When you pull the wires apart, the water film stretches, making its area bigger. A thin film has two surfaces (top and bottom), so the increase in area is: ∆A=2×L×∆d Let's define what we have: Length of each wire, L=8cm=0.08m Starting distance, d1=0.6cm=0.006m Final distance, d2=0.8cm=0.008m Change in distance: ∆d=d2−d1=0.008−0.006=0.002m Calculate the change in area: ∆A=2×0.08×0.002=3.2×10−4m2 The work done (energy needed) is given by: W=T∆A Here, surface tension T=0.07Nm−1 So, ‌W=0.07×3.2×10−4=0.224×10−4J ‌W=22.4×10−6J=22.4µJ
