1. The problem statement, all variables and given/known data In the atmosphere and at an altitude of 250 m, you measure the electric field to be 150 N/C directed downward, and you measure the electric field to be 170N/C directed downward at an altitude of 400 m. Calculate the volume charge density of the atmosphere in the region between altitudes of 250 m and 400m, assuming it to be uniform. 2. Relevant equations 3. The attempt at a solution I just want to know. When I substitute the values in E = (sigma/epsilon 0) and then sigma = Q/Area.. So I get Q = 150 (epsilon 0) A. Now this I will substitute in the formula rho = Q / V. Now my question is what will be Area? What value of r should I write in 4 pi r^2 ? Will it be radius of earth or the altitude of 250 m? If it's radius of earth, where will I write 250m then?