JEE Main & Advanced Physics Mathematical Tools, Units & Dimensions JEE PYQ - Mathematical Tools Units and Dimensions

  • question_answer
    If the capacitance of a Nano capacitor is measured in tern-is of a unit V made by combining the electronic charge 'e', Bohr radius \['{{a}_{0}}',\] Planck's constant 'h' and speed of light 'c' then:                    [JEE ONLINE 10-04-2015]

    A) \[u=\frac{hc}{{{e}^{2}}{{a}_{0}}}\]

    B) \[u=\frac{{{e}^{2}}h}{c{{a}_{0}}}\]

    C) \[u=\frac{{{e}^{2}}{{a}_{0}}}{hc}\]

    D) \[u=\frac{{{e}^{2}}c}{h{{a}_{0}}}\]

    Correct Answer: C

    Solution :

    [c] \[C={{e}^{x}}a_{0}^{y}{{h}^{z}}{{c}^{a}}\]
    Capacitance \[C=\left[ {{M}^{-1}}{{L}^{-2}}{{A}^{2}}{{T}^{4}} \right]\]
    \[e=\left[ AT \right]\]       \[{{a}_{0}}=\left[ L \right]\]
    \[c=\left[ {{L}^{1}}{{T}^{-1}} \right]\]            \[h=\left[ {{M}^{1}}{{L}^{2}}{{T}^{-1}} \right]\]
    \[\left[ {{M}^{-1}}{{L}^{-2}}{{A}^{2}}{{T}^{4}} \right]={{\left[ AT \right]}^{x}}{{\left[ L \right]}^{y}}{{\left[ {{M}^{1}}{{L}^{2}}{{T}^{-1}} \right]}^{z}}{{\left[ {{L}^{1}}{{T}^{-1}} \right]}^{a}}\]Compare x = 2
    \[z=-1\]
    \[\frac{{{M}^{-1}}{{L}^{-2}}{{A}^{2}}{{T}^{4}}}{{{A}^{2}}{{T}^{2}}{{M}^{-1}}{{L}^{-2}}{{T}^{1}}}={{\left[ L \right]}^{y}}{{\left[ {{L}^{1}}{{T}^{-1}} \right]}^{a}}\]
    \[T=\left[ {{L}^{y}} \right]{{\left[ {{L}^{1}}{{T}^{-1}} \right]}^{a}}\]So \[u=\frac{{{e}^{2}}{{a}_{0}}}{hc}\]
    \[\Rightarrow \] \[a=-1\]\[\Rightarrow \] \[y=1\]


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