A film of soap is formed between two straight parallel wires of length 10 cm each separated by 0.5 cm. If their separation is increased by 1 mm while still maintaining their parallelism. How much work will have to be done? (surface tension of = 65 × 10⁻² N/m) (a) 7.22 × 10⁻⁶ J (b) 13.0 × 10⁻⁵ J (c) 2.88 × 10⁻⁵ J (d) 5.76 × 10⁻⁵ J

Answer: (b). Film has two surfaces: W = 2TΔ A, Δ A = lΔ x. Δ A = 0.1 × 10⁻³ = 10⁻⁴ m² W = 2 × 65×10⁻² × 10⁻⁴ = 1.3×10⁻⁴ = 13.0×10⁻⁵ J So the answer is (b) 13.0×10⁻⁵ J.

A film of soap is formed between two straight parallel wires of… | MHT CET 2023 (9 May, Shift 2)

Q22MHT CET 2023 · 9 MAY · SHIFT 2Medium

Surface Energy & Work Done

A film of soap is formed between two straight parallel wires of length $10\text{ cm}$ each separated by $0.5\text{ cm}$ . If their separation is increased by $1\text{ mm}$ while still maintaining their parallelism. How much work will have to be done?

(surface tension of $= 65 \times 10^{-2}\text{ N/m}$ )

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Answer: (b)

Film has two surfaces: $W = 2T\Delta A$ , $\Delta A = l\Delta x$ .
$\Delta A = 0.1 \times 10^{-3} = 10^{-4}\text{ m}^2$
$W = 2 \times 65\times10^{-2} \times 10^{-4} = 1.3\times10^{-4} = 13.0\times10^{-5}\text{ J}$
So the answer is (b) $13.0\times10^{-5}\text{ J}$ .

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