BCECE Medical BCECE Medical Solved Papers-2015

  • question_answer
    An air bubble stats rising from the bottom of a lake. Its diameter is 3.6 mm at the bottom and 4 mm at the surface. The depth of the lake is 250 cm and the temperature at surface is\[{{40}^{o}}C\]. What is the temperature at the bottom of the lake? Given atmpospheric pressure = 76 cm of Hg and \[g=980\,\,cm/{{s}^{2}}\].

    A)  \[{{11}^{o}}C\]   

    B)  \[{{12.36}^{o}}C\]

    C)  \[{{13}^{o}}C\]  

    D)  \[{{10.37}^{o}}C\]

    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.                 \[=76\times 13.6\times 980+250\times 1\times 980\]                 \[=(76\times 13.6+250)980\] dyne/\[c{{m}^{2}}\] 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,                 \[{{p}_{2}}=\] atmospheric pressure                 \[=(76\times 13.6\times 980)\] dyne/\[c{{m}^{2}}\]                 \[{{T}_{2}}=273+40=313\,K\]                 Now, \[\frac{{{p}_{1}}{{V}_{1}}}{{{T}_{1}}}=\frac{{{p}_{2}}{{V}_{2}}}{{{T}_{2}}}\] \[\Rightarrow \] \[\frac{(76\times 13.6\times 250)\,980\times \left( \frac{4}{3} \right)\,\pi {{(0.18)}^{3}}}{{{T}_{1}}}\]                 \[=\frac{(76\times 13.6)980\left( \frac{4}{3} \right)\pi {{(0.2)}^{3}}}{313}\] \[\Rightarrow \] \[{{T}_{1}}=283.37\,K\] \[\therefore \] \[{{T}_{1}}=283.37-273={{10.37}^{o}}C\]


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