Question Details

The radii of two soap bubbles are R₁ and R₂ respectively. The ratio of masses of air in them will be

Options

A

R₁³/R₂³

B

R₂³/R₁³

C

(P+4T/R₁)R₁³/(P+4T/R₂)R₂³

D

(P+4T/R₂)R₂³/(P+4T/R₁)R₁³

Correct Answer :

(P+4T/R₁)R₁³/(P+4T/R₂)R₂³

Solution :

To find the ratio of the masses of air inside two soap bubbles of radii R1 and R2, we can use the ideal gas equation and the concept of excess pressure inside a soap bubble.

According to the ideal gas equation:
PV=nRT
where:
- P is the absolute pressure of the air inside the bubble,
- V is the volume of the bubble,
- n is the number of moles of air, which is proportional to the mass m (n=m/M, where M is the molar mass of air),
- R is the universal gas constant, and
- T is the absolute temperature.

Since the temperature T and the molar mass M of air are the same for both bubbles, the mass m of air inside a bubble is directly proportional to the product of its absolute pressure and volume:
mPV

For a soap bubble of radius R, surface tension T, and atmospheric pressure P outside:
The excess pressure inside the soap bubble is given by:
ΔP=4TR
Thus, the absolute pressure Pin inside the bubble is:
Pin=P+4TR

The volume of a spherical bubble of radius R is:
V=43πR3

Substituting the absolute pressure and volume into the relation for mass, we get:
m(P+4TR)·43πR3
Since 43π is a constant, we have:
m(P+4TR)R3

Therefore, the ratio of the masses of air in the two bubbles is:
m1m2=(P+4TR1)R13(P+4TR2)R23

This matches the correct option.

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