A work of 6.0 x 10⁻⁴ J is needed to enlarge a soap film from 10 cm x 6 cm to 10 cm x 11 cm. Determine the surface tension of the film (in N/m).

5 x 10⁻²
6 x 10⁻²
1.5 x 10⁻²
1.2 x 10⁻²

Correct answer: 6 x 10⁻²

Solution

Change in single-face area = (10*11 - 10*6) = 50 cm² = 50e-4 m². A soap film has two faces, so total area change = 2*50e-4 = 0.01 m². T = W/dA = 6.0e-4/0.01 = 0.06 = 6 x 10⁻² N/m.

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