Why is electricity so hard to understand? (1989) • Hacker News Board

Re: Why is electricity so hard to understand? (1989)

#181

Post by wbeaty » Wed, Nov 09, 2016, 4:10 AM UTC

CamperBob2 wrote: post #166

wbeaty wrote: post #160

Earlier quoted context omitted.

So, you're saying that capacitors store electrons?! Debunking this particular conception was the whole point of my capacitor article http://amasci.com/emotor/cap1.html Capacitors "store" electrons, like springs "store" steel, or rubber bands "store" rubber. A charged capacitor has exactly the same number of electrons as an "uncharged" capacitor. When "charging" a capacitor, charge is forced into one terminal, and exa…

Capacitors store joules, not coulombs. Fair enough -- a two-terminal capacitor that stored electrons supplied via one terminal could be charged without drawing any corresponding current at the other, violating Kirchoff. I do like your water-filled sphere analogy, and I agree that the word "charge" is an overloaded term. But what would you say is happening at the top electrode of a Van de Graaff generator? It represen…

Yep, a VDG machine is not a single-ended device. I tell people that there are always two spheres involved, although usually the second sphere is below our feet: planet Earth. Charge conservation says that, with a VDG, the e-field flux extends between the upper metal sphere and the ground below it. So, to concentrate attention on just the charged sphere, while ignoring the oppositely-charged ground surface, is much like concentrating on just one plate of any capacitor.

Better: hang many different metal spheres from insulating threads, then use a HV supply to deposit various charges upon them. "Capacitor" is always taken to mean a pair of opposite-charged objects. But miscellaneous "charged objects" aren't necessarily capacitors.

Also, this:

ENGINEER'S CAPACITOR, not physicists'

http://amasci.com/emotor/enCap.html

While employed at MOS in Boston I temporarily threw together a floating, double-ended VDG with a battery/motor inside one sphere. Like this: http://amasci.com/emotor/vdgdesc.html#diff

I thought it would much better communicate the true nature of electrostatic generators, but it never ended up in our exhibit. VDGs are just constant-current high-voltage power supplies. A long enough chain of 9V batteries would produce all the same phenomena ...aside from the 10amp short circuit current, and the megawatt arcing!

PS, weirdness

With VDGs I was triggering three separate kinds of spark. I've not seen this discussed anywhere. We have the usual kind, the thin straight "needle" that jumps between smooth spheres. Then we have the violet fractal tree. Attach a 1cm ball to a VDG sphere and watch in a darkened room. It periodically spits foot-wide lightning networks, just like the miles-wide kind. And third: occasionally I was getting "silent purple sausage" discharge about an inch thick and a couple feet long. In a lighted room they make a slight "thump" sound, so if you hear that noise from a VDG, try observing in total darkness. Sometimes the "sausage" would even produce branching (possibly nanosecond wave effects,) when it would leap out 1ft, then split into five branches from the tip, then proceed to the adjacent metal wall as five fuzzy pathways. Perhaps the particular "seed" at the micro-scale will determine the type of spark which propagates? Or maybe the "sausage" discharge was actually a relativistic effect seeded by MeV cosmic rays.

Re: Why is electricity so hard to understand? (1989)

#182

Post by solipsism » Wed, Nov 09, 2016, 4:11 AM UTC

thisrod wrote: post #157

seiferteric wrote: post #49

Earlier quoted context omitted.

Not really sure I like that link. He seems to suggest that capacitors store "energy" instead of charge, which is just as ambiguous really. It's not like there is some sort of energy particle either. Of course what's really happening is that you are creating an electric potential between two plates. It's true that the net charge is the same, but you are moving electrons from one plate and forcing them (doing work) int…

> water does not attract other water in any way Um ... once we've cleared up all the misconceptions about electricity, we might want to move on to this other thing called gravity :-) The usual reason that people are mislead by "water" analogies—more precisely, the analogy between height and electric potential—is because they misunderstood gravity to start with. If you start by writing Newton's law and Coulomb's law s…

where you have two tanks floating in outer space, and water gets sucked into them by gravitational attraction

As a person who has always struggled to understand electricity (but understand Newtonian gravity well enough), please tell me more! Water is going from where to where?

Re: Why is electricity so hard to understand? (1989)

#183

Post by wbeaty » Wed, Nov 09, 2016, 4:25 AM UTC

Aardwolf wrote: post #3

Dear author of the website: Nice article. Please fix the TOC links on "whyhard2.html" to point to "whyhard2.html" and not to "whyhard1.html", and remove the huge whitespace between TOC and content. This was a very confusing experience and likely others will fall in this trap. Thanks!

DOH!

Fixed now.

I messed it up last time I was getting slashdotted by reddit and ycombinator, and everyone was complaining that the raw unedited notes were just a bunch of raw unedited notes.

Heh, it's still a 1995 gopher-era design, back when we put all our chapters on a single long page, to compensate for 300/2400b modem speeds. If it was broken up into many separate pages, you'd be waaaaaaiting for the text to finally appear.

