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Exam
NEET
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Physics
Chapter
Mechanical Properties of Fluids

NEET · Physics · Mechanical Properties of Fluids

The gauge pressure inside a soap bubble of surface tension T is...

The gauge pressure inside a soap bubble of surface tension TT is equal to the pressure exerted by a liquid column of density ρ\rho and height hh. What is the work done in blowing this soap bubble from zero radius? (Let gg be the acceleration due to gravity)

Options

  1. A64πT3ρ2g2h2\dfrac{64\pi T^3}{\rho^2 g^2 h^2}
  2. B128πT3ρ2g2h2\dfrac{128\pi T^3}{\rho^2 g^2 h^2}
  3. C32πT3ρ2g2h2\dfrac{32\pi T^3}{\rho^2 g^2 h^2}
  4. D16πT3ρ2g2h2\dfrac{16\pi T^3}{\rho^2 g^2 h^2}

Correct answer

B. 128πT3ρ2g2h2\dfrac{128\pi T^3}{\rho^2 g^2 h^2}

Step-by-step solution

Excess pressure inside a soap bubble is given by ΔP=4TR\Delta P = \dfrac{4T}{R}, where RR is the radius of the bubble.

Gauge pressure exerted by a liquid column of height hh and density ρ\rho is P=ρghP = \rho g h.

Equating the two pressures as given:
4TR=ρgh  ⟹  R=4Tρgh\dfrac{4T}{R} = \rho g h \implies R = \dfrac{4T}{\rho g h}

The work done in blowing a soap bubble from zero radius is equal to its surface energy. Since a soap bubble has two free surfaces (inner and outer), the work done is:
W=T×2×4πR2=8πR2TW = T \times 2 \times 4\pi R^2 = 8\pi R^2 T

Substituting the value of RR into the work equation:
W=8πT(4Tρgh)2W = 8\pi T \left(\dfrac{4T}{\rho g h}\right)^2

W=8πT(16T2ρ2g2h2)=128πT3ρ2g2h2W = 8\pi T \left(\dfrac{16T^2}{\rho^2 g^2 h^2}\right) = \dfrac{128\pi T^3}{\rho^2 g^2 h^2}

Note: Using W=4πR2TW = 4\pi R^2 T (forgetting the two surfaces for work) leads to the incorrect option 64πT3ρ2g2h2\dfrac{64\pi T^3}{\rho^2 g^2 h^2}, and using ΔP=2TR\Delta P = \dfrac{2T}{R} (forgetting two surfaces for pressure) leads to the incorrect option 32πT3ρ2g2h2\dfrac{32\pi T^3}{\rho^2 g^2 h^2}.

Answer: 128πT3ρ2g2h2\dfrac{128\pi T^3}{\rho^2 g^2 h^2}
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