A very long cylindrical soap bubble in air has radius R. If the surface tension of the soap solution is S, find the excess pressure inside the bubble.
Options

Question

A very long cylindrical soap bubble in air has radius R. If the surface tension of the soap solution is S, find the excess pressure inside the bubble.

G1 Physics · Accepted Answer

Step 1: Young–Laplace law for one interface is ΔP = S(1/R₁ + 1/R₂). Step 2: For a long cylinder, principal radii are R₁ = R and R₂ → ∞, giving ΔPone = S/R. Step 3: A soap film has two surfaces (inner and outer), so pressure jumps add. Step 4: ΔP = 2(S/R) = 2S/R. Final answer: 2S/R (Option A).