<< Calculus`VectorAnalysis`
<< Miscellaneous`PhysicalConstants`

cm = 0.01;

SetCoordinates[Cylindrical[r, fi, z]]

eqn = 
      0 == (Laplacian[φ[r]] // Numerator)

dosfi = DSolve[{eqn, φ[r1] == 
              u1, φ[r2] == u2}, φ[r], 
          r][[1]]

dospar = {u1 -> 0, r1 -> 0.5  cm, r2 -> 1.5  cm, 
          u2 -> (22000/(Sqrt[3])), tgdelta -> 0.001, omega -> 314, 
          epsr -> 4, 
          eps0 -> 
            VacuumPermittivity[[1]]};
ecko = D[φ[r] /. dosfi, r] /. dospar

γ = 
        omega*eps0*epsr*tgdelta /. dospar;
kveve = γ*(ecko^2);
Integrate[2*Pi*r*kveve, {r, r1 /. dospar, r2 /. dospar}]

ClearAll[prechod, tau, pom];
prechod[odkud_, kam_, kde_, promenna_, tau_] := 
      Module[{a, b, res}, 
        res = Solve[{a - b == odkud, a + b == kam}]; 
        a + b*Tanh[((promenna - kde)/tau)] /. 
          res[[1]]]
Plot[prechod[1, 2, 3, x, 0.01], {x, 1, 5}]

ϵr1 = 1;
ϵr2 = 4;
d1 = 2  cm;
d2 = 2  cm;
ϵr = 
        prechod[ϵr1, ϵr2, d1, x, 
          0.01  cm];
rce = 
        D[ϵr*D[fi[x], x], x] == 
          0;
res[derfi_] := 
        fi[x] /. NDSolve[{rce, 
                fi[0] == 0, fi'[0] == derfi}, 
              fi[x], {x, 0, d1 + d2}, 
              MaxStepSize -> 
                0.01  cm][[1]];
pom = res[5];
Plot[pom, {x, 0, d1 + d2}, 
        AxesLabel -> {"x", "φ[x]"}, 
        PlotStyle -> {Hue[0.6], Thickness[0.008]}];

ClearAll[hodn]

    hodn[derfi_?NumericQ] := 
        res[derfi] /. x -> d1 + d2;

    derka[napeti_] := 
        dernase /. 
          FindRoot[
            hodn[dernase] == napeti, {dernase, 0.1, 
              100}];

    vyslfi = res[derka[100]];

    vyslE = D[vyslfi, x];

    Plot[vyslfi, {x, 0, d1 + d2}, 
        AxesLabel -> {"x", "φ[x]"}, 
        PlotStyle -> {Hue[0.6], Thickness[0.008]}];

    Plot[{vyslE, 0}, {x, 0, d1 + d2}, 
        AxesLabel -> {"x", "E[x]"}, 
        PlotStyle -> {{Hue[0.6], Thickness[0.008]}, 
            GrayLevel[0]}];