11th Class Physics Physical World / भौतिक जगत Question Bank 11th CBSE Physics Mathematical Tools, Units & Dimensions

  • question_answer
    Construct a new physical quantity having dimensions of length in terms of universal constants G, c and h. What is it called? What is the order of its value?

    Answer:

                    Let                                                                                    \[l=k{{G}^{x}}{{c}^{y}}{{h}^{z}}\] \[\left[ {{\text{M}}^{0}}{{\text{L}}^{\text{1}}}{{\text{T}}^{0}} \right]\text{ }=\text{ }{{\left[ {{\text{M}}^{-\text{1}}}{{\text{L}}^{\text{3}}}{{\text{T}}^{-\text{2}}} \right]}^{\text{x}}}\left[ \text{L}{{\text{T}}^{-\text{1}}} \right]{{\left[ \text{M}{{\text{L}}^{\text{2}}}{{\text{T}}^{-\text{1}}} \right]}^{\text{2}}}\] = \[=[{{\text{M}}^{-}}^{\text{x}+\text{z}}{{\text{L}}^{\text{3x}+\text{y 2 z}}}{{\text{T}}^{\text{y}+}}^{\text{2z}}{{\text{T}}^{-\text{2x }-\text{ y }-\text{ z}}}]\]                                                 ...(i) Applying the principle of homogeneity of dimensions, we get                                                                                             \[-\text{x}+\text{z}=0\] ,                                                                                  \[\text{3x}+\text{y}+\text{2z=1}\] ,                                                                                                        \[-2x-y-z=0\] . Solving these eqns., we get                                                                                                     \[x=\frac{1}{2}\] ,                                                                                                  \[\,y=\frac{-3}{2}\] ,                                                                                                   \[\,z=\frac{1}{2}\]                                                                                                       \[\therefore\]             From(i),   \[l=\text{k}{{\text{G}}^{\text{1}/\text{2}}}{{\text{c}}^{-\text{3}/\text{2}}}{{\text{h}}^{\text{1}/\text{2}}}=\text{ k}\sqrt{\frac{Gh}{{{c}^{3}}}}\] Taking                                                                                        \[\text{k}=\text{1};\text{ }\]   \[\text{l }=\sqrt{\frac{(6.67\times {{10}^{-11}})(6.63\times {{10}^{-34}})}{{{(3\times {{10}^{8}})}^{3}}}}\approx \text{1}{{0}^{-\text{35}}}\text{m}\] . This length is called Planck's length.


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