First slide

Surface tension and surface energy

Question

If the excess pressure inside a soap bubble is balanced by oil column of height 2 mm, then the surface tension of soap solution will be (r = 1 cm and density d = 0.8 gm/cc)

Moderate

A

3.9 N/m
B

3.9 ×10^–2 N/m
C

3.9 ×10^–3 N/m
D

3.9 dyne/m

Solution

$\begin{array}{l} \frac{4 T}{R} = hdg ∴ T = \frac{Rhdg}{4} \\ T = \frac{10^{- 2} \times 2 \times 10^{- 3} \times 0 .8 \times 10^{3} \times 9 .8}{4} = 3 .9 \times 10^{- 2} N / m \end{array}$

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