Question: Need a little help

Okay, I'm just starting to get the hang of Maple and I'm going through a tutorial that has a problem in it I'm having a little difficulty setting up. 

A charged disk of radius a having uniform charge density "sigma" (the actual symbol sigma) produces and electric field Ez along the z-axis given by the integral expression (I'll do the best I can to illustrate it here)

Ez(z) =  (sigma * z * int ( r/((r^2+z^2)^(3/2)) ,r=0..a))/(2*epsilon0)

both sigma and epsilon are displayed as greek letters.  So sigma multiplied by z multiplied by the integral expression from 0 to a multiplied by dr (so with respect to r) all over 2 * epsilon0

So it says find a simple algebraic expression for Ez(z), then use sigma=1e-6 and epsilon0=8.854e-12 and a=0.1 to make a plot of Ez from z=-5 a to z= 5 a .

So first of all I make an expression E:= (the mess I described above) and I get a warning : unable to determine if -(-z^2)^(1/2) is between 0 and a; try to use assumptions or set _EnvAllSolutions to true.

Then, if I didn't get the error I was going to try to make a taylor expansion out of it for the simple algebraic expression taylor(E,x,5) or something like that.   And plot(E,z=-5*a..5*a) I don't think is right probably because I haven't got the expression right, or maybe it should be setup as a function? 



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