First slide

Surface tension

Question

A water film is made between two straight parallel wires of length 10 cm each and at a distance of 0.5 cm from each other. If the distance between the wires is increased by 1 mm, how much work will be done ? Surface tension of water = 72 dynes/cm.

Moderate

A

288 ergs
B

72 ergs
C

144 ergs
D

216 ergs

Solution

Force due to surface tension of the film on the wire.
$F = 2 \times T \times 10 cm$
[Since, there is two free surface]
$∴ Work done in increasing the distance by 1 mm,$
W = F $\times (0.1)$
= $2 \times T \times 10 \times 10 (0.1) = 2 \times 72 \times 10 \times 0.1 = 144 ergs$

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