Question

A moving charged particle q travelling along the positive x-axis enters a uniform magnetic field B. When will the force acting on q be maximum?

Answer 1

force acting on moving charge particle is given by,
        F = Q(V x B)
here, F is magnetic force act on moving charge particle  of charge Q with velocity V , entering magnetic field B.

here it is clear that magnetic force depends upon cross product of velocity and magnetic field. if we talk about magnitude of magnetic force. then,
    |F| = BQVsinθ
force will be maximum, when sinθ  will be maximum.
and we know, sinθ  maximum at θ = 90°

hence, when motion of charge particle perpendicular upon magnetic field then force acting on q be maximum.