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RE: Re: Plot using DSolve HELP

• To: mathgroup at smc.vnet.net
• Subject: [mg104485] RE: [mg104412] Re: Plot using DSolve HELP
• From: "Becky" <noslowski at comcast.net>
• Date: Sun, 1 Nov 2009 17:54:45 -0500 (EST)
• References: <hce3uu$r15$1@smc.vnet.net> <200910310649.BAA13050@smc.vnet.net>

I just want to thank those that have helped me with my problem.  You were
right, I was missing " == ".  Once I fixed it, the problem worked out fine.

Thanks
Jake 

-----Original Message-----
From: Peter Breitfeld [mailto:phbrf at t-online.de] 
Sent: Saturday, October 31, 2009 2:50 AM
To: mathgroup at smc.vnet.net
Subject: [mg104485] [mg104412] Re: Plot using DSolve HELP

"Becky" wrote:

> I have a problem with what I am showing below.
>
> Step one is my equation.
> eq = i * v[t]/r +c v'[t]
>
> Step two I want to solve it, using DSolve, and put my values in as shown.
> Please note, that "i= 0", and my IC is v[0]== -70.
>
> sol=v[t]/.DSolve[{eq/.{c->3, i->0, r->2.5} ,v[0]==-70}, v[t], t]  {-70 
> *-0.133333 t} The results are shown above.
>
> Thus next step is to plot it.
> p1=Plot[sol,{t,-20,140}]
>
> My problem is that I get a line from -70 on the vertical axis going in 
> a curve up to the horizontal axis out to 140.  The vertical axis is 
> "v" and the horizontal is "t".
>
> What am I doing wrong?

I don't get your result. First, eq is not an equation, maybe you meant eq1=
i* v[t]/r+c v'[t]==0

If you set i->0, eq reduces to v[t]==const, so v[t]=v[0]=-70

The general solution is

DSolve[{eq1,v[0]==-70},v[t],t]
Out= {-70 Exp[-i t/(c r)]}


--
_________________________________________________________________
Peter Breitfeld, Bad Saulgau, Germany -- http://www.pBreitfeld.de