The Veritasium Electricity Debate: Answers to the Confusion
2022-Sep-03
These comments are in response to a pair of videos on the nature of electricity that have been making the YouTube rounds. Produced by Derek Muller with Veritasium, there are two parts and here are the links:
These are very well-produced and informative, and I should take my hat off to the creator. However, (you knew there had to be a “However,” right?) and unfortunately, the videos seem to have actually created a bit of confusion, as evidenced by the questions that appear repeatedly in the comments section of the second video.
Most prominent are questions about energy in electrical circuits being transferred outside of the wires by the fields, rather than within the wires by the electrons; that was the major claim in the first video, and reiterated strongly in the second. He even explains a physical quantity called the “Poynting vector” that tells quantitatively how much energy is flowing.
But afterward his viewers are asking questions about why the wires are needed if the energy is flowing through space, why do switches have any effect, and why the lamp needs to be connected to the wires.
The answers to these questions are simple. Electrical energy is directed by the flow of electrons inside the wire. End of Story. Mic drop. One can debate exactly where and in what form that energy is represented and how the electric and magnetic fields are involved, but the bottom line is that it is the electron flow that determines and controls where that energy goes. The field is strongest near and between the wires, following them like a sheath. Our power grid delivers energy through wires because it needs to AIM that energy precisely at the outlets in our homes, without it spraying all over the countryside. We protect ourselves from electrocution by wrapping wires in plastic insulation, which doesn’t affect the fields but does stop the electrons, keeping them from flowing into us and dragging their fatal energy behind. Lamps need the electron flow to extract energy to glow, and switches interrupt that flow to turn the light on and off. In other words, electricity works exactly the way you always thought, electromagnetic field and Poynting vector notwithstanding.
So are the Veritasium videos wrong? Technically, no. As far as I can tell, everything in them is correct in detail. The difficulty is the emphasis. Muller focuses on the idea, which I guess is new and exciting to him, that the fields are preeminent, and he seeks to discount the importance of the charges. And it is true that in some situations the field description is essential, such as for propagation of electromagnetic waves (light, radio, X-rays) through empty space without nearby charges to help out.
But for circuits, a charge-focused description is more useful. In that view, we pay attention to simple ideas such as that unlike charges attract each other, while like charges repel. This is, if one insists, a “field” interaction in which a charge creates a field around itself and that field pushes or pulls other charges. But mathematically it isn’t any different to just say the particles exert those forces directly on each other, at a distance.
(To be fair, the idea of action-at-a-distance may seem weird to many people, and the concept of a space-filling field that acts on things it contacts may be more comfortable. And I think the modern physics viewpoint is that the field is a real, palpable thing. My point is not to say that one shouldn’t think about the field, just that in certain situations it’s easier if you pay attention to something else first.)
If you’ve watched the videos you’ve seen the (hypothetical) demo of a very, very long pair of wires connecting a battery/switch to a lamp, but the wires are folded around so that the battery/switch and lamp are very close together. It is asserted that energy flows through space across the short gap, energizing the lamp sooner than current could flow around the whole loop (limited by the speed of light). Now you may be thinking, “Well, is that claim true? Does energy flow across the gap between the two wires, or not?” Absolutely, it does! When the switch is closed, the electrons rearrange their positions in the wire directly attached to the battery, and by attraction/repulsion through space they influence the electrons in the other wire that is connected to the lamp. Those charges then rearrange themselves in response, and that movement creates a small current through the bulb, which may be enough for it to light up. It should be noted, though, that this effect is short-lived: it only lasts while the wires are “charging up” immediately after the switch closing. Depending on whether the distant wire ends are connected or not (Derek discusses both options), either the main current takes over and the light gets brighter, or the current drops to zero and the light goes out.
So in answer to those questions is that wires and switches work exactly as you always thought they did: they manage the location and motion of electrons, and that controls the energy flow. This is a clear and very useful intuition. Simultaneously, it is also useful to keep in mind that wireless transmission does occur, and that it may occur in a circuit where it wasn't intended. But the possibility of energy crossing the gap between wires should not obscure that major actors are the electrons in the wires, and that a description based on them is most faithful to reality.
So that’s it for now, I’m happy to answer further questions about this.