Date: Mon, 17 Nov 2003 08:53:09 -0800
Reply-To: laurasdog@WEIRDSTUFFWEMAKE.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Steve Delanty <laurasdog@WEIRDSTUFFWEMAKE.COM>
Subject: Re: Fwd: RE: "Green" solar battery charger
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At 10:08 PM 11/16/2003, Daniel L Katz wrote:
>(1) as you well know, an ohm's law device is one in which current is
>proportional to voltage: I = V/R. double the voltage and the current
>doubles; halve the voltage and the current drops to half; reverse the
>voltage and the current changes sign. an ordinary copper wire is a good
>example of a practical ohm's law device.
Damn, I was gonna try and stay out of this...
I have here on my desk am ordinary 40 watt incandescent bulb.
I know from experience, and hope you will agree, that when run on it's
nominal voltage of 120V, that it WILL consume 40 watts of power,
within a *reasonable* percentage.
Since 40 watts divided by 120 volts = 0.333 amps (approximately)
we can assume that the bulb does indeed use about 1/3 of an amp.
We can take it farther and calculate that to pass 0.333 amps at 120
volts, the bulb MUST have a resistance of 360 ohms.
THIS is ohms law in action...
So far so good?
Now I take my trusty ohmmeter, which I know to be accurate within
a reasonable percentage. I measure the resistance of the bulb and
find that it is 25.3 ohms. UH OH! Ohms law sez our bulb should
actually draw 4.74 amps and consume over 569 watts of power !!!
So, we've had a failure of ohms law according to you?
No. We've simply demonstrated that some materials can change
their resistance when current is passed through them or voltage
applied across them. Indeed, the light bulb DOES consume
about 4.7 amps and 500+ watts of power for a very brief period
of time. But as the filament rapidly heats up under the inrush
of current, its resistance changes and goes up dramatically.
There's no failure of ohms law. Only the resistance of a material
rapidly changing in response to it's environment.
A piece of copper wire does the same thing, and resistance of
copper wire is always specified at a specific temperature.
Ohms law works fine on copper too.
>(2) a diode does not obey ohm's law. in particular, reverse the voltage on
>an initially forward biased diode and the current stops.
When a voltage is applied across the crystalline structure of the PN
junction, electrons migrate into different positions and the physical
resistance of the device changes. The resistance is still there, and
voltage and current follow it appropriately, it's just that the material
changes in more complex ways than an ordinary copper wire or
tungsten light bulb filament.
>(5) no device obeys ohm's law exactly,
Ahhh, here's the source of the trouble....
What is exactly? To how many decimal places?
What super-micro-quantum level do you want to dissasemble
it to, to make it not work...?
When you disassemble anything far enough, everything we
know breaks down. It can even be argued that nothing really
exists. Including you. And me.
A lot of us have had our Vanagons pretty thoroughly disassembled,
but I doubt any of us are ever going to take them so completely
apart that ohms law fails on *any* of the pieces.
> and ohm's law fails for all devices
>at sufficiently high voltages.
How high? I used to have a beautiful glass and ceramic resistor.
It was about 5 feet long and 3 inches in diameter. It was rated
at 12.2 Megohms, with a 1% tolerance. Maximum continuous
voltage rating was 250KV. Sounds like a pretty stable resistance
at a pretty high voltage...
Some materials, like air are easily ionized. Once you've ripped
the electrons loose from their atoms, the resistance of things
changes. I'll bet you'd be less resistant too if most of your
electrons were ripped loose!
Ohms law still works. Only the conditions have changed.
Applying the voltage has changed the physical properties of
the material, and so the resistance changed.
Neon tubes, and cold cathode lighting all use a high voltage to
pass current through a column of ionized gas. If you know the
types of gasses and their pressures and the current through
the tube and the diameter of the column of gas, and a few
other factors, the voltage across the tube at a particular
current can be accurately calculated.
You just need to realize that the physical resistance changes
with applied voltage and current. Ohms law still works, but you
need know the properties of the material and specify the exact
conditions under which measurement will be taken.
Just because it becomes complex doesn't mean it isn't working.
I've worked on quite a bit of electrical stuff, from microvolts to
100s of KV, and never seen anything where ohms law didn't
work. Sometimes I didn't have the right equipment, or wasn't
smart enough to interpret the results of measurements, but
there was no reason to assume that ohms law had crapped
out on me.
Use your meters with confidence folks, ohms law isn't
going to fail you on any of your Vanagon parts...
Steve
EJ22 -> '86 Westy "Escape Pod"