The surfaces of a lipid bi-layer forming the membrane around a cell with a radius of 1.5 μm has a residual charge qr = 9.1×10-15 C on outside of the bi-layer, and the same amount of negative charge on the inside. What is the force in pN (×10-12 N) on a singly-charged positive ion (q =1.6 ×10-19 C) located on the outer surface of this membrane? Hint: Use F = q E = q (σ/e) with σ = qr/A = qr/ (4π r2) and εo = 8.85 x 10-12 F·m-1.