Question: Plotting the result of a solve

I'm trying to plot directly the result of a solve, using the following syntax:

plot(solve(diff(E,N)=0,N),Z=0..20);

( E is dependant of Z and N )
 

it works for an equation that has only 1 solution. But, when using an equation with two solutions, it only plot one of the solutions. Of course, it is always the solution I don't want.

I've tried solving this problem by forcing N to be above 0 ( the unwanted, but always plotted solution is negative ):

plot(solve({Ep-E=0,N>0},N),Z=0..20);

( Ep is dependant of Z and N )
 

But this doesn't get plotted at all. I've figured that the problem is the output of the solve command. When assigning a fixed value to Z, here are the outputs for the different solve used:

solve(diff(E,N)=0,N);
9.243307870

solve({Ep-E=0,N>0},N);

{N = 3.241106003}

solve(E-Ep=0,N);
3.241106003, -1.384090609

Clearly, the plot command can't understand {N = 3.241106003}.

I've tried doing:

assign(solve({Ep-E=0,N>0},N));

plot(N,Z=0..20);

Which didn't work, since Maple can't solve the equation without a fixed value for Z. ( the output is nothing when not assigning a fixed value to Z )

So, how do I get the plot command to only show the positive result of the solve command?