WBJEE 2019 Physics Solved Paper

If velocity of particle, v is perpendicular to magnetic field, B i.e. θ=90∘, then particle will experience maximum magnetic force, i.e. Fmax=qvB. This force acts in a direction perpendicular to the motion of charged particle. Therefore the trajectory of the particle is a circle.
In this case path of charged particle is circular and magnetic force provides the necessary centripetal force,
i.e., qvB=‌

mv2

r

⇒ Radius of path, r=‌

mv

qB

r=‌

p

qB

  [∵p=mv]
where, p=p = momentum of the particle
∴r∝ momentum
Hence, option (d) is correct.