Find the surface tension in a soap bubble of 80 mm diameter, having an internal pressure 3 N/m² in excess of the outside pressure.

Concept:

Surface tension on a liquid droplet (Spherical droplet of water): Pressure intensity inside the droplet:

\(p = \frac{{4\sigma }}{d}\)

Surface tension on a hollow bubble (Soap bubble): Pressure intensity inside the droplet:

\(p = \frac{{8\sigma }}{d}\)

where σ is the surface tension of the liquid.

Calculation:

\({\rm{\Delta }}P = \frac{{8\sigma}}{d}\)

\(\sigma = \frac{{\Delta P\times d}}{8 } =\frac{{3\times 0.080}}{8 }= 0.03 \; N/m\)