First slide

Surface tension and surface energy

Question

The pressure of air in a soap bubble of 0.7cm diameter is 8 mm of water above the pressure outside. The surface tension of the soap solution is

Moderate

A

$100 d y n e / c m$
B

$68.66 d y n e / c m$
C

$137 d y n e / c m$
D

$150 d y n e / c m$

Solution

$\begin{array}{l} ΔP = \frac{4 T}{r} = hdg \Rightarrow T = \frac{rhdg}{4} = \frac{0 .35 \times 0 .8 \times 1 \times 10^{3}}{4} \\ = 70 dyne/cm \equiv 68 .66 dyne/cm \end{array}$

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