It's not so much Ohm's law does not apply, as you have characteristics
based on the device(s) you are using.  Take for example a FET
transistor.  Your drain current (when the device is in sat.) is iD = 1/2
K'n (W/L)(Vgs-Vt)^2 assuming no early voltage/effect.  Now, as you can
see, if you apply a constant Vgs (gate to source voltage) you can change
the amount of current the device will pass by altering the Width and
Length of the channels in the device.  These are all IC characteristics,
but this is how you produce a resistor from a transistor.

So the long and short of it is, Ohm's law is correct, but it really only
applies in specific instances.

If you really want to fold your head inside out, then imagine a wire
becoming a resistor, capacitor and inductor all at the same time.  This
is what happens when you run an extremely high frequency signal through
it.  So the value of the resistance of a wire changes with frequency.
But if you look in to the physics of it, you'll also see it can change
based on cross sectional area.

So as you can see, it's easiest and typically most useful to write Ohm's
law as V=IR (usually interested in voltage, and it's easier to write).
However, it really is quite simplistic and when it comes to computing
currents, etc. things become a tad convoluted when we move away from
your typical (low f) discrete based design.

I hope this answers some of your questions.

I encourage you to look in to:
BJTs (bipolar junction transistors, typical trans..)
FETs (Field Effect Transistors)
Transmission lines
Anything regarding frequency response in a circuit

All those things should keep you busy for at least a month.


Shawn Wilton
Junior in CpE
MicroBiologist

Phone: (503) 881-2707
Email: shawn@black9.net

http://black9.net


Rick Regan wrote:
>>>But aren't R and V the only two of the three variables
>>>that you can manipulate directly?
>>
>>No, not really.  There is a current source in the EE lab
>>that maintains a set current regardless of voltage or
>>resistance (within reason).  I think ordinary
>>transistors do this to some extent also, as they are
>>current amplifying devices.
>
>
> I think I'm getting in over my head here (I'm not an EE).
> Could you elaborate on how you can produce current, and
> how you can do so keeping R and V constant?  So in other
> words, are you saying Ohm's law does not apply in certain
> cases?
>
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