an "infinite" sheet with a thickness has a uniform density charge. however, my teacher gave it in rho=az^2. I know what rho is, and I know what z is. What is 'a'??? He's used it in an example or two, but never thought to ask what it means and if it's consistent for all distances of the electric field and whatnot... If I could just know this, then I think my life would be much easier and I think I could figure out this problem. I've looked online and throughout my book but I cannot find it's reference anywhere. I've found it one place in this forum, but it wasn't explained because it seemed everybody already knew. Thanks for any help.