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MathGroup Archive 2005

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Re: Re: RC circuit

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56862] Re: [mg56845] Re: [mg56844] RC circuit
  • From: DrBob <drbob at bigfoot.com>
  • Date: Sun, 8 May 2005 02:10:13 -0400 (EDT)
  • References: <200505071935.PAA26942@smc.vnet.net>
  • Reply-to: drbob at bigfoot.com
  • Sender: owner-wri-mathgroup at wolfram.com

To insert values for "data" at the last minute (even in the Plot legend), try this:

Block[{Plot, ToString, StringJoin},
  Plot[v@t, {t, 0, 50}, Frame -> True,
  FrameLabel -> {"t seconds", "v volts"},
  PlotLabel -> "Voltage in an RC Circuit",
  Epilog -> {Text["c = " <> ToString@c <> " Farads", {30, 4}, {-1, 0}],
    Text["i = " <> ToString@i <> " Amperes", {30, 3.5}, {-1, 0}],
    Text["r = " <> ToString@r <> " Ohms", {30, 3.0}, {-1, 0}]},
  ImageSize -> 450] /. data
];

That eliminates the need for Evaluate, too.

Bobby

On Sat, 7 May 2005 15:35:07 -0400 (EDT), David Park <djmp at earthlink.net> wrote:

> Here is one method to solve your exercise. You can use 'i' for current but
> not 'I', which is a reserved symbol. In general you should avoid symbols
> that start with capital letters. Also notice that 'equal' is represented by
> == in Mathematica.
>
> Here is your equation.
>
> Clear[v];
> deqn = i == v[t]/r + c v'[t];
>
> Now we solve the equation and add the initial condition that v starts at 0
> when t == 0.
>
> Clear[v];
> dsolutions = DSolve[{deqn, v[0] == 0}, v, t]
> v[t_] = v[t] /. Part[dsolutions, 1, 1]
> {{v -> Function[{t}, ((-1 + E^(t/(c*r)))*i*r)/E^(t/(c*r))]}}
> ((-1 + E^(t/(c*r)))*i*r)/E^(t/(c*r))
>
> Now we need a set of data for some particular case to plot.
>
> (data = {c -> 3, i -> 2, r -> 2.5}) // TableForm
>
> Now we can plot, We want to substitute the data into the expression for v[t]
> and we want to evaluate the expression in the Plot statement. I've added
> labels and text to make a more informative plot.
>
> Plot[Evaluate[v[t] /. data], {t, 0, 50},
>     Frame -> True,
>     FrameLabel -> {"t seconds", "v volts"},
>     PlotLabel -> "Voltage in an RC Circuit",
>     Epilog -> {Text["c = 3 Farads", {30, 4}, {-1, 0}],
>         Text["i = 2 Amperes", {30, 3.5}, {-1, 0}],
>         Text["r = 2.5 Ohms", {30, 3.0}, {-1, 0}]},
>     ImageSize -> 450];
>
> It does take a little time to learn the syntax and available commands in
> Mathematica before one can efficiently use it to solve practical problems
> and homework exercises. But it is worth it if you expect to do much of it.
>
> David Park
> djmp at earthlink.net
> http://home.earthlink.net/~djmp/
>
>
>
>
>
> From: pennsylvaniajake at netscape.net
To: mathgroup at smc.vnet.net
>
>
>
>  Two ?'s  I am trying to solve a simple RC circuit.  ?1  I can's use the
>  letter "i" for current, is there a way around this ?
>  ?2. How do I solve this equation for dv/dt,  eq:  i = v/r + C dv/dt
>  thanks
>
>>
>
>
>
>
>
>



-- 
DrBob at bigfoot.com


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