# Simple Ohms law question

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#### rif

Paid Member
I've always seen it as V=IR.

I've always thought about it as R=V/I (resistance is defined by the ratio)

But I've been thinking, is it more accurate to say R= (change in V) / (change in I) ?

It doesnt make a difference if the V vs I curve is linear and goes the V=0 and I = 0

Any other kind of curve (tube transfer function, transistor curves, diode curves, etc.) and you get different results.

#### JonSnell Electronic

Transfer function of valves is nothing to do with Ohms Law. You are correct V = I * R. That's it.

#### rayma

Is it more accurate to say R= (change in V) / (change in I) ? Any other kind of curve
(tube transfer function, transistor curves, diode curves, etc.) and you get different results.

Yes, for a nonlinear device this is the incremental (or differential) impedance, the slope of the tangent
to the V vs I curve at the operating point, or z = dv/di. This is in general different from the static resistance,
except for perfect resistors. https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance

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#### sreten

Hi,

Yes, for a diode you can only talk about the dynamic impedance,
as its not a resistor it does not follow ohms law, in any sense.

Generally you'd be talking about a devices transconductance variation
rather than the somewhat misleading dynamic resistance concept.

rgds, sreten.

#### Ron E

It's not a simple question. dV/dI being "more correct" is a false concept.
More generally V/I=Z, but once you get into things like tubes and transistors you aren't talking about a simple 2 port passive linear device anymore.

#### DF96

rif said:
But I've been thinking, is it more accurate to say R= (change in V) / (change in I) ?
Only when the slope or AC or dynamic resistance is the relevant parameter. In other cases V/I is correct. It all depends on context.

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