Electron flow vs flow of charge - Page 1
Forum section: electronics / general

author: FenderBender
author_rank: Super Contributor
timestamp: January 22, 2013, 12:10:52 am
content: Hello,Was digging through my physics notes, reviewing all of the coulombs and charge stuff, and now I'm suddenly confused.1A is defined as 1C/1sec.1C is equal to the charge of 6.2 x 10^18 electrons. One would easily assume that if one amp is flowing then 6.2 x 10^18 electrons are flowing past a certain point in the circuit in one second.I know however, that electrons actually move incredibly slow in a conductor. I know a popular analogy is that there are marbles in a straw. If one moves, they all move, sorta kinda.But how do you explain the whole 1A = 1C/1sec. How can 1C of charge flow past a certain point in second, yet the electrons might only move a millimeter or less? How can there be so much flow of charge, yet so little movement of electrons?Does anyone have an understandable explanation?

author: w2aew
author_rank: Super Contributor
timestamp: January 22, 2013, 12:30:43 am
content: The density of electrons is huge.Think of an enormously wide parallel bus. A very low transmission rate still leads to a huge number of bits per second.Does this help?

author: RJSC
author_rank: Regular Contributor
timestamp: January 22, 2013, 12:32:49 am
content: Think of the cross-section of a typical conductor:How many electrons are there?Several per atom and a huge number of atoms side by side.Even if the electrons are moving very slow, there are many moving in parallel.

author: mark-r
author_rank: Contributor
timestamp: January 22, 2013, 12:33:40 am
content: Consider, though, that a conductor is absolutely stuffed full of electrons!What if instead of 1 straw full of marbles you had 10^18 straws full of marbles. Then in any one straw the marbles only need to move at 6.2 marbles/sec but you still get 6.2x10^18 marbles per second pouring out on to the floor.Also, the electrons are subject to thermal motion, so the marbles aren't neatly parading along all the straws at 6.2 marbles/sec, they are sloshing about backwards and forwards but the average speed in all the straws is 6.2 marbles/sec

author: ftransform
author_rank: Frequent Contributor
timestamp: January 22, 2013, 12:48:34 am
content: This thread makes me want a picoameter that can measure 1 electron per second!!!

author: poodyp
author_rank: Regular Contributor
timestamp: January 22, 2013, 01:01:11 am
content: Quote from: ftransform on January 22, 2013, 12:48:34 amThat would require it could measure at least 100 zeptoamps.

author: Unknown
author_rank: Unknown
timestamp: January 22, 2013, 01:02:13 am
content: Some electrometers go down to the 100 aA, which is about a 1000 electrons per second. That's pretty close to counting individual electrons. I think some of the the newer ones might do 10 aA.

author: FenderBender
author_rank: Super Contributor
timestamp: January 22, 2013, 01:07:58 am
content: Ah that makes loads more sense. I guess I wasn't thinking about the whole "parallel" lines things. I seemed to have been thinking of electricity as "serial". Not sure why I would think that, but I did.For sanity check, if the conductor had a cross section with 3.1x10^18 electrons (half of 6.2...), and 1A flow, then you'd have 2 electrons pass (per "line") x 3.1E18 electrons?I wonder why all of the analogies I have seen make me believe that there's only one line of electrons.

author: free_electron
author_rank: Super Contributor
timestamp: January 22, 2013, 01:27:43 am
content: even worse. if you push a paralell bunch of marbles forward you will see that some even can move backward ! its like swirling milk in coffee. some 'wisps' move the wrong way.in other words on;t focus on the absolute electron or its movement... look how many pass a certain point per second.

author: Shuggsy
author_rank: Regular Contributor
timestamp: January 22, 2013, 01:33:52 am
content: It's much like the water analogy for a lot of electronics. Just as the water flowing through a hose is composed of billions and billions of individual water molecules, the current flowing through a wire is composed of billions upon billions of individual electrons.The ampere is a measure of charge going through a point in a circuit over time. So, if you somehow had a current path that was the size of a single electron, then 1A of current would mean 6.2x10^18 electrons flow through a point in that path every second. Similarly, with your example, 2 electrons (on average) would flow through each "electron sized" segment of the path per second. The actual movement of electrons is a fairly random process as they bang into molecules while they move along, generally causing heating in the current path which could be a wire, a resistor, etc.Even then, if I recall correctly, it's not necessarily that one electron moves from one end of the path to the other. The electrons will jump between the shells of nearby atoms, constantly trying to find a unity state where everything is stable. That is, when an electron in the outer shell of one atom gets excited enough by an applied electric field it shoots off to a nearby atom that just gave up an electron of its own. Similarly, the atom the first electron just left is now charged is and is looking to become neutral again, so another nearby electron eventually jumps to its shell. This action continues for the length of the current path causing an overall flow of electrons in some direction. As free_electron said, some may move backward, but the average results in a movement in the direction of current flow.Someone please correct me if I'm getting way off base here... it's been too long since I explored the physics of electron movement in terms of electric current and even then I didn't delve too far into it.

author: FenderBender
author_rank: Super Contributor
timestamp: January 22, 2013, 01:45:59 am
content: I think you are right from some other external resources I just looked at, though I'm the last person to validate anything on this matter.It's all about the net movement....well I guess the other stuff is important to someone, but not to you or me.It makes sense now.

author: ftransform
author_rank: Frequent Contributor
timestamp: January 22, 2013, 02:04:29 am
content: counting electronswww.keithley.com/data?asset=50390

author: FenderBender
author_rank: Super Contributor
timestamp: January 22, 2013, 03:33:25 am
content: Dave isn't kidding when he says they are the ultra low voltage/current measurement masters...

author: JuKu
author_rank: Frequent Contributor
timestamp: January 22, 2013, 06:53:02 am
content: Quote from: ftransform on January 22, 2013, 12:48:34 amCan be done, although prof. Heisenberg does not allow better than a few ten electrons per second accuracy. Budget is a few ten millions, and you need at least two professor level scientists on the team, one on low temperature physics and one on semiconductor manufacturing. But it can be done, I once helped solder one together.

author: Gall
author_rank: Frequent Contributor
timestamp: January 22, 2013, 11:26:39 am
content: Under certain circumstances it is possible to watch individual electrons moving. That's really annoying if you're trying to do some precision physical measurements. But not in an ordinary conductor.Another thing to consider is: electron is not like a ball or bullet. It is de-localized and may even occupy the whole wire.

author: smashedProton
author_rank: Frequent Contributor
timestamp: January 22, 2013, 08:21:59 pm
content: And now to complicate things even worseplease explain the skin effect with the parallel straw analogy