In the time between posting this and now, I have found my mistake. So, problem solved. 1. The problem statement, all variables and given/known data Earth's lower atmosphere contains negative and positive ions, created by radioactive elements in the soil and cosmic rays from space. In a certain region, the atmospheric electric field strength is 140 V/m and the field is directed vertically down. This field causes singly charged positive ions, at a density of 610 cm^-3, to drift downward and singly charged negative ions, at a density of 550 cm^-3, to drift upward. The measured conductivity of the air in that region is 2.70 multiplied by 10-14 (ohm·m)^-1. Calculate the magnitude of the current density. 2. Relevant equations J = conductivity * E J = nev (v here represents the drift speed) 3. The attempt at a solution I was writing this and lost what I wrote (alas, I was too verbose!), so here's it quick: J = conductivity * E = 2.7*10^-14 * 140 = 3.78*10^-12 <<------- correct v = J / ne Here we have two values for n, one of positive ions and one of negative ions. So I figured I could write: v = J / (e(n1-n2)) where n1 represents the density of the positive ions and n2 of the negative. I checked my math a whole lot, but I keep coming out with v = 0.393258427 m/s, which is wrong, says www.webassign.com. So apparently I can't just take the net drift?