A rectangular liquid film is enlarged from 4 cm × 2 cm to 5 cm × 4 cm. If the work done is 3 × 10⁻⁴ J, find the surface tension of the liquid.
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Question

A rectangular liquid film is enlarged from 4 cm × 2 cm to 5 cm × 4 cm. If the work done is 3 × 10⁻⁴ J, find the surface tension of the liquid.

G1 Physics · Accepted Answer

Step 1: Initial area of one face = 4 × 2 = 8 cm². Step 2: Final area of one face = 5 × 4 = 20 cm². Step 3: Increase per face = 12 cm² = 12 × 10⁻⁴ m². Step 4: A liquid film has two surfaces, so total ΔA = 2 × 12 × 10⁻⁴ = 2.4 × 10⁻³ m². Step 5: Work W = TΔA ⟹ T = 3 × 10⁻⁴/(2.4 × 10⁻³) = 0.125 N/m. Final answer: 0.125 N/m (Option D).