What Is Electricity? (2013) | Hacker News Reader

What Is Electricity? (2013)

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126 pointscalinf45 comments

I teach high school physics. Here's my notes/textbook on electricity with simulations and equations rendered in LaTex.

https://landgreen.github.io/physics/notes/circuits/electrici...

I haven't gone through all of your notes yet, but this would've been a godsend to go over in highschool before I started my EE classes in college. It was always obvious which students had been lucky enough to already cover the material.

This is amazing ! Really well put together!

Awesome notes. I haven't had my coffee yet so perhaps I'm just misreading this part:

"In 1746 Benjamin Franklin mistakenly assigned a negative value to the charge carriers that we now call electrons."

Shouldn't that read "mistakenly assigned a _positive_ value to the charge carriers that we now call electrons"?

Nope! It's kinda silly. We all call electrons negative because of how they were labeled 250 years ago. If he had labeled them positive the math involving electric current would be a bit easier.

https://xkcd.com/567/

Or 250 years from now... ;-)

https://en.wikipedia.org/wiki/CPT_symmetry

I’ve been hearing for years that the math involved would be easier if we switched signs on the electron but what equation specifically would be simpler? One would still have to deal with both polarities. I tutored electrical engineers for years and the biggest misconception by far was always considering the amount (or movement) of charge and the number of particles to be equivalent. Polarity was never really an issue.

A couple things off the top of my head from teaching high school physics:

It would make the direction of current the same as the flow of charge.

It would make electric fields predict the force on electrons.

It would make the right-hand rule for magnetic fields work for electrons and instead of the left-hand rule.

But existing theory predicts all those just fine, it just happens that some things are not in the same direction, but it’s not like one can ignore direction. The math is still always there. And I argue things going in different directions is a feature, not a bug, because it reinforces the point that net charge and net particles are not the same thing.

I see your point.

Right, the math is the same. We face this in physics all the time. You drop a ball and it accelerates downward. Should the distance it covers be a negative number? Well, it seems linear distance should be positive, but if it is, then does down become positive? Shouldn't up be positive? Or do we reverse from end-minus-start and make it start-minus-end?

It doesn't matter. The math is all the same with just different labeling conventions.

For the general physics of electric charges, the convention that a proton is +1 and electron is -1 is somewhat useful as a reminder of the larger range of phenomena: protons carry charge, too, and a movement of positively charged molecules makes the charge carriers and current move in the same direction.

But as soon as you start working with practical, artificial electrical circuits (as opposed to, say, neurons), the electron-is-negative is a nuisance, because the charge carriers are almost always simple electrons. To visualize "stuff" moving one way, you have to visualize "other stuff" moving the opposite direction. You have to keep thinking that when you add more you get less. Visualization of electric/electronic technology would have been a lot more convenient if the convention had been that electrons were positive, protons negative.

We're probably going to have to agree to disagree on this one as I just don't see the nuisance and I see some positive aspects to the existing convention which I highlighted previously.

The classic, and really intuitive, experiment that requires one to understand the movement of both the carriers and the charge at the same time is the Haynes Shockley experiment [1]. It has not been my experience that charge convention gets in the way of students interpreting this experiment.

For the vast majority of practical (artificial?) electronics, when designing one only needs to look at the movement of net charge. Passive sign convention (used by spice) is about the flow of charge, not carriers. Maxwell's equations don't even mention the carriers. It's not clear to me why one needs to visualise carriers to design circuits. In my experience (i.e. transmission lines) it can actually hinder students to consider the movement of carriers when designing a practical circuit.

[1] https://en.wikipedia.org/wiki/Haynes–Shockley_experiment [2] https://en.wikipedia.org/wiki/Passive_sign_convention

artificial: "made or produced by human beings rather than occurring naturally"

I ran into this recently with vacuum tube diodes. There's a cloud of electrons surrounding the heated ground-plate. The source plate is at high voltage. Increasing the voltage on the grid attracts electrons, which stream from the ground and fly towards the "source." And I'm calling it "source" because the stream of electrons flowing from ground to source is a positive current from source to ground. Thanks, Ben

Your notes are awesome. One thing I never thought of (and still have a doubt): can you really say that adding more resistance physically slows down the electrons? I mean, how slow can you make them move then by adding mega or giga ohm resistors? In metres per second? Isn't it more like that by adding more resistors there are less electrons which are able to pass it, which makes current flow less (not slower)? Just a random thought, without research.

Electrons move at the same speed. In a low-resistance material (metal crystal) they move in an almost strait line. In a higher resitance material they take a longer path, and must line up (stop) to make it through bottlenecks. They only seem to be moving more slowly at a macroscopic level.

I'm also speculating here, but I think resistance can manifest as slower electrons or as fewer electrons moving at the same speed.

On the wire, away from the resistors, the number of electrons that can move should stay constant, so the election drift velocity should drop as resistance increases.

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

https://codepen.io/lilgreenland/pen/xprGvr (not a mobile friendly link)

One more thing to add. Electron drift velocity isn't the same as signal propagation speed. Wires can transmit information at around 2/3 the speed of light, but the drift velocity of an electron in a wire is around walking speed.

Thanks, point taken. And of course there are approximations and simplifications necessary when you try to explain this subject to high school students. I mean, that is the age when you still draw parallels from the physical world and it is easier to digest electron as a very small fast moving "ball" even though it is probably not.

The analogy I use to explain this (which is, like all analogies of course broken at many levels): a tube full of marbles will produce a marble at the far end every time you push one new marble in, but the individual marbles travel really slow.

