An alpha particle enters into a uniform magnetic . . .

Question : An ⍺-particle enters into a uniform magnetic field with its initial velocity perpendicular to the direction of magnetic field. The particle will move along a 
a) straight path 
b) circular path 
c) spiral path 
d) helix

Doubt by Angad 

Solution : 

We know, the magnitude of force experienced by the charge particle in uniform magnetic field is given by 
F=qvBsinθ

When velocity is perpendicular to the magnetic field i.e. θ=90° and the force on the charge particle will be maximum 
F_max=qvB

According to Fleming's left rule the direction of this force will be perpendicular to both the velocity and magnetic field. So, no work is done by the magnetic field on the charge particle, hence the kinetic energy or the speed remains constant, which is only possible when the direction of motion is affected but not the speed. Hence, the motion of the charge particle must be uniform circular motion. Hence path followed must be circular.

b) circular path is correct option.