Re: Why doesn't Mathematica solve this simple differential equation?

*To*: mathgroup at smc.vnet.net*Subject*: [mg69268] Re: Why doesn't Mathematica solve this simple differential equation?*From*: Paul Abbott <paul at physics.uwa.edu.au>*Date*: Mon, 4 Sep 2006 04:47:09 -0400 (EDT)*Organization*: The University of Western Australia*References*: <eddqq8$3vq$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

In article <eddqq8$3vq$1 at smc.vnet.net>, Joseph Gwinn <JoeGwinn at comcast.net> wrote: > Here is the system I'm trying to solve. It's an electrical circuit > consisting of a capacitor C1 (with initial voltage 4.0 volts), a > resistor R1, and a diode in series. There is an NDSolve example involving a diode in the built-in documentation. Evaluate FrontEndExecute[FrontEnd`HelpBrowserLookup[ "DemosLink", "Chaotic Circuit"]] and click on the "Demos" link. > Approach 1: > > eqns11 = {Q1'[t] == -Iloop[t], Q1[t] == C1*Vc[t], Vr[t] == > R1*Is*Exp[Vd[t]/0.026], Vc[t] == Vr[t] + Vd[t], Vc[0] == 4.0} > > eqns12 = eqns11 /. {C1 -> 1.0*10^-6, R1 -> 16, Is -> 10^-13} > > eqns12soln = NDSolve[eqns12, Q1, {t, 0, 1}] > > Approach 2: > > eqns21 = {Vc'[t] == -Id[t]/C1, Vc[t] == 0.026*Log[Id[t]/Is] + R1*Id[t], > Vc[0] == 4.0} > > eqns22 = eqns11 /. {C1 -> 1.0*10^-6, R1 -> 16, Is -> 10^-13} You mean eqns22 = eqns21 /. {C1 -> 1.0*10^-6, R1 -> 16, Is -> 10^-13} ? Even so, your equations are inconsistent. > eqns22soln = NDSolve[eqns22, Vc, {t, 0, 1}] > > Both approaches fail with Mathematica complaining that "NDSolve::ndode: > Input is not an ordinary differential equation". Which is not suprising. Cheers, Paul _______________________________________________________________________ Paul Abbott Phone: 61 8 6488 2734 School of Physics, M013 Fax: +61 8 6488 1014 The University of Western Australia (CRICOS Provider No 00126G) AUSTRALIA http://physics.uwa.edu.au/~paul