First slide

Surface tension

Question

A soap film measure 10 cm x 6 cm. It is increased to 10 cm x 12 cm. If surface tension is 30 x $10^{- 3}$ newton per metre, then the work done is

Moderate

A

$3.6 \times 10^{6} J$
B

$3.6 \times 10^{- 2} J$
C

$3.6 \times 10^{- 4} J$
D

$3.6 \times 10^{3} J$

Solution

$W = 2 [120 \times 10^{- 4} - 60 \times 10^{- 4}] 30 \times 10^{- 3} J$
= $2 \times 60 \times 10^{- 4} \times 30 \times 10^{- 3} J = 3.6 \times 10^{- 4} J$

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