Solving an ODE for the path of an electron

Hello,

I am trying to plot an electron's trajectory through a magnetic field. However, I don't understand how to program the code to recognise that the magnetic field varies spatially.

Here is my go:

function xdot = electron(t,x);
  xdot = zeros(2,1);
  xdot(1) = x(2);

%my piecewise magnetic field

    function Mag = B(X)
      if (10 <= X) && (X <= 20)
      Mag = 100*10^(-4);
      else
      Mag = 0;
      end

    end

   xdot(2) = -(5.6933643*10^(-15)) * (x(2) * Mag(x(1)));

end

[t,x] = ode45('electron',[0, 300],[0, 1]);

Does it make sense to put Mag(x(1)) in my equation? I'm sure that's the problem, but I lack experience to know why and how to fix it.

Thank you!