What is the direction of the magnetic force on a positive charge moving parallel to a uniform magnetic field?
**Motion of charged particle perpendicular to uniform B** is circular because force F ⊥ v, no work done, speed constant, radius r = m v/(q B), period T = 2π m/(q B) independent of v. If v has component parallel to B, helical path results, pitch = v_parallel·T. The magnetic force is given by F = q (v × B) . If the velocity v is parallel to the magnetic field B , the angle between them is 0° , so sin 0° = 0 , and the force magnitude is zero. Thus, there is no force. Using F = q v B sinθ, F
Ref: NCERT > Physics Book > Moving Charge and Magnetism > Magnetic Force on Moving Charge - Lorentz Force and Motion