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KenP
12-04-2002, 05:15 AM
If not, what's the speed of electricity?

Cisco
12-04-2002, 05:22 AM
Electricity travels the speed of light in a vacuum but in everyday situations it is typically slowed down by what is carrying it (ie copper.)

Nishroch Order
12-04-2002, 05:22 AM
Erm.....erm.....*quick quick...get someone here to define electricity!!*

Cisco
12-04-2002, 05:32 AM
Upon further inspection I'd like to retract my statement. Search google for "speed of electricity" and you'll see why. This is a rather more complicated matter than I had thought, one of the reasons being, as Nirhroch Order mentioned, the word 'electricity' has no single definition.

Desmostylus
12-04-2002, 06:02 AM
The actual electrons moving in the conductor move at the "drift velocity", which depends on current and conductor cross-sectional area, but would typically be of the order of a few meters/sec. In an alternating current, of course, all they do is shuffle back and forth and never actually go anywhere.

The speed of the information carried by the electricity, which may be as simple as "switch is now on, make light work", travels at the speed of light in the dielectric that the conductors are operating in. For air, that speed is just under the speed of light in a vacuum (c). For a polymer insulator, that speed would be something like 2/3 c. So the answer, is "a large fraction of c".

Mort Furd
12-04-2002, 06:03 AM
If you are talking about the actual motion of electrons in the conductor, then no electricity does not move at the speed of light. If it di, all you would need to make an electron gun would be to make a sharp bend in a piece of wire with current flowing. The electrons wouldn't be able to make the turn and, presto, instant electron beam.
If you mean the propagation rate of the force that causes the elextrons to move, then the answer is "almost." The actual speed depends on the conductor and its density. RF cables actually have a factor that you can look up and use to calculate wavelengths for different frequencies in the cable. It is basically a percentage of c. A rough value for most 50 ohm cable is 0.9 - but that won't get you very far if the cable is connected to the wrong impedance. You'll also have to do a lot of fiddling to get the length just right if you make an antenna or balun using this value. To do it right, you need the correct value for the cable you are using.

Mort Furd
12-04-2002, 06:06 AM
Simulpost. Desmostylus explains it better.

Desmostylus
12-04-2002, 06:08 AM
Mort Furd: Almost right. It doesn't depend on the conductor but on the insulator. The length stuff and impedance matching has nothing to do with speed.

Desmostylus
12-04-2002, 06:11 AM
Sorry, Mort_Furd my last post didn't come across right. Those other factors you mention have to do with maximising power transfer rather than speed.

Mort Furd
12-04-2002, 06:37 AM
No, the length doesn't have anything to do with it. I was just trying to provide an example of why the speed of electricity in a conductor is significant. The balun is also just an example.
Impedance matching has an effect on the length of the antenna in that is changes the length at which the impedance of the antenna matches that of the cable - which is a must for a transmitter antenna if you want to avoid getting power reflected back at the transmitter. It is also important for a receiver, though not as critical. A receiver won't burn itself up because of a poor match the way a transmitter can.
Sorry. I tend to look at these things from a radio technician's point of view. Physics class is 17 years gone.:)

Mort Furd
12-04-2002, 06:39 AM
Originally posted by Desmostylus
Sorry, Mort_Furd my last post didn't come across right. Those other factors you mention have to do with maximising power transfer rather than speed.
Your last post was quite correct. We were discussing speed of electricity, not power transfer. I got rather far afield.:)

Desmostylus
12-04-2002, 06:42 AM
Mort Furd: Your first posting does make perfect sense. I just didn't read it carefully enough.:smack:

zut
12-04-2002, 06:58 AM
Originally posted by Desmostylus
Mort Furd: Almost right. It doesn't depend on the conductor but on the insulator. Can you 'splain this further? Why does speed depend on the insulator? As a practical matter, what would happen af a conducter were half-imbedded in polymer and half in air? Or, better yet, what happens in a stranded wire where the insulator doesn't touch the entire perimeter of each strand? Or did I misinterpret your statement?

Desmostylus
12-04-2002, 07:11 AM
The best example is a high frequency signal in a coaxial cable.

The energy transmitted is actually in the form of an electromagnetic field which exists in the insulator between the inner and outer conductor.

