General > General Technical Chat
Electroboom: How Right IS Veritasium?! Don't Electrons Push Each Other??
Sredni:
--- Quote from: rfeecs on June 22, 2022, 06:45:53 pm ---
--- Quote from: Sredni on June 22, 2022, 04:53:29 pm ---I was talking about the surface charge: the excess electrons or lack thereof that - along with the original external field generated by the battery - shape the electric field inside the conductor in such a way that it be directed along the conductor axis and will have a magnitude that satisfies Ohm's law in its local form.
--- End quote ---
So what happens with a superconducting wire? Presumably the surface charges make the field inside the wire zero. So what makes the current go?
--- End quote ---
Ah. Superconductors are quantum beasts. (Well, technically even ordinary conduction requires quantum theory to give quantitative agreement).
If we stay in the realm of classical ED, we can consider a perfect conductor as the limit of a resistive conductor for sigma->infinity. You need an infinitesimally small field to make the electrons move. But it can get tricky. Last year (?) Lewin posted a problem with a superconducting ring in a changing magnetic field. Basically what we call Lewin's ring but without resistors. What will happen? Only two people, among all those who were exposed to the problem (and they included several physics professors to which Lewin had emailed the problem) gave the correct solution. One is a professor in a University in Switzerland (IIRC) and the other is George Hniatuk (he has a youtube channel).
The solution is: no current inside the superconducting ring.
I had it wrong: my initial assumption was that the current would rise so rapidly - being a superconductor - that the small self inductance of the ring would act as a current limiter. Then I saw George Hniatuk's comment (Nope. No current inside) - and knowing how he knows EM - I realized he was right. (Math and Physics are different - in math you can create the induced electric field magically inside the superconductor, in physics you must justify its presence there. How do you place it in? Surface charge will redistribute in such a way as to prevent it from entering the ring).
But this is not the reason I am telling you this. In the video (I will add a link tomorrow, now I need to sleep), or in the comments, Lewin made a very interesting statement. That to initiate a current in a superconducting ring you need... a resistor. You start your magnetic mumbo jumbo with the resistor inserted - and it's the field in the resistor that makes the electron go - once in the superconducting material they continue 'by inertia', and only after the current is established, you switch to a full superconducting ring.
Pretty crazy, uh?
SiliconWizard:
--- Quote from: EPAIII on June 22, 2022, 11:32:48 am ---OK good old OHM's law tells us that the current is inversely proportional to the resistance.
--- End quote ---
Did you know that Ohm was considered a lunatic (to put it mildly) by his peers back in the days? He got rehabilitated only after a relatively long time.
I wonder how it would have unfolded these days. ::)
tszaboo:
At this point Veritasium just needs to admit that he has no ide what he is talking about and he was using a textbook with flawed physics in it.
All this discussion about current density in a conductor is totally solved formulas, which tell us if there is current inside the conductor or not:
Here is one:
Naej:
--- Quote from: Sredni on June 22, 2022, 07:15:36 pm ---
--- Quote from: rfeecs on June 22, 2022, 06:45:53 pm ---
--- Quote from: Sredni on June 22, 2022, 04:53:29 pm ---I was talking about the surface charge: the excess electrons or lack thereof that - along with the original external field generated by the battery - shape the electric field inside the conductor in such a way that it be directed along the conductor axis and will have a magnitude that satisfies Ohm's law in its local form.
--- End quote ---
So what happens with a superconducting wire? Presumably the surface charges make the field inside the wire zero. So what makes the current go?
--- End quote ---
Ah. Superconductors are quantum beasts. (Well, technically even ordinary conduction requires quantum theory to give quantitative agreement).
If we stay in the realm of classical ED, we can consider a perfect conductor as the limit of a resistive conductor for sigma->infinity. You need an infinitesimally small field to make the electrons move. But it can get tricky. Last year (?) Lewin posted a problem with a superconducting ring in a changing magnetic field. Basically what we call Lewin's ring but without resistors. What will happen? Only two people, among all those who were exposed to the problem (and they included several physics professors to which Lewin had emailed the problem) gave the correct solution. One is a professor in a University in Switzerland (IIRC) and the other is George Hniatuk (he has a youtube channel).
The solution is: no current inside the superconducting ring.
I had it wrong: my initial assumption was that the current would rise so rapidly - being a superconductor - that the small self inductance of the ring would act as a current limiter. Then I saw George Hniatuk's comment (Nope. No current inside) - and knowing how he knows EM - I realized he was right. (Math and Physics are different - in math you can create the induced electric field magically inside the superconductor, in physics you must justify its presence there. How do you place it in? Surface charge will redistribute in such a way as to prevent it from entering the ring).
But this is not the reason I am telling you this. In the video (I will add a link tomorrow, now I need to sleep), or in the comments, Lewin made a very interesting statement. That to initiate a current in a superconducting ring you need... a resistor. You start your magnetic mumbo jumbo with the resistor inserted - and it's the field in the resistor that makes the electron go - once in the superconducting material they continue 'by inertia', and only after the current is established, you switch to a full superconducting ring.
Pretty crazy, uh?
--- End quote ---
Lewin did a bit of a mess, and the question is easy to anyone who knows superconductors.
The solution is: there'll be a current inside the superconductor.
No one said "E=0 in a superconductor" for the last 90 years, cf. https://en.wikipedia.org/wiki/London_equations
Of course, there's such a thing called "skin effect" which is a bit more extreme in superconductors, so that the current will be concentrated within a London length, or less than a µm. This is always true (at low magnetic field, type I etc.), whether you put a resistor with the superconductor or not.
So what makes the current go: either you inject electrons from a normal conductor (and you can then close the superconducting loop after if you want), or you bring a magnet close.
Sredni:
--- Quote from: Naej on June 22, 2022, 09:10:57 pm ---So what makes the current go: either you inject electrons from a normal conductor (and you can then close the superconducting loop after if you want),
--- End quote ---
Which is what I wrote above: you need a resistor (the finite conductivity conductor).
--- Quote --- or you bring a magnet close.
--- End quote ---
And there will not be current or field inside. Basically the current due to the surface charge that kills the field inside is all you get in a superconductor. In classical electrodynamics we can consider an impossible sheet of current of zero thickness (we can also consider infinitesimal charge quantities, if we wish). In the real world, in a real superconductor, the surface current will be confined to a few atom layers. But this is a far cry from stating that there is a current inside. That fraction of a micron is the real world approximation of the zero depth surface sheet.
There is a discussion of the difference between perfect conductors and superconductors in Ramo, Whinnery, VanDuzer (sec. 13.4 Perfect conductors and superconductors, p. 676 on the second edition).
Anyway, I found the links to Lewin's statement of the problem:
The solution
And further discussion
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version