Q.A soap bubble of radius $1.0\ \text{cm}$ is blown in air using a soap solution of surface tension $2.5\times10^{-2}\ \text{N/m}$. Calculate the excess pressure inside the bubble over the pressure of the surrounding air, remembering that a soap bubble has two free surfaces (an inner and an outer surface), both contributing to the excess pressure.

Given: $r = 1.0\ \text{cm} = 0.01\ \text{m}$, $T = 2.5\times10^{-2}\ \text{N/m}$.

A soap bubble has two free surfaces (an inner and an outer film surface), so: …

Use $\Delta P = 4T/r$ for a soap bubble, since it has two free surfaces under tension, rather than $\Delta P = 2T/r$, which app …

• Using $\Delta P = 2T/r$ (the single-surface formula) for a soap bubble, forgetting that it has two surfaces, both contributing to the excess pressure. …