Physics. Urgent! Help please?

Question

In the air over a particular region, at an altitude of 500 m above the ground, the electric field is 150 N/C directed downward. At 600 m above the ground, the electric field is 80 N/C downward. What is the average volume charge density in the layer of air between these two elevations?

Answer 1

Use Gauss's Law http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gaulaw.html
Make the volume the distance between 2 spheres at 500 to 600 km. Then the surface integral EdA will equal the enclosed charge. divide by the volume to get charge density.

Note: Using spheres is a way to neglect edge effects. You could pick other volumes (assuming a flat earth and infinite layer would also work)