A rectangular film of liquid is extended from (4 cm × 2cm) to (5 cm × 4 cm). If the work done is 3 × 10–4 J, the value of the surface tension of the liquid is

  • 8.0 Nm–1
  • 0.250 N m–1
  • 125 Nm–1
  • 0.2 Nm–1
  • Explanation:

    Increase in surface area = (20 cm2-8 cm2)×2

    = 12×2  cm2

    = 24  cm2 (film has two surfaces)

    So work done = T Δ S = T × 24 × 10-4 = 3×10-4

    so T = 3/24 N/m = 1/8 Nm-1 = 0.125 N/m

A rectangular film of liquid is extended from (4 cm × 2cm) to (5 cm × 4 cm). If the work
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