Physics · Magnetism and Electromagnetic Induction

Force is given by F = qvBsinθ, where F, q, v and B are the force, charge on particle, velocity of particle and magnetic field strength respectively. θ is the angle between v and B.

If θ = 0 or 180, there is no force on the particle and it continues to move with uniform velocity.

If θ = 90, the force acting on the particle is maximum and perpendicular to the path of the particle. (force acts as a centripetal force and the particle moves along a circular path)

When θ has a value other than 0, 90 or 180, the direction of velocity is either perpendicular or parallel to B. If the component of velocity is parallel to B it will not be affected by the magnetic field. However if the component of velocity is perpendicular to B it will be subject to a force of the magnetic field which would result in uniform circular motion. Hence the particle would have a component along a straight line as well as a circular component. Therefore the path would be in the form of a helix. And so option D is correct.

The path will be circular when the angle between velocity of a charged particle and magnetic field is 90°. Hence, this option is incorrect.

A charged particle moves in a straight line when the angle between the velocity of the particle and magnetic field is 0° or 180°. It is not complimenting the given situation. Hence, this option is incorrect. 

A spherical path is not observed when a charged particle interacts with a magnetic field. Hence, this option is incorrect.

The path will be helical when the angle between velocity of a charged particle and magnetic field is any angle but 90°. It compliments the given statement. Hence, this option is the correct option.