A film of water is formed between two straight parallel wires each 10 cm long and at separation 0.5 cm. Calculate the work required to increase 1 mm distance between the wires. Surface tension of water=72×10−3N/m.
A
144×10−7J
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B
104×10−7 J
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C
72×10−7 J
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D
288×10−7 J
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Solution
The correct option is A144×10−7J Given:
As film has two layer.
Initial surface area=2×(length of wires × separation between them) =2×10 cm×0.5 cm =10×10−4 m2
Final surface area=2×(length of wires × separation between them) =2×10 cm×(0.5+0.1) cm =12×10−4 m2
The required work, W=TΔA =72×10−3×(12×10−4−10×10−4) J =144×10−7 J