Shown in Figure 19-18 is an array of six charges. The top charges are all positive and of the same magnitude, +Q, and there is a distance a between the charges. At a distance 2a below this line of charges are three other charges, with the outer two being +Q and the central one -Q. What is the net electric field at a point P at the centre of the charge array? a a a P a + +0 Figure 19-18 Charge array for Checkpoint 19-8