JEE Main & Advanced JEE Main Paper (Held On 12-Jan-2019 Morning)

  • question_answer
    A proton and an \[\alpha -\]particle (with their masses in the ratio 1 : 4 and charges in the ratio of 1 : 2) are accelerated from rest through a potential difference V. If a uniform, magnetic field is set up perpendicular to their' velocities, the ratio  of the radii \[{{r}_{p}}:{{r}_{\alpha }}\]of the circular paths described by them will be [JEE Main Online Paper Held On 12-Jan-2019 Morning]

    A) \[1:3\]                                       

    B) \[1:\sqrt{2}\]

    C) \[1:2\]                           

    D)   \[1:\sqrt{3}\]

    Correct Answer: B

    Solution :

    We know that \[r=\frac{mv}{Bq}=\frac{\sqrt{2mqV}}{Bq}=\frac{1}{B}\sqrt{\frac{2mV}{q}}\Rightarrow r\propto \sqrt{\frac{m}{q}}\] Given\[\frac{{{m}_{p}}}{{{m}_{\alpha }}}=\frac{1}{4};\frac{{{q}_{p}}}{{{q}_{\alpha }}}=\frac{1}{2}\] \[\frac{{{r}_{p}}}{{{r}_{\alpha }}}=\sqrt{\frac{{{m}_{p}}{{q}_{\alpha }}}{{{q}_{p}}{{m}_{\alpha }}}}=\sqrt{\left( \frac{1}{4} \right)\left( \frac{2}{1} \right)}=\frac{1}{\sqrt{2}}\]


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