BHU PMT BHU PMT (Screening) Solved Paper-2010

  • question_answer
    An electron moving with the speed                                                                                                        \[5\times {{10}^{6}}\] per sec is shooted parallel to the electric field of intensity                                                                                                   \[1\times 10s\text{ }N/C\] . Field is responsible for the retardation of motion of electron. Now evaluate the distance travelled by the electron before coming to rest for an instant (mass of                                                                                                \[e=9\times {{10}^{-31}}kg,\] charge                                                                                                 \[=1.6\times {{10}^{-19}}C\] )

    A)  7 m                                       

    B)  0.7 mm

    C)  7cm                                      

    D)  0.7cm

    Correct Answer: C

    Solution :

                     Electric force,                                                                                                                   \[qE=ma\]                                                                                                              \[\Rightarrow\]                                                                                                           \[a=\frac{qE}{m}\]                                                                                                               \[\therefore\]            \[a=\frac{1.6\times {{10}^{-19}}\times 1\times {{10}^{3}}}{9\times {{10}^{-31}}}=\frac{1.6\times {{10}^{5}}}{9}\]                                                                                                                     \[u=5\times {{10}^{6}}\] and                                                                                                                        \[v=0\]                                                                                                               \[\therefore\] From                                                                                                   \[{{v}^{2}}={{u}^{2}}-2as\]                                                                                                              \[\Rightarrow\]                                                                                                    \[s=\frac{{{u}^{2}}}{2a}\]                                                                                                               \[\therefore\] Distance,                                 \[s=\frac{{{(5\times {{10}^{6}})}^{2}}\times 9}{2\times 1.6\times {{10}^{15}}}\simeq 7\,cm\]


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