Bert Says That a Charged Particle in a Vacuum Can

Multiple Choice

Bert says that a charged particle in a vacuum can travel in a helix only if a uniform electric field and a uniform magnetic field are both present and both parallel to the axis of the helix. Stuart says that only a magnetic field with a component parallel to the axis of the helix is needed. Which one, if either, is correct, and why?

A) Bert, because the charged particle's velocity can have a vertical component only if an electric field in the vertical direction is present.
B) Stuart, because a component of velocity in the vertical direction is not changed by a vertical component of a magnetic field.
C) Bert, because a component of velocity in the vertical direction is changed by a vertical component of a magnetic field.
D) Stuart, because an electric field in the vertical direction would cause the particle to come to a complete stop.
E) Neither, because particles cannot move in helical paths in the presence of magnetic and electric fields.