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NEET PHYSICSMechanical Properties of FluidEasy

Question

A soap bubble, having a radius of 1 mm1 \text{ mm}, is blown from a detergent solution having a surface tension of 2.5×102 N/m2.5 \times 10^{-2} \text{ N/m}. The pressure inside the bubble equals the pressure at a point Z0Z_0 below the free surface of the water in a container. Taking g=10 m/s2g = 10 \text{ m/s}^2 and the density of water ρ=103 kg/m3\rho = 10^3 \text{ kg/m}^3, the value of Z0Z_0 is:

A

0.5 cm

B

100 cm

C

10 cm

D

1 cm

Step-by-Step Solution

  1. Excess Pressure in a Soap Bubble: A soap bubble has two free surfaces (inner and outer). Therefore, the excess pressure (PexcessP_{excess}) inside the bubble is given by the formula ΔP=4Sr\Delta P = \frac{4S}{r}, where SS is the surface tension and rr is the radius .
  2. Hydrostatic Pressure: The gauge pressure at a depth Z0Z_0 below the free surface of a liquid is given by Pgauge=Z0ρgP_{gauge} = Z_0 \rho g .
  3. Equating Pressures: The problem states that the absolute pressure inside the bubble (Patm+ΔPP_{atm} + \Delta P) equals the absolute pressure at depth Z0Z_0 (Patm+Z0ρgP_{atm} + Z_0 \rho g). Therefore, the excess pressure equals the gauge pressure due to the water column: 4Sr=Z0ρg\frac{4S}{r} = Z_0 \rho g
  4. Calculation:
  • Surface Tension, S=2.5×102 N/mS = 2.5 \times 10^{-2} \text{ N/m}
  • Radius, r=1 mm=103 mr = 1 \text{ mm} = 10^{-3} \text{ m}
  • Density, ρ=103 kg/m3\rho = 10^3 \text{ kg/m}^3
  • Gravity, g=10 m/s2g = 10 \text{ m/s}^2

Substituting the values: Z0=4SrρgZ_0 = \frac{4S}{r \rho g} Z0=4×(2.5×102)(103)×103×10Z_0 = \frac{4 \times (2.5 \times 10^{-2})}{(10^{-3}) \times 10^3 \times 10} Z0=10×10210=102 mZ_0 = \frac{10 \times 10^{-2}}{10} = 10^{-2} \text{ m} Z0=1 cmZ_0 = 1 \text{ cm}

Exam Context & Concepts Covered

This question aligns with the NEET PHYSICS syllabus, specifically targeting concepts from Mechanical Properties of Fluid. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

PHYSICSMechanical Properties of Fluidbubblehavingradiusdetergentsolution

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