Re: Why is electricity so hard to understand? (1989)

#184

Post by wbeaty » Wed, Nov 09, 2016, 5:06 AM UTC

roel_v wrote: post #143

Maybe this is the right occasion to re-ask a question I asked some years ago on stack exchange, but (despite several people trying their best to explain) still failed to understand the answer to. So: does electricity have 'mass'? What I mean is, when current flows, is there a transfer of electrons (or something else) from the power source to whatever it is send to? And is there a difference between AC and DC? The con…

In 1975, our physics teacher said he'd been a consultant to the international group who was working on electric utilities and import/export of energy across the US/CA border. Trying to define what "electricity" actually was!

:)

Electric circuits are much like drive-belts, but using charged particles rather than rubber or leather. If a drive-belt extends across the border between two countries, what is being transported? It's not mass, since for every KG of rubber belt going out, an exactly equal amount is coming back. (Go further and use rotating shafts. Then the mass just spins in place, and doesn't actually cross the border at all.)

With circuits, with AC, the charges are just wiggling back and forth, and not proceeding forward. But with DC the charges are taking a closed circular path, so still are not proceeding forward. The thing which proceeds forward is radio waves. Sixty-cycle radio waves, forced to follow a waveguide composed of two or three conductors. EM waves are being sold to us by the utility companies. We live in a "radio-powered" civilization, but where the 60Hz EM waves are forced to follow waveguides, rather than leaping across empty gaps.

Humorous "legal" chapter in Feynman: IS ELECTRICITY FIRE?

https://www.reddit.com/r/atheism/comments/ev3gy/excerpt_from...

Re: Why is electricity so hard to understand? (1989)

#185

Post by rubberstamp » Wed, Nov 09, 2016, 6:48 AM UTC

0x0 wrote: post #74

rubberstamp wrote: post #58

Earlier quoted context omitted.

shouldn't the cash be listed as capital instead of asset?

So the basic equation is "assets = capital + liabilities". Capital+liabilities explain how your assets are "covered". Either you owe someone for the assets (liabilities), or you own the assets yourself (capital). Sometimes it's easier to think of this as "assets - liabilities = capital". I.e. whatever is left after considering what you owe, you own. If you have some cash (an asset), then you need to also list it eith…

Thanks for explaining it well. I was caught up in the parent post's example and tried to equate it with the equation given in it. How would the parent's example look like in balance sheet? I looked at small business example given in wikipedia, but failed at writing down parents example into assets and liabilities balance sheet. (https://en.wikipedia.org/wiki/Balance_sheet).

Re: Why is electricity so hard to understand? (1989)

#186

Post by dotancohen » Wed, Nov 09, 2016, 7:02 AM UTC

seiferteric wrote: post #101

justifier wrote: post #90

Earlier quoted context omitted.

gp> because water does not attract other water in any way though i was merely addressing this quote your comment confuses me both in fillip and content from cohesion wiki(o): Water, for example, is strongly cohesive as each molecule may make four hydrogen bonds to other water molecules in a tetrahedral configuration. This results in a relatively strong Coulomb force between molecules. from coulomb force wiki(i): Coul…

Yes I knew water attracts water, but it is not relevant to the capacitor analogy. You might as well have said water attracts water via gravity, it's just not relevant in this situation.

Gravity does have an inverse-square attraction. Though obviously it is several orders magnitude less than electromagnetism, in fact the analogy holds.

Re: Why is electricity so hard to understand? (1989)

#187

Post by ZenoArrow » Wed, Nov 09, 2016, 7:41 AM UTC

agumonkey wrote: post #155

ZenoArrow wrote: post #119

Earlier quoted context omitted.

> ">ELECTRIC ENERGY is a wave that travels via a column of charge." I'm not an expert when it comes to electricity, so someone correct me if I'm wrong, but one thing that has made sense to me when it comes to trying to understand electricity is that not all electrons have equal potential for work. When studying electricity, you're often told the charge of an electron as a fixed quantity. However, if I've understood c…

How significant is this when you work at the level of Coulombs (high count of charges) ?

Sorry, I don't think I fully understand your question. Again, what I say could be wrong, but if there's a flow of charge it could mean the outer electrons have more charge than they can contain whilst remaining part of an atom, resulting in either electrons breaking free of atoms, or the energy being passed on to another atom (causing a chain reaction as each atom does the same). I'd suggest a higher count of charges just means this is happening more.

I think another useful mechanism to consider is that atoms want to return to an electrically neutral state. If they have energy that differs from this neutral state, they will either give out energy or take in energy to reach the neutral state. The movement involved in rebalancing the atoms can be thought of as electric current. Again, happy to be corrected if I'm wrong.

Re: Why is electricity so hard to understand? (1989)

#188

Post by notalaser » Wed, Nov 09, 2016, 8:38 AM UTC

fzzzy wrote: post #122

notalaser wrote: post #115

Earlier quoted context omitted.

