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A positively charged particle travels horizontally northward and enters a region where a field may exist. This region may contain only a magnetic field, only an electric field, or both a magnetic field and an electric field. When the particle enters the region, the particle does not speed up or slow down, but it does deflect in the downward direction. Which of the following statements is correct about the field or fields within the region and provides correct reasoning?

source: AP electromagnetism Only a southward magnetic field is present, because then a magnetic force will cause the particle to undergo circular motion. Apparently this is the answer but how does positively charged object travels horizontally northward????????

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If I'm reading it right, that solution seems wrong. Moving horizontally northward might make sense if "horizontal" means any direction other than up and down.

If a particle is moving northward and there is only a southward pointing magnetic field, there is no force by the Lorentz force law.

Motion downward would require a magnetic field pointing ease if all there is is a magnetic field.

If there's a combination of E and M fields, then there's several combinations that can work.

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  • $\begingroup$ I agree with you that is why I checked the answer with our teacher and got this which seems kind of weird and not sensible at all sin theta = 0, due to the fact that theta = 0. What to take as frame of reference $\endgroup$
    – Quicksilver BL3DE
    Commented Jan 24 at 19:06
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Take a world map with North, East, South and West marked on it and place it in the normal orientation on a horizontal table top. North is now horizontally away from you. Romero is correct that the solution is wrong. A Southward magnetic field would be pointing horizontally towards you and antiparallel to the motion so there would be no deflection due to the magnetic field which should be orthogonal to the motion to cause a deflection.

The point they are trying to make is only a magnetic field would cause the particle to deflect while maintaining constant speed so it starts traveling in a circle. An electric field on the other hand would deflect the particle until the the particle is travelling parallel to the electric field and its speed increases. The statement "particle does not speed up or slow down" is the clue that there is no electric field.

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