Chhattisgarh PMT Chhattisgarh PMT Solved Paper-2007

  • question_answer
    The rest energy of an electron is 0.511 Me V. The electron is accelerated from rest to a velocity 0.5c. The change in its energy will be

    A)  0.026 MeV                        

    B)  0.051 MeV

    C)  0.079 MeV                        

    D)  0.105 MeV

    Correct Answer: C

    Solution :

    Mass in moving condition and rest mass are related as \[m=\frac{{{m}_{o}}}{\sqrt{\left( 1-\frac{{{v}_{2}}}{{{c}^{2}}} \right)}}\]                 or            \[m{{c}^{2}}=\frac{{{m}_{o}}{{c}^{2}}}{\sqrt{1-\frac{{{v}^{2}}}{{{c}^{2}}}}}\]                 \[\Rightarrow \]               \[E={{E}_{0}}{{\left( 1-\frac{{{v}^{2}}}{{{c}^{2}}} \right)}^{-1/2}}\] Given,      \[v=0.5c\] \[\therefore \]  \[E={{E}_{0}}{{\left( 1-\frac{0.25{{c}^{2}}}{{{c}^{2}}} \right)}^{-\frac{1}{2}}}\]                 \[={{E}_{0}}{{(0.75)}^{-1/2}}\]                 \[=\frac{{{E}_{0}}}{\sqrt{0.75}}\] \[\therefore \] Change in energy \[=E-{{E}_{0}}\]                 \[\Delta E=\frac{{{E}_{0}}}{\sqrt{0.75}}-{{E}_{0}}\]                 \[{{E}_{0}}\left( \frac{1-\sqrt{0.75}}{\sqrt{0.75}} \right)\] Given,    \[{{E}_{0}}=0.511MeV\] \[\therefore \]  \[\Delta E=0.511\times \left( \frac{1-0.5\sqrt{3}}{0.5\sqrt{3}} \right)\]                 \[=0.079MeV\]


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