Date:         Fri, 14 Nov 2003 19:08:15 -0500
Reply-To:     David Brodbeck <gull@GULL.US>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Brodbeck <gull@GULL.US>
Subject:      Re: Fwd: RE: "Green" solar battery charger
Comments: To: "Daniel L. Katz" <katzd54@YAHOO.COM>
In-Reply-To:  <vanagon%2003111417251202@GERRY.VANAGON.COM>
Content-Type: text/plain; charset=us-ascii; format=flowed

Daniel L. Katz wrote:
> tom and david:
>
> i thought you guys were believers in ohm's "law". if 1 MV is applied
> across an ideal ohms law resistor with a value of, say, 500,000 ohm (100
> ohm), characteristic of a very dry (soaked with salt water) human being,
> the current would be 2 A (10,000 A), and the corresponding thermal power
> would be 2 MW (10 GW). note that the average hydroelectric output of the
> entire bonneville power administration is some 6 GW. of course, ohm's law
> fails in such unfortunate circumstances.

How does Ohm's law fail in that situation?  2 MW will, in fact, be
consumed *if* your source can really provide 1 MV at 2 amps!  If your
source can't, well, that just proves your source has resistance too.
Every real current source has an internal resistance that acts exactly
like a resistor in series with an "ideal" current source.  In fact, if
you put a heavy load on your car battery and measure how much the
voltage drops, you can calculate the internal resistance of the battery.
  This is a pretty good indicator of its health.  A good 12V battery
will have a very low internal resistance.  If you try this with, say, a
12V dry cell lantern battery, you'll see a significant voltage drop
because the internal resistance is high.  (Internal resistance also
dissipates heat like a normal resistor, which is why batteries get hot
when you discharge them fast.)

Additionally, if you really try to put 2 MW through your resistor you
may find it'll cease to exist almost instantly, but before it does it
*will* act according to Ohm's law. ;)


--

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si -- For Sale
'86 Volvo 240DL wagon