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 (a) 0.250 N m ⁻¹ (b) 0.125 N m ⁻¹ (c) 0.2 N m ⁻¹ (d) 8.0 N m ⁻¹

Answer: (b). Film has two surfaces: W = 2TΔ A. Δ A = 20 - 8 = 12 cm² = 12×10⁻⁴ m² T = 3×10⁻⁴2 × 12×10⁻⁴ = 0.125 N m⁻¹ So the answer is (b) 0.125 N m⁻¹.

A rectangular film of liquid is extended from (4 cm × 2 cm) to (5 cm… | NEET 2016 (Phase 2)

Q29NEET 2016 · PHASE 2Medium

Surface Energy & Work Done

A rectangular film of liquid is extended from (4 cm $\times$ 2 cm) to (5 cm $\times$ 4 cm). If the work done is 3 $\times$ 10 $-$ 4 J, the value of the surface tension of the liquid is

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Answer: (b)

Film has two surfaces: $W = 2T\Delta A$ .
$\Delta A = 20 - 8 = 12\text{ cm}^2 = 12\times10^{-4}\text{ m}^2$
$T = \dfrac{3\times10^{-4}}{2 \times 12\times10^{-4}} = 0.125\text{ N m}^{-1}$
So the answer is (b) $0.125\text{ N m}^{-1}$ .

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