On Thu, 28 Sep 2006, Rick Glazier wrote:
Gary VanderMolen wrote:
Ohm's Law tells us that current equals voltage divided by
resistance.
From: "Pete Holsberg"
Actually, it's voltage divided by IMPEDANCE for AC. :-)
I think Ohm had died before we made AC so he could not put
it in his law...
True, but the law was "moderized". :)
For DC V = I * R (R=Resistance)
For AC V = I * Z (Z=Impedance)
The second equation is a simplified version of the following
(From Wikipedia) "For an AC circuit Ohm's law can be written
{V} = {I} * {Z}, where V and I are the oscillating phasor
voltage and current respectively and Z is the complex impedance
for the frequency of oscillation."
Also Ohm's Law is constrained to "ohmic" devices. As Wikipedia
states "Ohm's law applies to conductors whose resistance is
(substantially) independent of the applied voltage (or
equivalently the injected current). That is, Ohm's law only
applies to the linear portion of the I vs. V curve centered
around the origin. The equation is just too simple to encompass
devices described by a more complicated I vs. V relationship."
Wiki also states "Ohm's work long preceded Maxwell's equations
and any understanding of frequency-dependent effects in AC
circuits. Modern developments in electromagnetic theory and
circuit theory do not contradict Ohm's law when they are
evaluated within the appropriate limits."
Wiki article mentioned above is at:
<http://en.wikipedia.org/wiki/Ohm's_law>
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