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Question

A film of water is formed between two straight parallel wires each 10 cm long and at a separation of 0.5cm. Calculate the work required to increase the distance between them by 1mm. The surface tension of water =72×10-30N/m


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Solution

Step 1:Given data:

Length of wires =10cm=0.1m

Separation between wires=0.5cm=5×10-3m

Distance to be increased between wires=1mm=1×10-3m

The surface tension of water=72×10-30N/m

Change in surface area = ΔS

Surface Tension = T

Step 2: Formulae used

Change in surface area=Area of films ×no.of films

Work done=ΔS×T

Step 3:Solution

Work done by water to increase the separation= Change in surface area ×Surface Tension

Change in surface area=ΔS=Area of films ×no.of films

=0.1×1×10-3×2=2×10-4( Water has 2 films )

Work done=ΔS×T

==2×10-3×72×10-30=144×10-7J=1.44×10-2J

So, Work done to increase the separation between 2 parallel wires by 1mm is equal to 1.44×10-2J


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