Question Detail
Question
A soap bubble has two surfaces: an inner surface and an outer surface. How does the gauge pressure P inside a soap bubble relate to its surface tension γ and radius r?
Correct Answer
Option C is the correct answer.
Detailed Explanation
A soap bubble has two liquid surfaces in contact with air, both contributing to the inward pull. The force balance leads to a pressure difference that is twice that of a simple droplet or a bubble in a liquid. The gauge pressure inside a soap bubble is given by P = 4γ / r.
Hint
Consider how many surfaces are contributing to the surface tension force for a soap bubble compared to a liquid droplet.