Haryana PMT Haryana PMT Solved Paper-2007

  • question_answer
    A beam of protons with velocity \[4\times {{10}^{5}}m/s\] enters a uniform magnetic field of 0.3 T at an angle of \[60{}^\circ \] to the magnetic field. Find the radius of the helical path taken by the proton beam.

    A)  0.2 cm                                 

    B)  1.2 cm

    C)  2.2cm                                  

    D)  0.122cm

    Correct Answer: B

    Solution :

                    When the charged particle is moving at an angle to the field (other than \[{{0}^{o}},\,{{90}^{o}}\] and \[{{180}^{o}}\]), in this situation resolving the velocity  of the particle along and perpendicular to the field, we find that the particle moves with constant velocity \[v\text{ }cos\theta \]along the field and at the same time it is also moving with velocity v sin 6 perpendicular to the field due to which it will describe a circle (in a plane perpendicular to the field) of radius \[r=\frac{m(v\,\sin \theta )}{qB}\] Here,    \[m=1.67\times {{10}^{-27}}kg\]                 \[v=4\times {{10}^{5}}m/s\]                 \[\theta ={{60}^{o}}\]                 \[q=1.6\times {{10}^{-19}}C\]                 \[B=0.3T\] \[\therefore \]  \[r=\frac{1.67\times {{10}^{-27}}\times 4\times {{10}^{5}}\times (\sqrt{3}/2)}{1.6\times {{10}^{-19}}\times 0.3}\]                 \[=0.012m\]                 \[=1.2cm\]


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