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);