State the condition under which a charged particle . . .

Question : State the condition under which a charged particle moving with velocity v goes undeflected in a magnetic field B?

Doubt by Saif 

Solution : 

We know when a charged particle move in magnetic filed B with velocity v then the force experienced by the charge particle (also know as Magnetic Lorentz Force) is given by 
F=q(v×B)

OR
In scalar form 
F = qvBsinθ

If q≠0, v≠0, B≠0 but still F=0 it means sinθ=0
which is only possible when θ=0° or θ=180°

Hence, we can say that the charge particle moving in electric field will not experience any force if it is moving parallel or antiparallel to the direction of magnetic field.