William Beaty -- Electrical Engineer 35yrs, electrostatics hobbyist, Amasci siteUpdated Mon
A Veritasium video said that a light bulb with the wires a light second away from the battery will light up in 1/c seconds if the light bulb and the battery were a meter apart. Doesn't the light bulb physically require electrons to pass through it?
Edit: “Z Y” changed his mind, agrees with me, getting the same numbers I did: Aha, Electroboom also agrees, getting the same numbers:
.
So, electrical engineers can figure it out, while physicists believe that signals can travel inside solid copper at approximately c velocity. (Doesn’t happen. Physics actually says that 100% of the electrical energy travels only in empty space outside the wires. Within the copper, the speed of light is roughly ten meters per second, and this low V causes the skin-effect. So, don’t look at Feynman Lectures or at Purcell physics, instead look at JD Kraus’ book “Electromagnetics.”)
Note that Veritasium’s video is COUNTERINTUITIVE, intended to embarrass even some of the experts (the many who never had their grade-school “lies” get debunked later. Some “experts” incorrectly think that electrical energy travels inside wires. Veritasium points this out. )
So, even experts who SHOULD know better, still angrily insist that electrical energy flows *inside* solid copper. Nope. Doesn’t. In physics class, perhaps they were sick that day, and missed it during college coursework? Or, their textbook didn’t get into Maxwell’s equations regarding waveguides (coax cables, etc.)
Note well that anything moving at c through solid copper would generate x-rays at least, or perhaps fusion explosions. In circuits, electrical energy can move at c because it “leapfrogs” between electrons on the surface of wires. The energy travels as EM flows in space, located entirely outside the metal surface, but firmly guided by the moving charges.
Maxwell and Heaviside say that the amount of electrical energy inside any copper wire is zero, and 100% of “wattage” is in the fields.[1] We’ve known this for about 120 years, so it should only surprise people who never had college physics.
What’s the “for dummies” rule of thumb?
Just tell yourself that, inside capacitors, 100% of the energy is in the e-field, not inside the metal plates. And in coils, 100% of the energy is in the b-field, not circulating inside the wires. The same also applies to all circuits, since even a simple circuit is a combination of 1-turn inductor and 2-plate capacitor: a loop of current, plus two adjacent wires with opposite surface-charge.
However, even with the truth being well known, it’s absolutely NOT taught in high school …and even then, only taught to a portion of the physicists and engineers.
The rest of the population, when they first hear about it, will “see red” and start arguing and getting personal …unknowingly attacking Maxwell, and sneeringly rejecting a century of very conventional RF engineering. Their wrong beliefs are painful to watch, embarrassing. They thought they were ridiculing Veritasium, when actually they were ridiculing their own textbooks on Electrodynamics, which apparently they’d since forgotten.
So, Veritasium is correct, and the ones who insist that the energy flows *inside* the copper are trapped in embarrassing childhood misconceptions they should have lost during their undergrad sci/eng courses. (How will they ever live it down? Hee!)
Here’s the key to everything: transmission lines are not RF-only devices. After all, the physics for transmission lines has no lower bound on frequency! The physics/math applies equally well to DC as to GHz waves. 2-wire lines turn out to be a weird type of waveguide, where we can reduce the frequency all the way down to zero, yet the wave-energy is still outside the wires, and guided by the wires.
The key again: coax cables (etc.) employ the same math and physics at GHz as they do at 60Hz, and at DC. Wires are waveguides, ALL wires are waveguides, regardless of frequency. Hint: inside solid copper the speed of light is quite low: roughly tens of M/S. In circuits, the axial-directed EM energy is entirely flowing 100% outside the wires, and also the energy-flow outside the wires is the cause if of skin-effect, and crosstalk, as well as explaining the operation of capacitors and transformers (where the same occurs, and the EM energy is located entirely outside the metal parts.)
Actually, all this answers a question I had as a kid. Microwaves flow through huge damn rectangular hollow waveguides. But they also go right through extremely narrow coaxial cable, less than a hundredth of a wavelength! HOOWWWW?!! It’s because in 2-wire lines, the fields compress themselves down if the lines come together, and there is no lower bound to this process, as long as the two lines don’t touch each other.
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