Q=1e-11;
eps0=8.854e-12;
a=0.02;


z=0:0.001:2*a;
% Polje tockaste elektrine
EQ=Q./(4*pi*eps0*z.^2);

% Polje naelektrenega prstana
q=Q/(2*pi*a)
Eq=q*a*

plot(z,EQ)