KVPY Sample Paper KVPY Stream-SX Model Paper-25

  • question_answer
    An isolated and charged spherical soap bubble has a radius 'r' and the pressure inside is atmospheric. If 'T' is the surface tension of soap solution, then charge on drop is:

    A) \[2\sqrt{\frac{2\,r\,T}{{{\varepsilon }_{0}}}}\]

    B) \[8\,\pi \,r\sqrt{2\,r\,T\,{{\varepsilon }_{0}}}\]

    C) \[8\,\pi \,r\sqrt{r\,T\,{{\varepsilon }_{0}}}\]

    D) \[8\,\pi \,r\sqrt{\frac{2\,r\,T}{{{\varepsilon }_{0}}}}\]

    Correct Answer: B

    Solution :

    Inside pressure must be \[\frac{4T}{r}\] greater than outside pressure in bubble. This excess pressure is provided by charge on bubble. \[\frac{4T}{r}=\frac{{{\sigma }^{2}}}{2{{\varepsilon }_{0}}}\]
    \[\frac{4T}{r}=\frac{{{Q}^{2}}}{16{{\pi }^{2}}{{r}^{4}}\times 2{{\varepsilon }_{0}}}\]\[......\left[ \sigma =\frac{Q}{4\pi {{r}^{2}}} \right]\]
    \[Q=8\pi r\sqrt{2rT{{\varepsilon }_{0}}}\]

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