JEE Main & Advanced Physics Photo Electric Effect, X- Rays & Matter Waves Question Bank Critical Thinking

  • question_answer
    In order to coincide the parabolas formed by singly ionised ions in one spectrograph and doubly ionized ions in the other Thomson?s mass spectrograph, the electric fields and magnetic fields are kept in the ratios 1 : 2 and 3 : 2 respectively. Then the ratio of masses of the ions is  

    A)            3 : 4                                          

    B)            1 : 3

    C)            9 : 4                                          

    D)            None of these

    Correct Answer: C

    Solution :

                       Using \[{{Z}^{2}}=k\left( \frac{q}{m} \right)y;\] where \[k=\frac{{{B}^{2}}LD}{E}\]. For parabolas to coincide in the two photographs, the \[\frac{k\,q}{m}\] should be same for the two cases. Thus,                \[\frac{B_{1}^{2}\ LDe}{{{E}_{1}}{{m}_{1}}}=\frac{B_{2}^{2}LD\,(2e)}{{{E}_{2}}{{m}_{2}}}\]                    Þ \[\frac{{{m}_{1}}}{{{m}_{2}}}={{\left( \frac{{{B}_{1}}}{{{B}_{2}}} \right)}^{2}}\times \left( \frac{{{E}_{2}}}{{{E}_{1}}} \right)\times \frac{1}{2}\]\[=\frac{9}{4}\times \frac{2}{1}\times \frac{1}{2}=\frac{9}{4}\]


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