y obs. location)-(source (x.0)-(0,y0 x,y △O Magnitude: △E Unit vector(direction x,y, x+y x Q r  y E   , ,0 ,0,0 0, ,0 obs. location source x y x y r = − = − = −  2 2 r = x + (−y)  Magnitude: 2 2 , ,0 ˆ x y x y r r r + − =  =  Unit vector (direction):