JEE Main & Advanced Physics Mathematical Tools, Units & Dimensions Question Bank Critical Thinking

  • question_answer
    The speed of light\[(c)\], gravitational constant \[(G)\] and Planck's constant \[(h)\] are taken as the fundamental units in a system. The dimension of time in this new  system should be                                               [AMU 1995]

    A)             \[{{G}^{1/2}}{{h}^{1/2}}{{c}^{-5/2}}\]

    B)                      \[{{G}^{-1/2}}{{h}^{1/2}}{{c}^{1/2}}\]

    C)             \[{{G}^{1/2}}{{h}^{1/2}}{{c}^{-3/2}}\]

    D)                      \[{{G}^{1/2}}{{h}^{1/2}}{{c}^{1/2}}\]

    Correct Answer: A

    Solution :

                    Time\[\propto {{c}^{x}}{{G}^{y}}{{h}^{z}}\Rightarrow T=k{{c}^{x}}{{G}^{y}}{{h}^{z}}\]             Putting the dimensions in the above relation \[\Rightarrow \]  \[[{{M}^{0}}{{L}^{0}}{{T}^{1}}]={{[L{{T}^{-1}}]}^{x}}{{[{{M}^{-1}}{{L}^{3}}{{T}^{-2}}]}^{y}}{{[M{{L}^{2}}{{T}^{-1}}]}^{z}}\] \[\Rightarrow \]  \[[{{M}^{0}}{{L}^{0}}{{T}^{1}}]=[{{M}^{-y+z}}{{L}^{x+3y+2z}}{{T}^{-x-2y-z}}]\]                      Comparing the powers of \[M,\,L\] and \[T\] \[-y+z=0\]                               ?(i) \[x+3y+2z=0\]                                   ?(ii) \[-x-2y-z=1\]                            ?(iii)             On solving equations (i) and (ii) and (iii) \[x=\frac{-5}{2},\,y=z=\frac{1}{2}\] Hence dimension of time  are \[[{{G}^{1/2}}{{h}^{1/2}}{{c}^{-5/2}}]\]


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