If the insulator is all air, you'll get one speed. If it's all, say, polyethylene, you get a lower speed. If it's a combination of the two, and lots of cables are deliberately made with air spaces in polyethylene, you'll get a speed in between that of air and polyethylene.

Ask again if you want more.

Algernon
12-04-2002, 07:40 AM

"The energy transmitted is actually in the form of an electromagnetic field which exists in the insulator between the inner and outer conductor."????

Could you please explain this again, in a different way? Inner and outer conductors? Doesn't the energy "flow" in the conductor itself?

Desmostylus
12-04-2002, 08:04 AM
A coaxial cable has two concentric conductors. Like the cables from you TV antenna to your VCR, from the VCR to the TV. Like the stuff "cable TV" comes in.

Electric current has to "flow" in the conductors, but that's really just electrons shuffling back and forth a short distance.

At low frequencies, like the 50 or 60 Hz used for electric power transmission, it's usual for people to talk about power flowing through the conductors, etc., because that's a useful and logical sounding way of describing it.

But at higher frequencies, and this is where the "speed of electricity" question becomes important as Mort Furd noted, those useful and logical sounding descriptions aren't so useful anymore.

The real situation, at any frequency, is as I've described it, with energy being transmitted through the dielectric. Sounds silly, but that's how it is.

I'm struggling to keep the discussion simple.

Algernon
12-04-2002, 08:12 AM
Thanks for attempting to dumb it down for me Desmostylus.

In a simple insulated copper wire, what exactly is the dielectric?

Mort Furd
12-04-2002, 08:18 AM
The insulation.

Desmostylus
12-04-2002, 08:24 AM
Dielectric basically equals insulator, but it's slightly complicated.

You can't just have one insulated copper wire and transmit energy through it. You need a return path. The dielectric is that combination of insulators that exist between the two paths.

So, if you have two insulated copper wires running side by side, the dielectric is the combination of the insulation surrounding the wires, any air between the copper and the insulation, and the air surrounding the two insulated copper wires.

Algernon
12-04-2002, 08:27 AM
Wow. This is fascinating. It's like when I discovered that Santa really didn't exist.

The energy is transmitted through the insulation rather than the conductor. Could they have been more misleading with the noun selections? (OK. I know this is sounding quite sarcastic, when that's not really my intent.)

I'm quite confident they didn't cover this in my college physics courses.

So if the energy transmission is through the dielectric, and insulated copper wire is slower than air, then what difference does it make what material the conductor is made out of?

Desmostylus
12-04-2002, 08:28 AM
None whatsoever to the speed question.

zut
12-04-2002, 08:38 AM
OK, I think that makes sense. We're talking about how the electric field around the conducter propagates. This winds up being some kind of space integral involving dielectric constants of the material "between" the conductors. That is darn interesting. I concur with Algernon; I don't recall it being explained quite that way before.

I suppose that this concept goes hand-in-hand with the idea that electrons flow on the surface of the conductor, something that I just took on faith before.

Algernon
12-04-2002, 08:40 AM

So, as they say, could you drop the other shoe?

The conducting material is irrelevant to the speed of the energy transfer. The speed is dependent upon the insulators.

What then is dependent upon the conducting material? Power transfer? Efficiency? (I'm probably using all the wrong words here. My electricity experience consists of wiring my basement.)

Is there any relationship between speed and efficiency? Does speed matter at all?

Mort Furd
12-04-2002, 08:45 AM
Originally posted by Desmostylus
None whatsoever to the speed question.
My mistake. Sorry for confusing things.

andy_fl
12-04-2002, 08:49 AM
Originally posted by zut
... electrons flow on the surface of the conductor ....

Are you sure on that ?

My basic electrical knowledge tells me that, that the resistance (R) of a length is related to the specific resistance (<rho>), the lenght of the conductor (L) and the area of cross-section (A), by :

R = <rho> L / A

Agreed that when the frquency is very high, current does tend to flow near the surface (skin effect) but for DC currents, I believe the flow is through the entire cross section.

If the flow was only through the surface, we would have seen hollow tubes being used for power transmission, which would have saved us a lot.

But I maybe totally wrong on that.

