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.

Question

Quizplus · Accepted Answer

Stuart is correct because a charged particle can travel in a helical path if there is a uniform magnetic field with a component parallel to the axis of the helix. This is because the magnetic force will act perpendicular to the velocity vector, causing the particle to move in a circular path. An electric field is not necessary for this motion, and even if present, it would not affect the particle's motion as long as it is not also perpendicular to the velocity vector. Bert's statement is incorrect because a vertical component of a magnetic field would not change the particle's motion in the vertical direction, only the direction of the circular motion. Therefore, Stuart's statement is more accurate.

Bert Says That a Charged Particle in a Vacuum Can