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Re: R = V/I ?



At 2:30 PM -0700 5/9/00, Chuck Britton wrote:

WORDS, gotta LOVE 'em or HATE 'em.



At 1:59 PM -0700 5/9/00, Leigh Palmer wrote:


>A linear I-V plot does not make a resistor. An I V plot of a battery
>is linear (at least for small current draws) but it is not a
>resistor. For a resistor, the graph must be proportional, linear
>with no intercept.

I said that? I don't recall doing so. I wouldn't have in a fully
awake condition. Someone else said that.

The last EE course which I took made the point that the ONLY
'linear' relations were those that were straight lines passing
through the origin.

other definitions may apply, but this one was considered
important back in the '80's.

I think your EE course was nonstandard. I consider y = mx + b to be
a linear dependence of y upon x, given b and m constant.

>
>I wish texts would make clear that "internal resistance" of a battery
>is a dynamic resistance, and that this model of a battery only works
>for limited amounts of current drawn from the battery.

The internal resistance r of a battery is not a dynamic resistance. It
is a model parameter that yields the approximate potential difference
V across a battery rated at E emf when current I is drawn from it:

V = E - Ir

Again, we will find that EE's use the term 'dynamic resistance' (or
impedance) as the SLOPE of the I V plot (or rather its inverse) as
opposed to the slope of the straight line drawn from the origin to
the point in question.

In this sense the internal resistance of a battery or power supply
seems to be a dynamic impedance and would be appropriate for
calculating AC characteristics thereof.

That's fine, Chuck. I will continue to call it the internal
resistance of the battery and to treat it as a model parameter. I
bet most teachers will continue to do so.

Leigh