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I need to get the numeric solution of a pde but the solution diverges

$k \frac{\partial^2 u(x,t)}{ \partial x^2} = \frac{\partial u(x,t)}{\partial t}$

where $k = 2$

and

$u(x,0)=\begin{cases} 0 & x< -1 \\ -100 & -1<x<0\\ 100 & 0 < x < 1\\ 0 & x>1 \end{cases}$

here is my code:

trozos[x_] = Piecewise[{{0, x < -1}, {-100, -1 < x < 0}, {100, 0 < x < 1}, {0, x >= 1}}]

NDSolve[{k D[u[x, t], x, x] == D[u[x, t], t], u[x, 0] == trozos[x]}, u[x, t], {x, -2, 2}, {t, 0, 5}] 

any help is appreciated

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In 12.2, you can follow the link to HeatTransferPDEComponent to set up a transient heat transfer workflow:

trozos[x_] = 
  Piecewise[{{0, x < -1}, {-100, -1 < x < 0}, {100, 0 < x < 1}, {0, 
     x >= 1}}];
vars = {Θ[t, x], t, {x}};
Ω = Line[{{-2}, {2}}];
pars = <|"MassDensity" -> 1, "SpecificHeatCapacity" -> 1, 
   "ThermalConductivity" -> 2|>;
ics = Θ[0, x] == trozos[x];
eqn = HeatTransferPDEComponent[vars, pars] ==
  0
Tfun = NDSolveValue[{eqn, ics}, Θ, {t, 0, 5}, 
   x ∈ Ω];
Plot[Tfun[0.001, x], x ∈ Ω]

enter image description here

If you want additional refinement to sharpen the edges, you could do:

Tfun = NDSolveValue[{eqn, ics}, Θ, {t, 0, 5}, 
   x ∈ Ω, 
   Method -> {"MethodOfLines", "TemporalVariable" -> t, 
     "SpatialDiscretization" -> {"FiniteElement", 
       "MeshOptions" -> {"MaxCellMeasure" -> 0.01}}}];
Plot[Tfun[0.00001, x], x ∈ Ω]

enter image description here

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