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