Ohms Law and Its Practical Limits: Investigating a 10um Steel Wire

[jim hardy]

and Francis Bacon :

Read not to contradict and confute, nor to believe and take for granted... but to weigh and consider.

 

[sophiecentaur]

Like the Curate's Egg, this thread has some very good parts in it. However, they are mixed with some naive and badly informed bits. As with many of the questions that get asked on these fora, there are several possible levels of treatment and this has surely led to some confusion here.
There is no essential contradiction in the Macroscopic and QM approaches but I think some of the contributors are glorying in the prospect of there being some conflict. A very counter - productive attitude.

 

[jim hardy]

there are several possible levels of treatment and this has surely led to some confusion here.

amen

when i re-read thread and realized "QM" effects was relating to nanostructures , well,
okay;
i don't deal with anything smaller than surface mount resistors.
QM is outside my experience.

consider another proportionality

Rate = Distance/Time

As a member of 'unwashed masses' i won't embrace QM unless it'll get me out of a speeding ticket.


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if i offended anyone please accept this apology.

 

[sophiecentaur]

i don't deal with anything smaller than surface mount resistors.
QM is outside my experience.

consider another proportionality

Rate = Distance/Time

As a member of 'unwashed masses' i won't embrace QM unless it'll get me out of a speeding ticket.
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if i offended anyone please accept this apology.

Amen to that. You are obviously a practical person and Ohm's Law is one of the best practical descriptions of a physical process in the book. It beats the 'SUVAT' equations of motion into a cocked hat for getting things right.

"Unwashed" is the way forward.

 

[Kholdstare]

there are several possible levels of treatment and this has surely led to some confusion here.

I'm sorry. But when someone asks me limit of something I usually look for the validity of that thing in more difficult levels where it might fail. If we only considered the easier levels we would not be looking for its 'limits'. Would we?

There is no essential contradiction in the Macroscopic and QM approaches but I think some of the contributors are glorying in the prospect of there being some conflict. A very counter - productive attitude.

There is contradiction. The application of macroscopic and QM approach on same structure gives different results.

PS: Rate = 1/Time and Velocity = Distance/Time

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If I had been rude to anyone or offended anyone please accept my apology.

 

[sophiecentaur]

I guess you haven't read what's been written in the same way as I have read it.
I take your point about 'limits' but many of these posts are referring to a strictly practical case yet ignoring the fact that Ohm's Law specifies constant temperature. A hot wire does not 'disobey' Ohm's Law because its conditions are not those which are specified in the terms of Ohm's Law. Treating the Original Question in macroscopic terms is quite valid and some of the misconceptions, at that level, need to be sorted out first - before bringing in QM. (You clearly never had a problem at that level so you may not appreciate that some mere mortals actually struggle, even there.)
There is no question that QM and macroscopic treatments will give different answers for small numbers and the 'limits' are, of course, worth discussing. It is as well, however, to avoid confusion which can (and does) frequently occur when a thread continues on two levels with the two sets of contributors continually mis-interpreting the contributions of the other set.
I would challenge you to find a Classical Law that does anything like as well, in practical applications, over such a vast range as Ohm's Law manages to.

Incidentally, "rate" is frequently used in other contexts than 1/time. Take the financial interest 'rate' and the atmospheric temperature lapse 'rate', for instance.

 

[jim hardy]

might this QM effect be at all related to the observable physical phenomenon of "noise" generated by resistors? Carbon I'm told is worst and wirewound the best..

 

[sophiecentaur]

might this QM effect be at all related to the observable physical phenomenon of "noise" generated by resistors? Carbon I'm told is worst and wirewound the best..

Well. Carbon is not a metal so it can't be expected to follow Ohm's Law!
In a metal resistor, the noise is largely due to the distribution of KE amongst the electrons, I think, giving a varying value of thermally generated volts. The thermal noise power is kT.