this is getting ridiculous.

dividing voltage by current to compute resistance is not ohm's law. ohm's
law applies to devices with constant resistance, thus providing a linear
relationship between current and voltage: I = V/R; in other words, a plot
of current vs voltage would be a straight line with slope = 1/R passing
through the point (I=0, V=0).

some devices obey ohm's law, while others do not; no device obeys ohm's
law exactly; ohm's law fails for all devices at sufficiently high voltage.

the light bulb filament alluded to below obeys ohm law over small ranges
in current value, but R evidently increases in value by a factor of
roughly 14 from zero current (cold) to working current (hot). the filament
therefore does not obey ohm's law over such a large range in current
value.

dlk

On Mon, 17 Nov 2003 08:53:09 -0800, Steve Delanty
<laurasdog@WEIRDSTUFFWEMAKE.COM> wrote:

>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"