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06-03-2008
03:00 AM

06-03-2008
03:00 AM

Help with ODE

Hi, I am having trouble solving for this ODE or at least graphing a ODE. I am looking for voltage as a function of time v(t) or v(x) (i've tried both here) I keep getting unit errors, even when I solve this using unitless parameters. Also, is there any way to display the equation or does Odesolve only give graphical solutions. I uploaded in Mathcad 13 & 14.

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06-03-2008
03:00 AM

06-03-2008
03:00 AM

Help with ODE

There is no equation. ODESolve is a numeric ODESolver (just a wrapper around rkfixed or Rkadapt, depending on options).

Generally the adaptive integrator is better than the fixed step integrator.

You need to have an understanding of your system. Integrating over a thousand seconds when the system stabilizes in about five milliseconds is not only meaningless, it results in major numerical problems. Note that the ODE is actually evaluated at a fixed number of points (as you request, default is one thousand) and the function returned by ODESolve is not a function that solves the ODE but rather a cubic spline (poorly chosen) that interpolates the calculated points of the solution. If all the calculated points are long after the system quiesces, this interpolation will have no useful information.

__________________

� � � � Tom Gutman

Generally the adaptive integrator is better than the fixed step integrator.

You need to have an understanding of your system. Integrating over a thousand seconds when the system stabilizes in about five milliseconds is not only meaningless, it results in major numerical problems. Note that the ODE is actually evaluated at a fixed number of points (as you request, default is one thousand) and the function returned by ODESolve is not a function that solves the ODE but rather a cubic spline (poorly chosen) that interpolates the calculated points of the solution. If all the calculated points are long after the system quiesces, this interpolation will have no useful information.

__________________

� � � � Tom Gutman