Question

A soap bubble contains two surface layers. What is the excess pressure just inside the first layer when seen from outside the bubble?

(a) 2S/R

(b) 4S/R

(c) -2S/R

(d) -4S/R

The question was asked in final exam.

This interesting question is from Fluids Mechanical Properties topic in division Mechanical Properties of Fluids of Physics – Class 11

Answer 1

Right option is (a) 2S/R

The best explanation: Let the pressure outside the bubble be P1, between the two layers be P2 and pressure inside the bubble be P3.

P2 – P1 = 2S/R

P3 – P2 = 2S/R                                                                                                                                                                 ∴ P3 – P1 = 4S/R

This shows that excess pressure just inside the first layer is 2S/R                                                                              while excess pressure completely inside the bubble is 4S/R.