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

  • question_answer
    Number of particles is given by \[n=-D\frac{{{n}_{2}}-{{n}_{1}}}{{{x}_{2}}-{{x}_{1}}}\] crossing a unit area perpendicular to X-axis in unit time, where \[{{n}_{1}}\] and \[{{n}_{2}}\] are number of particles per unit volume for the value of \[x\] meant to \[{{x}_{2}}\] and \[{{x}_{1}}\]. Find dimensions of \[D\] called as diffusion constant                                   [CPMT 1979]

    A)             \[{{M}^{0}}L{{T}^{2}}\]

    B)                      \[{{M}^{0}}{{L}^{2}}{{T}^{-4}}\]

    C)             \[{{M}^{0}}L{{T}^{-3}}\]

    D)                      \[{{M}^{0}}{{L}^{2}}{{T}^{-1}}\]

    Correct Answer: D

    Solution :

                                [n] = Number of particles crossing a unit area in unit time = \[[{{L}^{-2}}{{T}^{-1}}]\]             \[\left[ {{n}_{2}} \right]=\left[ {{n}_{1}} \right]=\]number of particles per unit volume = [L?3]             \[[{{x}_{2}}]=[{{x}_{1}}]\]= positions             \ \[D=\frac{[n]\ \left[ {{x}_{2}}-{{x}_{1}} \right]}{\left[ {{n}_{2}}-{{n}_{1}} \right]}=\frac{\left[ {{L}^{-2}}{{T}^{-1}} \right]\times [L]}{[{{L}^{-3}}]}\]=\[\left[ {{L}^{2}}{{T}^{-1}} \right]\]


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