General > General Technical Chat
"Veritasium" (YT) - "The Big Misconception About Electricity" ?
adx:
--- Quote from: penfold on March 22, 2022, 10:43:20 am ---...
Engineer and physicist are just job titles, there's nothing more to it, surely? ...
--- End quote ---
That might be the surprising answer. It shouldn't be any surprise if we're all trained in roughly the same stuff and could be at least considered for the same work, but the world seems so preoccupied with pigeon holes that it's hard to see beyond ours. Perhaps that's why they keep us in them!
I too wondered about power supply topologies, and all this patenting and cleverness. Whether it is fundamental enough to count. But all of the transistor team were payrolled scientists effectively, I'm wondering if there's some misdirected envy, grass is always greener styles. I wouldn't have minded being a theoretical physicist, impractical (or even impossible) as that might have been.
TimFox:
Forcing a fixed value of current through a wire and then reducing the diameter of the wire to zero is bad engineering.
It is also bad physics: the lattice constant for copper is 0.36 nm (rather small, but not zero), so a cylindrical wire that small is absurd.
Engineering: try forcing a measly Ampere down a #50 AWG wire, 0.025 mm diameter. The tabulated fusing current for #40 AWG copper is 1.77 A.
Physics: the fundamental variable is the (vector) current density J. A sensible calculation for small diameter wires would keep the magnitude J constant as you reduce the diameter--nothing weird happens.
As noted above, the magnitude of the field adjacent to a zero-diameter wire is not the important result, anyway: the laws of magnetic induction calculate the field at a finite distance.
When using limits as x goes to 0, be careful how you set up the problem to avoid mathematically absurd results that are not physical.
adx:
--- Quote from: HuronKing on March 22, 2022, 05:43:25 pm ---Hey,
I'm only going to respond to this tidbit below because you've written a lot, the discussion is getting kind of unwieldy, and I think we've both said what we want to say. But I don't wish to diminish the effort you put into writing your reply! I read it and consider my reply to it as "nod thoughtfully and smile." :D
--- End quote ---
Agreed! It was getting too much :)
--- Quote from: HuronKing on March 22, 2022, 05:43:25 pm ---Have you done any work in power engineering? The concept of 'reactive power' is immensely important and is mathematically described perfectly by the usage of sqrt(-1):
...
--- End quote ---
Not work per se, the power industry paid too much, jobs too secure and too many way too good opportunities :). I just wanted to do electronics. But I did the courses, either because we all had to, or it was seen as a relatively low workload course as some sort of unwritten inducement carrot dangled in the hope of slurping more over to the (then) state-run energy sector. Software would have been another good way to go. Yet here I am ...
Yes there was a lot of j around. I don't know how much of it there would be in the industry, I suspect there are a lot of vars and little squiggle drawings on control panels to keep power industry workers irreplaceable. (Actually I lie - I had 2 student jobs in the industry.)
We also had a lot of js in machines lab, and of course circuit theory stuff. I have seen little of it since then - maybe the odd appnote for something like a mains measuring chip, or motor driver, will trundle out the j box in a range of styles including metric fine and Whitworth.
I don't know about RF, but I suspect they don't believe any of it, so won't think of j as real (bad choice of word) anyway. Similar to DSP - I close my eyes and j goes away.
My problem (and I'll bet many others will share in the confusion, if surveyed) is that the rapid and frequent appearance of j in all of this, lends someone to think that there is some deeper meaning to it: That sqrt(-1) has physical meaning. Especially with the match of "imaginary" power to its identically-named counterpart.
Lecturers just romp straight into it, as though it's a thing. A little warning could have gone a long way for me.
Thanks for the Steinmetz link - I can see where it all came from. It's a bit sad, he had it working with vectors then took it just that little bit further to complex phasors and BANG there goes the minds of countless students for generations (no pun intended) to come. We've all had those sorts of ideas, that seemed like a good idea at the time, but can't be put back in the box. No doubt it was an extremely useful idea in the day, when "expressions" were more powerful than data.
TimFox:
In RF, you will see Z = R + jX all the time in impedance calculations.
HuronKing:
--- Quote from: adx on March 23, 2022, 02:56:45 pm ---I don't know about RF, but I suspect they don't believe any of it, so won't think of j as real (bad choice of word) anyway. Similar to DSP - I close my eyes and j goes away.
--- End quote ---
Oh I assure you - RF engineers believe it. Unfortunately my RF mentor left the school and the extent of my formal education and training in the RF field stopped after a course in microwave engineering and another in antenna engineering plus the HFSS simulations I did. I haven't stopped learning and picking up what I can though (I was an intern for an RF sales company for a while).
--- Quote ---My problem (and I'll bet many others will share in the confusion, if surveyed) is that the rapid and frequent appearance of j in all of this, lends someone to think that there is some deeper meaning to it: That sqrt(-1) has physical meaning. Especially with the match of "imaginary" power to its identically-named counterpart.
Lecturers just romp straight into it, as though it's a thing. A little warning could have gone a long way for me.
--- End quote ---
Now this I absolutely agree with. The difficulties are in the pedagogy. sqrt(-1) is called an 'imaginary' number or a 'complex' variable but these names are strictly historical. We can blame Rene Descartes for coining the term 'imaginary' as a derogatory term to imply they are not useful numbers. Those names have no bearing on what the sqrt(-1) actually represents - and it IS a physical phenomena. It's no less 'real' than negative numbers are 'real...' or how some ancient mathematicians regarded zero as a meaningless number...
Like, what if I asked you to calculate the power supplied by a voltage source? But then you did everything right and discovered the value of the wattage is negative! Is that not a 'real' answer? Of course it is. All it means is that I tricked you in the problem statement - the voltage source is absorbing power instead of delivering power.
In the world of sqrt(-1), where AC lives, all it means is that incident power is not the same as absorbed power. It can get phase-shifted by the reactance of components which means some of the incident power is reflected back up the line and not completely transmitted through. This is the meaning of the Reflection Coefficient:
https://en.wikipedia.org/wiki/Reflection_coefficient
This whole business about voltage reflections isn't just a problem in AC power. You have to have terminating resistors in bus network motor control equal to the characteristic impedance of the transmission cable or the digital control signals will hit a mismatched impedance, get reflected back up the line, and cause distortions in the new incoming bits which the end-user sees as 'packet losses':
https://support.maxongroup.com/hc/en-us/articles/360009241840-CAN-bus-topology-and-bus-termination
https://e2e.ti.com/blogs_/b/industrial_strength/posts/the-importance-of-termination-networks-in-can-transceivers
This is something I work hard at when teaching my students about AC power, power factor correction, and what it really means to say a circuit is 'leading or lagging.' I get seniors of EE coming through my classroom who are still fuzzy about what the j actually means.
Funnily enough, they get it better once I explain it in terms of the physics. ;D
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