It doesn't sound too bad when you put it that way, and it certainly made sense back when Ampere introduced the term, but: * It doesn't match the way we define electrostatics, magnetostatics and electrodynamics. What defines electrodynamics isn't the fact that charges are moving (they're moving if the currents are constant, too, but the magnetic fields produced by steady currents are in magnetostatics' yard) but the i…

Thanks for writing that out! Very interesting. I guess it all comes down to the fact that a magnetic field does not exist without an electric current. One way of thinking about it sees it as charges moving, and the other way of thinking about it sees it as a static magnetic field. I guess that's why the terms are archaic!

Indeed. That's why I think it might have made sense back in Ampere's time. The classification of these regimes (electrostatic, magnetostatic, electrodynamic) is more recent, and Ampere's own theory of electrodynamics deals more with what we term "magnetostatic" today.

Re: Why is electricity so hard to understand? (1989)

#189

Post by thisrod » Wed, Nov 09, 2016, 8:42 AM UTC

solipsism wrote: post #182

thisrod wrote: post #157

Earlier quoted context omitted.

> water does not attract other water in any way Um ... once we've cleared up all the misconceptions about electricity, we might want to move on to this other thing called gravity :-) The usual reason that people are mislead by "water" analogies—more precisely, the analogy between height and electric potential—is because they misunderstood gravity to start with. If you start by writing Newton's law and Coulomb's law s…

where you have two tanks floating in outer space, and water gets sucked into them by gravitational attraction As a person who has always struggled to understand electricity (but understand Newtonian gravity well enough), please tell me more! Water is going from where to where?

Picture a pair of tanks shaped like a parallel plate capacitor, a circuit shaped pipe connecting them, and a mixture of water and air filling the pipe. (The air lets the water move around without creating a vacuum.) All other things being equal, the water gets sucked into the plate shaped tanks to minimise the gravitational energy; the resulting gravitational field is identical to the electric field you would get if you forced positive charge onto both plates of a capacitor.

Some of the differences with electricity are:

Like charges repel, so you have to force the positive charge to go where the water would pull itself.

Mass is always positive (dark energy aside), but charge can be negative, so you can have "negative mass pipes" that cancel out the mass of the water.

You can squash positive charge into one tank, and negative into the other. Unlike the parallel water tanks, this takes work, because the charge on each plate repels itself-mass can't do that. But less work than when there is positive charge on both plates. The resulting dipole field is something that you can't get with gravity. This is how real capacitors work.

Re: Why is electricity so hard to understand? (1989)

#190

Post by notalaser » Wed, Nov 09, 2016, 9:07 AM UTC

bandrami wrote: post #86

notalaser wrote: post #16

I think more than half the time I spent earning my EE degree has been spent unlearning the intuitive (but wrong) things that I was liberally taught. I was fortunate enough to have an exceptional Physics teacher in high school, who managed to avoid a lot of the bullshit that less fortunate students were fed; sadly, I compensated that with some of my own (misguided) self-study. This experience also taught me to activel…

Intuitive but wrong (and doggedly persistent) idea #1: "electric current is moving electrons". It is moving photons, exciting (largely) stationary electrons. I can't stress how crucial overcoming that misconception was when I was doing EE.

It may be useful to understand where this error is coming from. Back when the theory of electrical current was developed, we had no good model of the atom. We had no idea about electrons, protons and neutrons, no idea about photons, quarks, nothing. Kirchoff developed his equations without knowing anything about electrons. When Ohm discovered his law, he knew nothing about electrons. Classical electromagnetism, in its entirety, is formulated without any knowledge of electrons (except with some of the late stuff that was developed in order to extend it to relativistic cases, but at that point we were already operating on "legacy code"). The modern idea of the atom, as we know it now, was developed more or less at the same time with classical electromagnetism; various theories about matter were being thrown around at the time, and for a long time, it was thought that electricity would be caused by some sort of charged fluid. Only beginning with the 1870s, at which time Maxwell was already revising his all-encompassing treaty, did the idea that it's all about particles really begin to take off.

The electrical charges in the classic theory of electromagnetism are not electrons, nor ions, nor any other particles that we know of. They're ideal models of "something" that carries charge, but they do not model all the inherent behaviour of electrons (e.g. the electrical charges of classical electromagnetism do not have any magnetic moments, unlike real-life electrons). They are somewhat like the point particles in kinematics, in that they capture an essential feature (electrical charge) of an object while dispensing with other features that are not essential for the study of some (but not all!) phenomenons.

It's ok to model electric current as "the flow of charge", as long as you don't give this model more physical meaning than it's due and attempt to equate the charges of classical electromagnetism with electrons.

Certain phenomenons can be studied within this frame: for instance, the motion of an electron's motion through static electrical and magnetic fields is generally (low enough frequencies, strong enough magnetic fields etc.) OK to do while equating the electron with the charge carrier of classical electromagnetism.

But back when the concept of electrical charge was elaborated, we knew nothing of electrons. The phenomenons gave enough clues for us to hypothesize that whatever "supports" electricity has some of the properties that "real" charge carriers have, but that's all there is to it.