Too Fast Too Curious: Electron Drift

I’m a bit of an old fart and not collage educated so, please be gentle. Subject of my question: speed of light & electricity. For some reason that I don’t remember I have always thought that electricity traveled at the speed of light. Today I have my doubts. My education is obviously lacking so bear with me. I remember being thought that electricity moving through a conductive element was a stream of electrons set in motion by whatever energy source and as long as a circuit remained closed the electricity moved through the circuit at the speed of light. Now! The rub! Electricity is an energy. Electrons are a particle. So, in effect do the electrons travel? Or are they transferring energy one to another throughout the circuit, and does this happen at the speed of light. Or, does the material of the circuit I.e. Copper, water, iron, etc… have an effect on speed of transfer?

-Martin

***

Hi Martin,

You are impressively close! You’ve hit the main point already, namely the difference between an electric signal (energy, as you put it), and the electron itself.

To start I’m going to define electricity and then never use it again: electricity is the presence and flow of electric charge.

Got it? Great. This word is so mind-numbingly broad that it can be applied to everything. I personally don’t like it, and will instead be using “electrons” and “electromagnetic energy” to keep the concepts you are asking about distinct. And before we start in on any of these – energy, electrons, electricity, electoral college, Electabuzz – we need to talk about light.

Light as an Electromagnetic Wave

Visible light is a wave.* This wave pulses through the air at the speed of… well, light! The amount of distance a wave travels between its pulses is its wavelength. The faster the pulsing, the shorter the distance is between pulses, and likewise if the pulsing is less frequent, the distance traveled between pulses will be longer.

Visible light is defined by waves that have a wavelength of around 400-700 nm. For comparison, your wireless Internet has a wavelength of about 5 inches; your favorite radio station is about 10 feet. Anything longer than 700 nm is infrared, meaning “below red.” Anything shorter than 400 nm is ultraviolet, meaning “above violet.” These are bands of light that we can’t see but still have impacts on our daily life — warm invisible infrared light keeps our fast food fast, and ultraviolet light turns me into a lobster during the summer months.  X-rays that you get at the doctor’s office are really, really tiny: about 10 nm, or one fortieth the size of ultraviolet, which also means roughly 40 times more cancer than UV light. All of these together make up the electromagnetic spectrum, and all of these things travel at the speed of light.

Check out that electromagnetic spectrum (diagram by Inductiveload and NASA under GFDL or CC-BY-SA-3.0 sharing licenses, via Wikimedia Commons)

All electromagnetic waves travel at 299,792,458 meters per second in a vacuum. So for the first part of your question, if “electricity” means any electromagnetic wave, then yes, it zips along at the same speed as light.**  That number, by the way, is not an approximation, nor is it rounded. In 1975 scientists redefined the length of a meter to make the speed of light an integer. That’s right – you’re all about 4 billionths of a meter taller then you would’ve been 42 years ago, just to make the math come out clean. ***

The speed of light is always the same, but notice that the number I gave you is in a perfect vacuum – that is, space with nothing at all in it. Perfect vacuums are not exactly the norm here on Earth. Electromagnetic waves moving through atmosphere or water or hockey rinks are coming at the same speed, but have to go farther to get out of the way of all this non-vacuum stuff.

Imagine we have two boxes that are the same size. The first box has nothing in it, and the second box is filled with dangerous shards of solid glass. If we shine a light through both of these boxes, the light going through the empty box will come out first.

The first one is an empty box; the second one’s a portable burglar alarm. (my drawing)

The light travels at the same speed for both boxes, it’s just that the box of glass is chock full of atoms. These are obstacles that light has to bend itself around. It is still traveling at 299,792,458 m/s, but it takes longer to get through the box because it has to dodge and bounce off of all of that pesky matter to get out.

Electrical Power/Signaling in Circuits

For circuits, these electromagnetic waves are still present, but rather than flying through the air they are stuck in a conductive metal, such as copper. This is handy for us, because rather than thinking of the wave in terms of some not-wave not-particle philosophical nightmare, we can start approaching it as the interaction between the electromagnetic wave and the matter it’s traveling through. Specifically the individual charges of electrons in the matter.

Direct Current

Direct current is a current that flows in one direction. Think a battery and a light bulb: you connect the battery to the light bulb, and charge begins to flow in. The light bulb releases the electromagnetic energy as visible light and thermal energy (heat).

These currents start off when an electron from one atom becomes excited with energy. So excited that it can’t bear to live in its dingy ol’ atom anymore. So it decides to leave home, only to realize it didn’t really plan this out very well and crash at the neighbors. This electron is now a homewrecker, because not only did it leave a hole in its old home, but it dislodged an electron in the new one. The electron kicked out of the second atom goes to find a third atom and kicks someone out of there, setting off a chain reaction with each atom passing an electron down to the next atom, occasionally skipping an atom or so in line.

Electrons: they just aren’t great planners. (my drawing)

So here’s the tricky part. This chain reaction is moving at the speed of light, but if we went back to check on the very first electron, it would still be only one atom over from its starting point. Imagine you had a tube completely filled with marbles. If you shove a new marble into the tube, another marble immediately pops out of the other end. Even though this isn’t the same marble we stuck in, it looks to the observer as if the marble instantly traveled the whole length of the tube. That’s what’s happening in our circuit. The electrons all bump into each other and our electromagnetic wave effectively moves at the speed of light even though the electrons haven’t really gone far at all.

For DC**** individual electrons move at about 80 cm per hour.***** That’s literally slower than molasses.

As long as the material is a good conductor, the electromagnetic wave will go through it at nearly the speed of light. We’re talking 99.9-till-you-get-bored-of-writing-9’s percent the speed of light. Just like the light in the box of glass, the tiny amount scraped off the max speed is because the signal has to interact with the matter in the circuit itself.

Now, if you or anyone you know is looking for an internship at the League of Nerds, we’ve got an opening in our broken glass disposal team. Seriously, this stuff is everywhere.

Yours,

The Electromagnetics Nerd

* It’s not really a wave or a particle but rather something else that exhibits characteristics of both. This is called wave-particle duality and is mathematically handled by approaching light as a vector potential field that both oscillates and decays radially given a particular direction of propagation. If those words mean nothing to you, then just trust me when I say that light is kinda peculiar.

** Before any mega-nerds get mad, this assumes a uniform and semi-infinite medium that has a linear wave impedance, and this doesn’t take into account electromagnetic boundary conditions.

*** https://sites.google.com/site/amyandsabrina/home/length-of-a-meter-1983

**** For the scope of this question, AC circuits are fairly similar. The big difference is that the signal doesn’t flow in only one direction but rather switches back and forth many many many times per second. The electric signal is passed back and forth at the speed of light, but individual electrons only move an atom or so before the signal reverses and they have to come back. So the electrons in an AC circuit more or less vibrate in place.

***** https://en.wikipedia.org/wiki/Drift_velocity

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