BHU PMT BHU PMT (Mains) Solved Paper-2007

  • question_answer

    Directions : In the following question more than one of the answers given may be correct. Select the correct answer and mark it according to the code:

    Two identical charged particles enter a uniform magnetic field with same speed but at angles\[30{}^\circ \]and\[60{}^\circ \]with field. Let a, b and c be the ratio of their time periods, radii and pitches of the helical paths then (1) \[abc>1\]                                      (2) \[abc=1\] (3) \[abc<1\]                                      (4) \[a=bc\]

    A)  1, 2 and 3 are correct

    B)  1 and 2 are correct

    C)  2 and 4 are correct

    D)  1 and 3 are correct

    Correct Answer: C

    Solution :

                     \[T=\frac{2\pi m}{Bq}\] \[\therefore \]  \[a=\frac{{{T}_{1}}}{{{T}_{2}}}=1\]                 \[r=\frac{mv\,\sin \theta }{qB}\] \[\therefore \]  \[b=\frac{{{r}_{1}}}{{{r}_{2}}}=\frac{\sin {{30}^{o}}}{\sin {{60}^{o}}}=\frac{1}{\sqrt{3}}\]                 \[p=(T)(v\cos \theta )\] \[\therefore \]  \[c=\frac{{{p}_{1}}}{{{p}_{2}}}=\frac{\cos {{30}^{o}}}{\cos {{60}^{o}}}=\sqrt{3}\] Therefore,          \[abc=1\times \frac{1}{\sqrt{3}}\times \sqrt{3}=1\] and                        \[a=bc\]


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