NEET 2019 · PhysicsExcess pressurePrevious Year Question
A soap bubble of radius 1 mm is blown from a detergent solution of surface tension 2.5 × 10⁻² N m⁻¹. The pressure inside the bubble equals the pressure at a point Z₀ below the free surface of water in a container. Taking g = 10 m s⁻² and density of water = 10³ kg m⁻³, the value of Z₀ is:
Answer: (C) 1 cm. Excess pressure in a soap bubble = 4T/r = 4 × 2.5×10⁻² / 10⁻³ = 100 Pa.
- A.100 cm
- B.10 cm
- C.1 cm✓
- D.0.5 cm
Correct Answer
(C) 1 cm
Solution & Explanation
Excess pressure in a soap bubble = 4T/r = 4 × 2.5×10⁻² / 10⁻³ = 100 Pa. This equals the hydrostatic gauge pressure ρgZ₀: 100 = 10³ × 10 × Z₀ ⇒ Z₀ = 0.01 m = 1 cm.
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