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JEE Advanced 2020Paper 1
Exam
JEE Advanced
Year
2020
Paper
Paper 1
Subject
Physics
Chapter
Mechanical Properties of Fluids

JEE Advanced 2020 · Paper 1 · Physics · Mechanical Properties of Fluids

When water is filled carefully in a glass, one can fill it to a...

When water is filled carefully in a glass, one can fill it to a height hh above the rim of the glass due to the surface tension of water. To calculate hh just before water starts flowing, model the shape of the water above the rim as a disc of thickness hh having semicircular edges, as shown schematically in the figure. When the pressure of water at the bottom of this disc exceeds what can be withstood due to the surface tension, the water surface breaks near the rim and water starts flowing from there. If the density of water, its surface tension and the acceleration due to gravity are 103kgm−3,0.07Nm−1{10}^{3}kg{m}^{- 3}, 0 . 07N{m}^{- 1} and 10ms−210m{s}^{- 2} respectively, the value of hh (in mmmm) is ________.

When water is filled carefully in a glass, one can fill it to a height h above the rim of the glass due to the surface

Correct answer

0

Step-by-step solution

Pressure at the bottom of disc == pressure due to surface tension ρgh=T(1R1+1R2)R1≫>R2 So 1R1≪<1R2 and  R2=h/2∴ρgh=T[1R1+1R2]=T[0+1h/2]h2=2Tρgh=2Tρg=2×0.07103×10=14×100104×100h=14mm=3.741\begin{aligned} &\rho g h=T\left(\frac{1}{\mathrm{R}_1}+\frac{1}{\mathrm{R}_2}\right) \\ &\mathrm{R}_1 \gg>\mathrm{R}_2 \\ &\text{ So } \frac{1}{\mathrm{R}_1} \ll<\frac{1}{\mathrm{R}_2} \text{\text{ and } } \mathrm{R}_2=\mathrm{h} / 2 \\ &\therefore \rho \mathrm{gh}=\mathrm{T}\left[\frac{1}{\mathrm{R}_1}+\frac{1}{\mathrm{R}_2}\right]=\mathrm{T}\left[0+\frac{1}{\mathrm{ h} / 2}\right] \\ &\mathrm{h}^2=\frac{2 \mathrm{ T}}{\rho \mathrm{g}} \\ &\mathrm{h}=\sqrt{\frac{2 \mathrm{ T}}{\rho \mathrm{g}}}=\sqrt{\frac{2 \times 0.07}{10^3 \times 10}}=\sqrt{\frac{14 \times 100}{10^4 \times 100}} \\ &\mathrm{ h}=\sqrt{14} \mathrm{ mm}=3.741 \end{aligned})$
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