Date:         Thu, 13 Nov 2003 22:27:43 -0500
Reply-To:     "Daniel L. Katz" <katzd54@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         "Daniel L. Katz" <katzd54@YAHOO.COM>
Subject:      Re: Fwd: RE: "Green" solar battery charger
Comments: To: Jack Cook <john.cook58@VERIZON.NET>

jack:

ohm's law really isn't a law, in the sense that newton's laws, for
example, are physical laws. that is, if motion is studied in finer and
finer detail, the limit is complete agreement with newton's laws. on the
other hand, if current in a resistor is studied with increasing detail,
the picture gets more and more complicated, with all kinds of empirically
determined parameters, and so on.

dlk

On Thu, 13 Nov 2003 19:05:38 -0800, Jack <john.cook58@VERIZON.NET> wrote:

>re: "... ohm's law implies that a current of 12 V/300 Ohm = 0.04 A
>will flow ..."
>
>No!  It doesn't "imply".  It's the LAW!
>
>Sorry.  Couldn't resist.
>
>//Jack
>
>
>On 13 Nov 2003, at 16:59, Daniel L. Katz wrote:
>
>> list:
>>
>> kindly note that current and voltage are not independent, and that 100
mA
>> through the heart for 1 s would probably be fatal.
>>
>> in the simplest cases, current and voltage are related fairly precisely
>> through ohm's law:
>>
>> I = V/R
>>
>> where I = current and V = potential difference across R. for example,
if 12
>> volts is applied across a 300 ohm 1 W resistor, ohm's law implies that a
>> current of 12 V/300 Ohm = 0.04 A will flow through the resistor. it
makes no
>> difference whether we say the voltage is 12 V on one side of the
resistor and
>> 0 volts on the other, or 1,000,012 volts on one side and 1,000,000
volts on
>> the other, we get the same 12 V potential difference across the
resistor, and
>> the same amount of current, 0.04 A, will flow. in this case, the power
>> developed by the resistor would be 0.48 watts, within its 1 W power
rating.
>> now, what if we substitute a 30 ohm 1 W resistor instead? well ohms law
would
>> predict a 0.4 A curren and heat produced at the rate of 4.8 W, nearly 5
times
>> the power rating of the resistor; in this case, ohm's law does its job
in so
>> far as letting us know that the resitor would fry, but the resistor
itself
>> would change value significantly as it burned up.
>>
>> dlk
>>
>>
>>
>> >The charge LED flashes fine even in pretty low light, but it's only
>> >indicating that there's voltage present... however there's nearly no
>> >current, and it's current, not voltage, which will charge a
>> >battery... or an electrocution! (a 100mA current of 1,000,000V will
>> >not even be felt).