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

  • question_answer
    If electronic charge e, electron mass m, speed of light in vacuum c and Planck's constant h are taken as fundamental quantities, the permeability of vacuum \[{{\mu }_{0}}\]can be expressed in units of:   [JEE MAIN ONLINE 11-04-2015]

    A) \[\left( \frac{hc}{m{{e}^{2}}} \right)\]

    B) \[\left( \frac{h}{m{{e}^{2}}} \right)\]

    C) \[\left( \frac{h}{c{{e}^{2}}} \right)\]

    D) \[\left( \frac{m{{c}^{2}}}{h{{e}^{2}}} \right)\]

    Correct Answer: C

    Solution :

    [c]        
    \[\left[ e \right]=IT\]                     \[\left[ m \right]=M\]
    \[\left[ c \right]=L{{T}^{-1}}\]               \[\left[ h \right]=M{{L}^{2}}{{T}^{-1}}\]
    \[\left[ {{\mu }_{0}} \right]=ML{{I}^{-2}}{{T}^{-3}}\]            If \[{{\mu }_{0}}={{e}^{a}}{{m}^{b}}{{c}^{c}}{{h}^{d}}\]
    \[ML{{T}^{-2}}{{T}^{-3}}={{\left[ IT \right]}^{a}}{{\left[ M \right]}^{b}}{{\left[ L{{T}^{-1}} \right]}^{c}}{{\left[ M{{L}^{2}}{{T}^{-1}} \right]}^{d}}\]by equating powers, we get
    \[a=-2,b=0,c=-1,d=1\]
    \[\therefore \] \[\left[ {{\mu }_{0}} \right]=\left[ \frac{h}{c{{e}^{2}}} \right]\]


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