Properties of Matter 4 Question 4
6. An open glass tube is immersed in mercury in such a way that a length of $8 cm$ extends above the mercury level. The open end of the tube is then closed and sealed and the tube is raised vertically up by additional $46 cm$. What will be length of the air column above mercury in the tube now? (Atmospheric pressure $=76 cm$ of $Hg$ )
(2014 Main)
(a) $16 cm$
(b) $22 cm$
(c) $38 cm$
(d) $6 cm$
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Answer:
Correct Answer: 6. (b)
Solution:
- Decrease in surface energy = heat required in vaporisation.
$$ \begin{aligned} & & S & =4 \pi r^{2} \\ & \therefore & d S & =2(4 \pi r) d r \\ & \therefore & T(d S) & =L(d m) \\ & \therefore & T(2)(4 \pi r) d r & =L\left(4 \pi r^{2} d r\right) \rho \\ & \therefore & r & =\frac{2 T}{\rho L} \end{aligned} $$
