The collection of charged aerosol particles by an isolated collector sphere, under the influence of electric field, is investigated numerically. The limiting trajectories of inertial aerosol particles are calculated both when the electric field is aligned with the flow direction and when the field is transverse to the flow. The collection efficiencies associated with these field orientations are evaluated for a wide range of Stokes numbers and electric field intensities for the Stokes and potential flow approximations. Imposing either a field parallel to the flow direction or one perpendicular to the flow increases the collection efficiency. The relative merit of the two field orientations depends on the field strength and on the particle inertia.