JEE Main & Advanced Physics Mathematical Tools, Units & Dimensions Question Bank Self Evaluation Test - Physical World, Units and Measurements

  • question_answer
    A physical quantity of the dimensions of length that can be formed out of c, G and \[\frac{{{e}^{2}}}{4\pi {{\varepsilon }_{0}}}\] is [c is velocity of light, G is universal constant of gravitation and e is charge]  

    A) \[{{c}^{2}}\left[ G\frac{{{e}^{2}}}{4\pi {{\varepsilon }_{0}}} \right]\]

    B)  \[\frac{1}{{{c}^{2}}}{{\left[ \frac{{{e}^{2}}}{G4\pi {{\varepsilon }_{0}}} \right]}^{1/2}}\]

    C)  \[\frac{1}{c}G\frac{{{e}^{2}}}{4\pi {{\varepsilon }_{0}}}\]

    D)  \[\frac{1}{{{c}^{2}}}{{\left[ G\frac{{{e}^{2}}}{4\pi {{\varepsilon }_{0}}} \right]}^{1/2}}\]

    Correct Answer: D

    Solution :

    [d] Let dimensions of length is related as, \[L={{[c]}^{x}}{{[G]}^{y}}{{\left[ \frac{{{e}^{2}}}{4\pi {{\varepsilon }_{0}}} \right]}^{z}}\Rightarrow \frac{{{e}^{2}}}{4\pi {{\varepsilon }_{0}}}=M{{L}^{3}}{{T}^{-2}}\] \[L={{[L{{T}^{-1}}]}^{x}}{{[{{M}^{-1}}{{L}^{3}}{{T}^{-2}}]}^{y}}{{[M{{L}^{3}}{{T}^{-2}}]}^{z}}\] \[[L]=[{{L}^{x+3y+3z}}{{M}^{-y+z}}{{T}^{-x-2y-2z}}]\] Comparing both sides \[z=y=\frac{1}{2},x=-2\]    Hence, L= \[{{c}^{-2}}{{\left[ G.\frac{{{e}^{2}}}{4\pi {{\varepsilon }_{0}}} \right]}^{1/2}}\]


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