Bert Says That a Charged Particle in a Vacuum Can

question 50

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.

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