 1000 equal droplets. Calculate the gain in surface energy. Surface tension of water is 0.075 N/m.

Sol. The volume of the original drop is 6.    Find the excess pressure inside a mercury drop of radius 2.0 mm. The surface tension of mercury = 0.464 N/m.

Sol.  The excess pressure inside the drop is

P2 = P1 = 2S/R 7.    A 0.02 cm liquid column balances the excess pressure inside a soap bubble of radius 7.5 mm. Determine the density of the liquid. Surface tension of soap solution = 0.03 N/m.

Sol. The excess pressure inside a soap bubble is 8.    A capillary tube of radius 0.20 mm is dipped vertically in water. Find the height of the water column raised in the tube. Surface tension of water = 0.075 N/m and density of water = 1000 kg/m3. Take g = 10 m/s2.
Sol.    We have,