# Find the surface tension in soap bubble of 40 mm diameter when the inside pressure is 2.5 N/m2 above atmospheric pressure.

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Find the surface tension in soap bubble of 40 mm diameter when the inside pressure is 2.5 N/m2 above atmospheric pressure.
1. 0.0064 N/m
2. 0.860 N/m
3. 1.265 N/m
4. 0.0125 N/m

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Correct Answer - Option 4 : 0.0125 N/m

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:

Given:

d = 40 mm, ΔP = 2.5 N/m2

${\rm{Δ }}P = \frac{{8\sigma}}{d}$

$\sigma = \frac{{Δ P\times d}}{8 } =\frac{{40\times 0.0025}}{8 }= 0.0125$ N/m.