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

Work done = Surface tension of film × Change in area of the film or, W=T×ΔA Here, A1=4 cm×2 cm=8 cm2A2=5 cm×4 cm=20 cm2ΔA=2A2-A1=24 cm2=24×10-4 m2W=3×10-4 J,T=?∴  T=WΔA=3×10-424×10-4=18=0.125 N m-1