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Comment by causality0 | Hacker News Reader

>but electron speed in cupper is about 15-20cm per nanosecond so it is usually not a big factor.

I'm going to be extremely pedantic for a moment, but this is a case where there's a bit of a difference between electricity and electrons. The speed of electricity, aka electrical energy, aka electric field propagation through copper is 15-20cm per nanosecond. The speed of electrons, aka drift velocity, is far slower and governed by the current and conductor cross-section. Vd=I/neA. For a 91w TDP desktop CPU at maximum current draw, the drift velocity through the power connector would be about 245cm per hour. Yes, per hour. For AC circuits the drift velocity is effectively zero because the electrons vibrate back and forth around their starting position.

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This blew my high-school-physics mind; is there a quick primer / ELI5 on this somewhere? Are you saying that if I connect a battery to a lightbulb, "electricity" moves at speeds we're accustomed to, but "electrons" will only move from one side terminal of battery, through a bulb, to the other side, way way slower than laypeople think? Or did I misread and oversimplify your message? :|

yeah, electrical power propagates way faster than electrons drift (and the power flux is primarily outside the wire, but that's a fun story for a different day).

One way to think about it is as a newton's cradle sorta deal. It's not perfect, but fairly close. Also maybe flicking a jump rope - energy propagates, but the bits of rope don't move from beginning to end.

Before I read your comment, I thought the description sounded like Newton’s Cradle.

Flicking a jump rope sounds more akin to AC to me, though. Maybe, pulsed DC?

What about a train or bumper car analogy (not joking)?

I would really like to see this as a visualization.

Absolutely. Electrons are accelerated by the field until they hit something. The average velocity of all the electrons due to the competing acceleration and collisions is the drift velocity and it's very slow.

On the other hand, electric fields move quickly. At some large proportion of the speed of light depending on the particular materials of the wire and some other factors.

Think of a large tube filled with tiny balls. If you push on one end, the other end moves almost immediately, but the balls themselves are barely moving.

Basically correct. If you could attach a magical GoPro to a single electron in a conductor and applied voltage to it, you would see that it zips around at very high speeds in various directions across very small distances and only averages out as moving in the right direction. This average velocity is the drift speed. If you magically labeled all the electrons in the lamp's wiring, you would find it contains (almost) all the same electrons it started with. The company just bills you for the energy it took to move those electrons back and forth like the teeth on a sawblade.

You can also think of it like water in a filled tube. If you pump more water into one end the person on the other end will get your signal long before the physical water you put in to create the signal reaches him. You could put a water wheel or propeller in the tube to convert the energy of the moving water into work, and just like the lamp you don't have to wait until the physical molecules of water you pumped in reaches the wheel to start using energy from the moving water.

> You can also think of it like water in a filled tube. If you pump more water into one end the person on the other end will get your signal long before the physical water you put in to create the signal reaches him.

This made it click for me, thanks!

This is appropriate: https://www.askamathematician.com/2012/04/q-can-you-poke-som...

http://amasci.com/miscon/eleca.html#light

>In metals, electric current is a flow of electrons. Many books claim that these electrons flow at the speed of light. This is incorrect. Electrons in an electric current actually flow quite slowly; at speeds on the order of centimeters per minute. And in AC circuits the electrons don't really "flow" much at all, instead they sit in place and vibrate. It's the energy in the circuit which flows fast, not the electrons.

> Metals are always full of movable electrons. In a simple circuit, all of the wires are totally packed full of electrons all the time. And when a battery or generator pumps the electrons at one point in the circuit, electrons in the entire loop of the circuit are forced to flow, and energy spreads almost instantly throughout the entire circuit. This happens even though the electrons move very slowly.

I'll add one more example to the others: picture a crowded hallway full of people all trying to move in the same direction. The flow of people is quite slow, but if one person in the back trips and pushes the next person and so on, the domino effect will travel much faster than the individual people.

You might want to look into "phase velocity" and "group velocity", for example on wiki: https://en.wikipedia.org/wiki/Phase_velocity#Relation_to_gro... .

Won’t even get that far, because it’s alternating current. It changes direction a hundred times a second.

“A batteri” is direct current.

Exactly: the signal propagation speed from turning on the switch arrives at the lightbulb at a high fraction of the speed of light. The actual charge carriers take a lot longer. My own preferred analogy is a bicycle chain: pressing on the pedals transfers energy to the back wheel almost immediately, you don't have to wait for individual links to reach the back wheel. And the disposition of electrons is a lot closer to the rigidity of a chain. Just as with the links, each exerts a force on the next.

On electronics.stackexchange we battle this misconception a lot, e.g. https://electronics.stackexchange.com/questions/245610/is-vo...

For almost all practical purposes you should ignore electrons when doing electronics. They're only relevant in detailed theoretical analysis of semiconductors, or (as per article) in vacuum tubes and related items (VFD displays, CRTs).

Thanks for all the replies, that is fascinating; I intuitively understand that for A/C, it all kinda sorta averages out without much movement. But for DC, I assumed electrons actually moved at a "meaningful and impressive velocity on human scales" - not as fast as the electricity propagation, I appreciate and understand the analogies and they make sense - but I imagined it'd still be way, way faster than "centimeters per hour" :O

It astonishes me that much energy can be transmitted / work can be done, with so little actual movement of electrons... I'll follow the links / articles suggested and see what I can gleam; thanks all! :)

In a lightning the electrons move a considerable distance, but even there the speed is relatively low.

Also astonishing is to calculate the weight, or mass, of the electrons participating in a lightning discharge. So little mass, so much energy.

The analogy from my physics class long ago was we each hold one end of a broomstick, if I shove the stick, you'll feel the stick move at a delay consistent with the speed of sound (speed of sound in wood, different than air...) yet the stick actually moves physically a good deal slower than the speed of sound when it transmits the force that moves at the speed of sound. If I whack the broomstick you'll feel it vastly sooner than I can hand the entire stick to you. Speed of sound in wood is hundreds of miles per hour, but I can't physically move a wood broomstick faster than several dozen miles per hour.

Bulk material moves slower than forces move... usually. The physics of shock waves and supersonic stuff is interesting.

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