Electromagnetism Problem, electric charge per unit volume.
1. The problem statement, all variables and given/known data
The atmostspheric electric fields at heights of 100m and 1000m above the surface of the eartg are 110 v/m and 25 v/m respectively and are directed downwards. Evaluate the averge electric charge per unit volume in the atmostsphere between these heights. Express your result as the number of ions per unit volume assuming that each ion carries a charge of magnitude 1.6x10^-19
2. Relevant equations
Value of Epsilon nought
3. The attempt at a solution
I am a little clueless about this problem. I think i may need to calculate the volume of the hollowed sphere between the two values (ie the volume of the 1000m sphere minus the volume of the 100m sphere giving me the total volume inbetween). But this doesnt sound right to me. Then how do i calaculate the electric charge per unit vol. from the electric field strength? Help would be appreciated muchly, i think i only need a prod in the right direction. Thankyou