First slide

Surface tension

Question

A wire ring of diameter 14 cm is gently lowered on to a liquid surface and then pulled up. When the film just breaks, the force required is 0.0616 N. The surface tension of the liquid is

Moderate

A

$70 N m^{- 1}$
B

$7 {Nm}^{- 1}$
C

$70 dyne {cm}^{- 1}$
D

None of these

Solution

$2 \times 2 πr \times σ = 0.0616 \times 10^{5} (in CGS)$
or $σ = \frac{6160}{2 \times 2 \times (\frac{22}{7}) \times 7} dyne {cm}^{- 1}$
= 70 dyne ${cm}^{- 1}$

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