DRomm
12-04-2002, 08:54 AM
I encountered this question while trying to determine network signal propagation. This site http://www.networkmagazine.com/article/NMG20010416S0006 has a good discussion of the subject: "In comparison, electric waves or signals in commonly used copper wire travel at speeds between 55 percent and 80 percent of c."

Mort Furd
12-04-2002, 08:59 AM
Originally posted by andy_fl
If the flow was only through the surface, we would have seen hollow tubes being used for power transmission, which would have saved us a lot.
Which actually is the case in some high frequency systems.

Mort Furd
12-04-2002, 09:02 AM
Originally posted by Algernon

So, as they say, could you drop the other shoe?

The conducting material is irrelevant to the speed of the energy transfer. The speed is dependent upon the insulators.

What then is dependent upon the conducting material? Power transfer? Efficiency? (I'm probably using all the wrong words here. My electricity experience consists of wiring my basement.)

Is there any relationship between speed and efficiency? Does speed matter at all?
The resistance of the conductor makes a difference in the efficiency of the power transfer. More resistance=more power loss over the length of the conductor.

FriendRob
12-04-2002, 09:04 AM
andy_fl is right that it depends on whether you're talking AC or DC. More specifically, it depends on the frequency of the AC current. The skin depth (i.e. how far into the conductor the motion of the electrons penetrates) depends on the frequency: lower frequency equals more penetration. DC current is zero frequency, so maximum penetration.

Desmostylus
12-04-2002, 09:05 AM
The conductor material is relevant to all sorts of things. Principally efficiency, cost, strength and size.

Materials with low electrical resistivity "waste" less energy through heating. (In some applications, though, like an electric heater, that heating isn't considered waste.)

Gold has low resistivity. Copper's is higher. Aluminium's is higher still. Gold would be terriffic for making conductors, but it's expensive. Copper is cheaper, so copper is used a lot. Aluminium is cheaper still, but its resistivity is so much higher than copper's that you need to use more of it. So much more, that the aluminium struggles to support its own weight when strung between power poles and needs steel reinforcement.

All sorts of trade-offs depending on the situation.

The speed question in most situations comes last as far as trade-offs go. You only need to know what the speed is, rather than wish for more.

In some situations, especially chip design, more speed=better. So if proposed the use of a new dielectric, its speed would be factored into any decision about its use.

Mort Furd
12-04-2002, 09:12 AM
Originally posted by Desmostylus
Gold has low resistivity. Copper's is higher. Aluminium's is higher still. Gold would be terriffic for making conductors, but it's expensive. Copper is cheaper, so copper is used a lot. Aluminium is cheaper still, but its resistivity is so much higher than copper's that you need to use more of it. So much more, that the aluminium struggles to support its own weight when strung between power poles and needs steel reinforcement.
According to a coworker (who grew up in the DDR,) aluminum has another bas property. It "flows" under pressure - meaning that a connection that was screwed down nice and tight loosens with time, until one day you plug in appliance and turn it on and the outlet goes "Kaboom."

Algernon
12-04-2002, 09:13 AM
Thanks all. I am now less ignorant than I was an hour ago.

Popup
12-04-2002, 09:52 AM
Well explained Desmostylus! I've only got one small nit to pick:
Originally posted by Desmostylus
Gold has low resistivity. Copper's is higher. Aluminium's is higher still.

Oft repeated, but not true!
In fact the best conductor is Silver followed very closely by Copper. Gold is rather far behind, just before Aluminium. Then there's nothing, nothing, and the other metals.
Here's a table:

Silver 1.59
Copper 1.673
Gold 2.35
Alumium 2.65
..
Iron 10

(Units in 10-8 Ohm m)

The reason Gold is used on connectors etc is that it is very soft, and inert.

andy_fl
12-04-2002, 09:54 AM
Originally posted by Algernon
The energy is transmitted through the insulation rather than the conductor.

The energy is not transmitted but propogated through the insulation. You can consider it like fluids flowing through pipelines. If you increase the pressure at one end, the other end comes to know of this at the speed of light. Now how much flow results due to this is dependent on the resistance at the other end and the resistance in the path itself.

Originally posted by Mort Furd
aluminum has another bas property. It "flows" under pressure - meaning that a connection that was screwed down nice and tight loosens with time, until one day you plug in appliance and turn it on and the outlet goes "Kaboom.....

