A rectangular film of liquid is expanded from (5 cm × 4 cm) to (7 cm × 8 cm). If the work done is 3 × 10⁻⁴ J, the surface tension of the liquid is (nearly) (a) 0.4 N/m (b) 0.04 N/m (c) 0.4 dyne /cm (d) 4.0 N/m

Answer: (b). Film has two surfaces: W = 2TΔ A. Δ A = 56 - 20 = 36 cm² = 36×10⁻⁴ m² T = 3×10⁻⁴72×10⁻⁴ ≈ 0.042 ≈ 0.04 N/m So the answer is (b) 0.04 N/m.

A rectangular film of liquid is expanded from (5 cm × 4 cm) to (7 cm… | MHT CET 2025 (22 Apr, Shift 1)

Q12MHT CET 2025 · 22 APR · SHIFT 1Medium

Surface Energy & Work Done

A rectangular film of liquid is expanded from $(5\text{ cm} \times 4\text{ cm})$ to $(7\text{ cm} \times 8\text{ cm})$ . If the work done is $3 \times 10^{-4}\text{ J}$ , the surface tension of the liquid is (nearly)

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Answer: (b)

Film has two surfaces: $W = 2T\Delta A$ .
$\Delta A = 56 - 20 = 36\text{ cm}^2 = 36\times10^{-4}\text{ m}^2$
$T = \dfrac{3\times10^{-4}}{72\times10^{-4}} \approx 0.042 \approx 0.04\text{ N/m}$
So the answer is (b) $0.04\text{ N/m}$ .

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