Question Detail
Question
A positive charge moving with a constant velocity v enters a region of a uniform magnetic field pointing to the top of the page. What is the direction of the magnetic force on the charge?
Correct Answer
Option A is the correct answer.
Detailed Explanation
If the charge's velocity is directed along the same line as the field (parallel or antiparallel to B), the cross product v×B is zero, so there is no magnetic force.
Hint
Recall that F = qv×B is zero whenever velocity is parallel (or antiparallel) to the magnetic field.