?DSolve

rce = R*i[t] + L*i'[t] - U1 == 0
poc = i[0] == 0

res = DSolve[rce, i[t], t]

Solve[(i[t] /. res[[1]] /. {t -> 0}) == 0, 
      C[1]]

res2 = DSolve[{rce, poc}, i[t], t] // Simplify

dos = {R -> Rzmen, L -> 1, U1 -> 5}

i[t] /. res2[[1]] /. dos

Plot[i[t] /. res2 /. dos, {t, 0, 0.01}, 
        PlotRange -> All];

U2 = L*D[i[t] /. res2, t] /. dos

U2[[1]] /. t -> 0.001

mrizka = {{0.001, 0.002, 
          0.004}, {(U2[[1]] /. t -> 0.001), (U2[[1]] /. 
              t -> 0.002), 
(U2[[1]] /. 
              t -> 0.004)}}

graf=
Table[
Plot[U2,{t, 0, 0.01},
PlotRange -> All,
PlotStyle -> {Hue[1], Thickness[0.02]},
\(AxesLabel -> {"t[s]", \*""(Subscript[U, 2])[V]""}\),
PlotLabel -> {"U2", U2[[1]], "R", 
                      Rzmen},
TextStyle -> {FontFamily -> "Times", 
                      FontSize -> 12},
PlotPoints -> 100,Frame -> True,
GridLines -> mrizka
],{Rzmen, 1000, 2000, 500}];

graf2 = graf /. Hue[1] -> Hue[0.6]

Show[graf]

Show[graf2]

Export["c:/grafik.gif",graf,ImageSize -> 500,ConversionOptions -> {"AnimationDisplayTime" -> 
                0.5, "Loop" -> True}]

FullForm[res]

FullForm[Frame -> True]

graf2/.Rule[Frame, True]->Rule[Frame, False]

<< Graphics`Animation`

ShowAnimation[graf2]

rce1 = -u1 + R1*ia[t] + L*D[ia[t] - ib[t], t] + 
          R2*(ia[t] - ib[t]) == 0
rce2 = 
      R3*ib[t] + R2*(ib[t] - ia[t]) + L*D[ib[t] - ia[t], t] == 
        0
poc2 = ia[0] - ib[0] == 0

res = 
      DSolve[{rce1, rce2, poc2}, {ia[t], ib[t]}, t] // 
        FullSimplify

dos = {R1 -> 1  k, R2 -> 2  k, R3 -> 500. , 
        L -> 4. , 
        Um1 -> 10. , ϕ -> -π/4, ω -> 1000, 
        u1 -> Um1*Sin[ω*t + ϕ], k -> 1000}

iacko = ia[t] /. res[[1]];
ibecko = ib[t] /. res[[1]];
ixko = Simplify[(iacko - ibecko) //. dos]

Plot[ixko, {t, 0, 0.012}]

rce3 = -u1 + R1*ic[t] + R3*(ic[t] + id[t]) == 
        0
rce4 = 
      R3*(id[t] + ic[t]) + R2*(id[t]) + L*D[id[t], t] == 
        0
poc3 = id[0] == 0

res = 
      NDSolve[{rce3, rce4, poc3} //. dos, {ic[t], id[t]}, {t, 0, 0.012}] // 
        FullSimplify

res = 
      DSolve[{rce3, rce4, poc3}, {ic[t], id[t]}, t] // 
        FullSimplify

Plot[-id[t] /. res //. dos, {t, 0, 0.012}]

napeti = 
      L*D[(-id[t] /. res[[1]] //. dos), t] //. dos

Plot[napeti, {t, 0, 0.012}]

Plot[
      Evaluate[L*D[(-id[t] /. res[[1]] //. dos), t] //. 
          dos], {t, 0, 0.012}]

Clear[a, b, c]

a + b + c /. {a -> 2  kilo, 
        kilo -> 1000}
a + b + c //. {a -> 2  kilo, kilo -> 1000}