>>Actually, I would go a tad further and say that none of the three exists
>>without the other two.

(I think when the OP said "you can/can't have x without y and z" I took
that to mean "you can/can't have the value of x without the values of y and
z" and responded that way.)


>>>>   - You can have V without I
>>
>>In order to have a voltage without a current, you would have to
>>postulate a resistance with the value "infinity" -- something possible in
>>theory, but not really in practice.
>
> You better hope it's achievable, the program memory storage in your pic works that way.

I suppose you are talking about Flash memory. Are you sure that there is no
current? I mean, not "no current for practical purposes in most cases" but
/no/ current? Why do Flash devices have limited (and for long-term or
high-temperature applications actually relevant) data retention times?
Somehow the charges seem to be slowly moving out of their location (or
charges moving in, depending on the POV). Maybe as slowly as a few
electrons per second, but isn't that more than nothing?


>>Again, how can you measure a resistance without moving any current (charge
>>carriers) through the resistance?
>
> It doesn't stop being a resistance when there's no current flowing.

How do you know? So far, I thought that resistance was being defined by the
ratio between a voltage and a current. What would be the value of 0/0 (if
there really was a 0)?

>> Keep in mind that the concept of a fixed
>>resistance (independent of anything else) is a simplification, helping in
>>quick and dirty engineering, but is not correct. A resistance is only the
>>proportional constant between the current and the voltage. There is no
>>resistance without a current (and a voltage), as the resistance is only
>>defined by the relationship between those two.
>
> No.  That's how you MEASURE resistance, but resistance itself is an intrinsic property of the material, the free electron mobility.

Don't you have to measure the resistance in order to know it? I agree that
every material has a resistance. But the value of it under certain
conditions is not known until it has been measured. Assuming it to be
constant is, as we all should know, a simplification that has its limits.
So in order to really know whether that assumption is correct, you'd have
to measure the resistance. And I think that in order to measure resistance,
you would have to measure both voltage and current. You don't measure, you
have an assumption of a resistance value, not a resistance value.

(I'm sure I'm not the only one who ever has forced a current through a
device and measured the voltage across it, to find out whether this device
was still the resistor it maybe once was -- or maybe never was. I wouldn't
trust the "intrinsic properties" when troubleshooting a device -- the last
word is a measurement to find out what is and what is just illusion :)


>>>>   - You CAN'T have I without V and R
>>
>>Now this one is the only one that's correct.
>
> Yes, you can. Messiner effect in superconductors demonstrates this quite nicely.

I'm not familiar enough with superconductors to be able to discuss this
with any depth. But in my (engineering and other) experience there is not
really much if anything in nature (which includes technology, in this
context) that's literally 0 or infinity. The two are mathematical concepts,
not physical ones. In physics, they tend to be simplifications that are
valuable and helpful within their limits. Both usually mean "outside the
range of interest (or measurability)".

Actually, if you take it by face value, no scientist would ever say that
there's 0 (or infinity) of anything. In order to be able to say so, you
have to measure. You can't measure neither. The only thing you can say is
that it's less or more than what you can detect with your experiment
configuration. So yes, there are currents less than 1 aA, and generally
they are /considered/ 0 for most practical purposes. But /are/ they 0?

Back to superconductors: The fact that there is no potential difference in
the first place that makes the charge carriers move when applying a
magnetic field to a superconductor doesn't necessarily mean there is no
potential difference once they started moving.

Gerhard

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