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Question

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=72×103 N/m.

A
144×107J
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B
104×107 J
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C
72×107 J
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D
288×107 J
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Solution

The correct option is A 144×107J
Given:
As film has two layer.
Initial surface area=2×(length of wires × separation between them)
=2×10 cm×0.5 cm
=10×104 m2
Final surface area=2×(length of wires × separation between them)
=2×10 cm×(0.5+0.1) cm
=12×104 m2

The required work,
W=TΔA
=72×103×(12×10410×104) J
=144×107 J

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