Needs["VectorAnalysis`"]

SetCoordinates[Cylindrical[r,fi,z]];

Jvect={0,0,Jz[r]};
rce=Thread[Laplacian[Jvect]==I*ω*γ*μ*Jvect][[3]]

res=(DSolve[rce,Jz[r],r][[1]]//Simplify[#,{r>0,γ>0,μ>0,ω>0}]&)/.C[2]->0

(*BesselY[0,(((-1))^(3/4)) r (Sqrt[γ μ ω])]/.{γ->57*(10^6),ω->314,μ->4.*Pi*(10^(-7))}
Plot[{Re[%],Im[%]},{r,0,0.05},PlotStyle->{{Thick,Red},{Thick,Blue}},AxesLabel->{"r[m]","BJ"}]*)

Bvect=((Curl[((Jvect/.res)/γ)])/(-I*ω));

resC1=Solve[Isum==Integrate[r*Jz[r]/.res,{r,0,rmax}],C[1]]

dos={γ->57*(10^6),ω->314,μ->4.*Pi*(10^(-7))};
hustota[rmax_]=((Jz[r]/.res)/.resC1[[1]]/.dos)/.Isum->1;

p[rmax_]=((hustota[rmax]*Conjugate[hustota[rmax]])/(2γ))/.dos;

vykon[rmax_,proud_]:=2*Pi*(proud^2)*delka*NIntegrate[r*p[rmax],{r,0,rmax}]//Chop;

Plot[{((vykon[rmax,3*(10^4)])/1000)/.delka->1.5,400},{rmax,0.01,0.05},PlotRange->All,AxesLabel->{"rmax[m]","P[kW]"},AxesOrigin->{0,0},PlotStyle->Thick]

volbarmax=0.03;
voldelka=1.5;
volvykon=400;
dosproud=Solve[volvykon==((vykon[volbarmax,proud])/1000)/.delka->voldelka][[-1]]

Eckopovrch=(1/γ)((proud*hustota[volbarmax]/.r->volbarmax)/.dosproud[[1]])/.dos;
napeti=Abs[voldelka*Eckopovrch];
Print["napeti je ",napeti,"  V"]
Print["proud je ",proud/.dosproud,"  A"]

hustota