I used to use the scene in "the two towers" where they light the fire to let Rohan know that Gondor is in need of aid. I thinks I have to stop referencing Lord of the Rings because most high schoolers haven't seen the movie these days.

https://www.youtube.com/results?search_query=beacons+of+mina...

> I thinks I have to stop referencing Lord of the Rings because most high schoolers haven't seen the movie these days.

I'm old enough to remember when they were 'just' books... and I thought you were going to write 'most high schoolers havne't read the books' :)

At least you aren't referencing the Twilight Bark from 101 Dalmatians - https://www.youtube.com/watch?v=xp-DW-2bq2U.

HA!

Beautifully presented! Thanks for sharing, these notes are inspiring, I especially like the simplicity of the presentation's design paired with interactive elements. Too often I find material with the latter to be extremely busy. As someone interested in instruction, I'm taking notes from this!

One aspect of electricity I think is useful to emphasize is that electrons in a conductor move more like the steel balls Newton's cradle (that toy you may have seen where a ball strikes one end of a line of balls, and nothing moves except the end ball, or you can pull back two balls and then only two balls will move, and so on). This can help with intuition about the distinction between "holes" and electrons, and how energy can flow with holes (which is quite counter-intuitive, I think). In terms of mental model, the ratio of holes to ambient electrons is sometimes useful to think about, as this is fundamental to computing current limits of conductors.

There is a diagram (an animated GIF) about half-way down that sort of implies the newton's cradle method of moving charge, but I don't like it because it's not clear enough. Also, I see current as more of a fixed cloud of valence electrons with holes moving through it - which is a very different intuition than what this diagram implies.

Looks like what you are describing is a model of the conductivity in the valence band (of a semiconductor). In the conduction band of a metal electrons move freely, their forward mobility being impeded only by heat.

Well, maybe the part about the holes. But even wikipedia mentions newton's cradle! https://en.wikipedia.org/wiki/Electrical_resistivity_and_con...

The distinction is between the speed of propagation (e.g. how fast between a light switch flipping until lighting the room) and the physical movement of subatomic particles through the wire.

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

Here's my take on a cartoony conductivity simulation. I've got three models: conductors, semiconductors, and semiconductors but with holes.

https://landgreen.github.io/physics/notes/electromagnetism/c...

I like these diagrams. Now I want to see what superconductivity looks like in your framework. An unimpeded stream of electrons flowing as if the red dots aren't even there?

My simulation just runs classical physics (Coulomb's law + Newton's laws). It's missing any mechanism for super conductivity. Although, I could just turn off friction.

I think superconductivity means the electrons pair up and stop colliding through the Pauli exclusion principle. I didn't code any collisions, so maybe it's already modeling a crappy version of superconductivity!

Yes, that looks much closer to what I have in mind, although I also tend to think of electrons as not visible individually and doing this as part of a large cloud over a 3-D crystalline substrate.

Just amazing! Imagine all the physics and knowledge from multiple sciences, disciplines and fields are used when someone tells Amazon Echo to turn on a charger or socket that’s connected to a phone or laptop.

Echo -> ISP wires —> Undersea Cables —> Amazon Servers -> Back home -> Electricity flow

And this is so super high level

All of these devices on the way are powered by electricity from dams, solar, wind etc. Before voice to speech hits or data is read. Processors and hard disks must do so much heavy lifting

It’s surreal the level of abstraction we are surrounded by..

Quite fascinating and mind-boggling. What really blows my mind is that technology is still extraordinarily less complicated than (bi|ec)ology.

A thread from 2015: https://news.ycombinator.com/item?id=10400123

It's interesting to me when I read it. We take it down to the proton neutron and electron stating they are charged with positive negative or neutral charge.... but what are they charged with ?

Electricity ? What is it made of ?

It's made of the electromagnetic force. We can measure it and characterize it very accurately with mathematics. We can say what it isn't (gravity or the weak force or the strong force), but we cannot say what the electromagnetic force is. Sorry. If you figure it out you'll get a Nobel Prize.

> We can say what it isn't (gravity or the weak force or the strong force)

Isn't the electric and the weak force considered the same force these days? See also https://en.m.wikipedia.org/wiki/Electroweak_interaction

They aren't "charged with" anything. Some elementary particles happen to interact with one another in a particular and consistent way, and we label them according to how they behave with something we call charge.

Electric charge is fundamental.

We say that a macroscopic body is “electrically charged” when there is an excess (or a lack) of electrons in it - which merely creates a disbalance between the electrons and the protons. We say that an elementary particle is electrically charged to indicate that there is a nonzero probability of it emitting or absorbing a photon (which leads to creation of the electromagnetic force).

Physics runs out of "why"s somewhere around the question-asking ability of a clever 5-year-old, and becomes "here's the math to describe what happens" the rest of the way. Electric charge and all the other forces (AFAIK) just are.

For an exercise in clear (and perhaps provocative) thinking, try:

> What is electricity? This question is impossible to answer because the word "Electricity" has several contradictory meanings. These different meanings are incompatible, and the contradictions confuse everyone. If you don't understand electricity, you're not alone. Even teachers, engineers, and scientists have a hard time grasping the concept.

> Obviously "electricity" cannot be several different things at the same time. Unfortunately we've defined the word Electricity in a crazy way. Because the word lacks one distinct meaning, we can never pin down the nature of electricity. In the end we're forced to declare that there's no such stuff as "electricity" at all!

http://amasci.com/miscon/whatis.html

Analyzing the meaning of words is characteristic to philosophy and is not a sign of “clear thinking.”

This is trivial for somebody who studies electrical engineering in highschool

Basic introductions of a topic are trivial for people that are studying that topic? Shocking!

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