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JEE Main 202621 JanuaryEvening Shift
Exam
JEE Main
Year
2026
Date
21 January
Shift
Evening Shift
Subject
Physics
Chapter
Mechanical Properties of Fluids

JEE Main 2026 · 21 January · Evening Shift · Physics · Mechanical Properties of Fluids

Surface tension of two liquids (having same densities), T_ 1 and T_...

Surface tension of two liquids (having same densities), T1T_{1} and T2T_{2}, are measured using capillary rise method utilizing two tubes with inner radii of r1r_{1} and r2r_{2} where r1>r2r_{1}>r_{2}. The measured liquid heights in these tubes are h1h_{1} and h2h_{2} respectively. [Ignore the weight of the liquid about the lowest point of miniscus]. The heights h1h_{1} and h2h_{2} and surfaces tensions T1T_{1} and T2T_{2} satisfy the relation:

Options

  1. Ah1>h2 and T1=T2h_{1}>h_{2} \text{ and } T_{1}=T_{2}
  2. Bh1=h2 and T1=T2h_{1}=h_{2} \text{ and } T_{1}=T_{2}
  3. Ch1>h2 and T1<T2h_{1} > h_{2} \text{ and } T_{1} < T_{2}
  4. Dh1<h2 and T1=T2h_{1} < h_{2} \text{ and } T_{1}=T_{2}

Correct answer

D. h1<h2 and T1=T2h_{1} < h_{2} \text{ and } T_{1}=T_{2}

Step-by-step solution

For capillary rise, the height of liquid in a tube is given by: h=2Tcos⁡θρgrh = \frac{2T\cos\theta}{\rho g r}, where TT is surface tension and rr is the tube radius.

For two liquids with equal density and contact angles:
h1=2T1cos⁡θρgr1h_1 = \frac{2T_1\cos\theta}{\rho g r_1} and h2=2T2cos⁡θρgr2h_2 = \frac{2T_2\cos\theta}{\rho g r_2}

Taking the ratio: h1h2=T1T2×r2r1\frac{h_1}{h_2} = \frac{T_1}{T_2} \times \frac{r_2}{r_1}

Since r1>r2r_1 > r_2, we have r2r1<1\frac{r_2}{r_1} < 1. The capillary rise is inversely proportional to tube radius, so the smaller radius tube will have greater height. Since the liquids are measured to have heights h1h_1 and h2h_2 with r1>r2r_1 > r_2, and they have the same density, the empirical relation shows that both liquids rise to different heights inversely proportional to their radii. For liquids with the same density measured in different tubes, if the heights and radii satisfy the capillary rise equation, the surface tensions must be equal: T1=T2T_1 = T_2, and h1<h2h_1 < h_2 since r1>r2r_1 > r_2.
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