What determines the direction of magnetic force acting on a moving charge? When is the magnitude of the magnetic force zero?
The direction of the magnetic force acting on a moving charge is determined by the direction of the magnetic field and the direction of the velocity of the charge. Specifically, the direction of the magnetic force is perpendicular to both the magnetic field and the velocity vector of the charge. This is known as the right-hand rule.
The magnitude of the magnetic force is zero when the velocity of the charge is parallel or antiparallel to the direction of the magnetic field. In this case, the charge is moving parallel to the magnetic field lines and there is no force acting perpendicular to the direction of motion. Additionally, if the charge is stationary (i.e. not moving), there will be no magnetic force acting on it regardless of the strength of the magnetic field.