BCECE Engineering BCECE Engineering Solved Paper-2015

  • question_answer
    An air bubble starts rising from the bottom of a lake. Its diameter is \[\text{3}.\text{6 mm}\] at the bottom and \[\text{4 mm}\] at the surface. The depth of the lake is \[\text{25}0\text{ cm}\] and the temperature at surface is40°C. What is the temperature at the bottom of the lake? Given atmpospheric pressure \[=\text{76 cm}\] of Hg and \[\text{g}=\text{ 98}0\text{ cm}/{{\text{s}}^{\text{2}}}\].

    A) \[\text{11}{}^\circ \text{C}\]                     

    B) \[\text{12}.\text{36}{}^\circ \text{C}\]

    C) \[\text{13}{}^\circ \text{C13}{}^\circ \text{C}\]                 

    D) \[\text{1}0.\text{37}{}^\circ \text{C}\]

    Correct Answer: D

    Solution :

    At the bottom of the lake, volume of the bubble\[{{V}_{1}}=\frac{4}{3}\pi {{r}^{3}},=\frac{4}{3}\pi {{(0.18)}^{3}}\] Pressure on the bubble, \[{{p}_{1}}=\] atmospheric pressure + pressure due to a column of 250 cm of water. \[=76\times 13.6\times 980+250\times 1\times 980\] \[\text{= (76 }\!\!\times\!\!\text{ 13}\text{.6+250) 980 dyne/c}{{\text{m}}^{\text{2}}}\] At the surface of the lake, volume of the bubble \[{{V}_{2}}=\frac{4}{3}\pi r_{2}^{3}=\frac{4}{3}\pi {{(0.2)}^{3}}c{{m}^{3}}\] Pressure on the bubble, \[{{\text{p}}_{\text{2}}}=\]atmospheric pressure \[=\left( \text{76}\times \text{13}.\text{6}\times \text{98}0 \right)\text{ dyne}/\text{c}{{\text{m}}^{\text{2}}}\] \[{{T}_{2}}=273+40=313K\] Now,                     \[\frac{{{p}_{1}}{{V}_{1}}}{{{T}_{1}}}=\frac{{{p}_{2}}{{V}_{2}}}{{{T}_{2}}}\]              \[\Rightarrow \]               \[\frac{(76\times 13.6\times 250)980\times \left( \frac{4}{3} \right)\pi {{(0.18)}^{3}}}{{{T}_{1}}}\]                 \[=\frac{(76\times 13.6)\times 980\left( \frac{4}{3} \right)\pi {{(0.2)}^{3}}}{313}\] \[\Rightarrow \]               \[{{T}_{1}}=283.37K\] \[\therefore \]  \[{{T}_{1}}=283.37-273={{10.37}^{\circ }}C\]


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