A neutral particle moves parallel to a wire. When a current is suddenly turned on in the wire, no force will act on the neutral particle. While a positively charged particle would feel a magnetic force in the direction of the cross product of its velocity and the magnetic field generated by the current. Specifically, with the magnetic field pointing along the positive z-axis and the particle's velocity along the positive y-axis, a positive particle would feel a force along the positive x-axis.

1. 19. (5pts) A neutral particle moves parallel to a wire. If a current is suddenly turned on, in which
direction will the force act? +q
Solution
As current is switvhed on, magnetic field is generated in space. Magnetic field applies force on
moving charge particles.
Particle in problem is given to be neutral , so no force will act on that.
In figure, particle is shown to be positively charged, there is force acting on this particle.
Magnetic force is given by
q ( v x B)
that is if particle is +ve, direction of force is same as direction of v x B , but if it is -ve charge
partcle force is along - (v x B).
Field due to current at the position of charge is along +ve z direction and velocity is along +ve y
direction. Hence direction of force on the =ve charge particle is
j x k
that is along =ve x direction.