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NEET PHYSICSMechanical Properties of FluidMedium

Question

Three liquids of densities ρ1,ρ2\rho_1, \rho_2 and ρ3\rho_3 (with ρ1>ρ2>ρ3\rho_1 > \rho_2 > \rho_3), having the same value of surface tension TT, rise to the same height in three identical capillaries. The angles of contact θ1,θ2\theta_1, \theta_2 and θ3\theta_3 obey:

A

π2>θ1>θ2>θ30\frac{\pi}{2} > \theta_1 > \theta_2 > \theta_3 \ge 0

B

0θ1<θ2<θ3<π20 \le \theta_1 < \theta_2 < \theta_3 < \frac{\pi}{2}

C

π2<θ1<θ2<θ3<π\frac{\pi}{2} < \theta_1 < \theta_2 < \theta_3 < \pi

D

π>θ1>θ2>θ3>π2\pi > \theta_1 > \theta_2 > \theta_3 > \frac{\pi}{2}

Step-by-Step Solution

  1. Capillary Rise Formula: The height hh to which a liquid rises in a capillary tube of radius rr is given by the formula: h=2Tcosθrρgh = \frac{2T \cos \theta}{r \rho g} where TT is surface tension, θ\theta is the angle of contact, ρ\rho is the density of the liquid, and gg is acceleration due to gravity .
  2. Analyze Dependencies: We are given that h,T,r,h, T, r, and gg are the same for all three liquids. Rearranging the formula for the variable terms: cosθ=hrg2Tρ\cos \theta = \frac{h r g}{2T} \cdot \rho Since hrg2T\frac{h r g}{2T} is a constant (KK), we have: cosθ=Kρ\cos \theta = K \rho This implies cosθρ\cos \theta \propto \rho.
  3. Apply Conditions: We are given densities ρ1>ρ2>ρ3\rho_1 > \rho_2 > \rho_3. Therefore: cosθ1>cosθ2>cosθ3\cos \theta_1 > \cos \theta_2 > \cos \theta_3
  4. Behavior of Cosine: For capillary rise (h>0h>0), the angle of contact θ\theta must be acute, i.e., 0θ<π20 \le \theta < \frac{\pi}{2}. In this range, the cosine function is a decreasing function (as angle increases, cosine value decreases). Therefore, the inequality for the angles reverses: θ1<θ2<θ3\theta_1 < \theta_2 < \theta_3
  5. Conclusion: The correct relationship is 0θ1<θ2<θ3<π20 \le \theta_1 < \theta_2 < \theta_3 < \frac{\pi}{2}.

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 Fluidliquidsdensitieshavingsurfacetension

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