Creep in Aluminum is just one of the problems. There are other problems too and prominent is the formation of Aluminum Oxide and dissimilar expansion when coupled with Copper. More Here (http://tis.eh.doe.gov/docs/sn/nsh9001.html)

Popup
12-04-2002, 10:00 AM
And I did preview it at least three times...
Aluminium, Aluminium, Aluminium.

(Or Aluminum, if you're thus inclined.Both spellings (http://www.quinion.com/words/articles/aluminium.htm) have been around for a long time.)

bobk2
12-04-2002, 11:39 AM
Originally posted by Desmostylus
The actual electrons moving in the conductor move at the "drift velocity", which depends on current and conductor cross-sectional area, but would typically be of the order of a few meters/sec.

Do you (or anyone) have an actual number for the drift velocity? I fuzzily remember hearing it was actually quite a bit slower...crawling along. It came up in either Physics or Chemistry class when we were figuring how many electrons had to flow past a point per second to constitue an amp. There are a whole lot of (mobile) electrons in a small amount of copper.

Thanks!

Originally posted by Desmostylus
In an alternating current, of course, all they do is shuffle back and forth and never actually go anywhere.

Per Dave Barry, that's the beauty of the scam run by the Electric Utilities - they sell you the same electrons over and over.

Ring
12-04-2002, 11:51 AM
You might find this interesting.....but probably not.

The propagation speed for a 1 MHz radio wave in a copper conductor is only 400 meters per second. If nothing else this pretty much shows why the electromagnetic wave must propagate via the fields outside the conductor.

ZenBeam
12-04-2002, 12:30 PM
And I did preview it at least three times...
Aluminium, Aluminium, Aluminium.And here I just thought you were really, really old:

In 1807, a British Chemist, Sir Humphrey Davy suggested to call this mysterious metal "Alumium," and later agreed upon its change to "Aluminum." (http://www.alcan.co.kr/eng/techcenter/al_history.asp)

bbeaty
12-04-2002, 03:41 PM
An analogy might help.

Electric circuits are like a system of pulleys and drive belts. If you turn one pulley, the belts all move and the other pulleys turn too.

We get confused about the difference between the speed of the "belt-stuff", versus the WAVES which spread along the belts. After all, if you turn one pulley, the other pulleys don't move instantly. Instead there are (very fast) waves which spread along the belts, and when the waves reach the distant pulleys, only then do those pulleys get the message that they have to start moving.

In a circuit, the wires are all full of "invisible belts" made of electrons. (Or from the textbook definition, a 'conductor' is a substance which contains a population of mobile charged particles.) Like drive belts, the population of electrons in a metal circuit moves slowly in a complete circle, with no electrons being gained or lost anywhere. And like with the drive belts, the speed of the moving belt has nothing to do with the speed of the waves which race along the belt.

See:

ELECTRICITY MISCONCEPTIONS
http://amasci.com/miscon/elect.html

ELECTRICITY FAQ
http://amasci.com/elect/elefaq.html

Algernon
12-04-2002, 04:34 PM
bbeaty, from your links (bolding mine)...Here's one way to clarify the muddled concepts: if electric current is like wind, then electrical energy is like some sound waves, and electrons are like the molecules of the air. For example, sound can travel through a pipe if the pipe is full of air molecules, and electrical energy can flow along a wire because the wire is full of movable charges. Sound moves much faster than wind, correct? And electrical energy moves much faster than electric current for much the same reason. Air in a pipe can flow fast or slow, while sound waves always move at the same very high speed. Charges in a wire can flow fast or slow, while electrical energy always flows along the wire at the same incredibly high speed. Whenever sound is flowing through a pipe, the air molecules in that pipe are vibrating back and forth. When waves of AC electrical energy are flowing along a wire, the electrons in that wire are vibrating back and forth 60 times per second.

Some books teach that, in a simple battery/bulb circuit, each electron carries energy to the bulb, deposits its energy in the hot filament, and then returns to the battery where it's re-filled with energy. This is wrong. Some books give an analogy with a circular track full of freight cars waiting to be filled with coal. This picture is wrong too. The energy in electric circuits is not carried by individual electrons. Instead the electrons move very slowly while the electrical energy flows rapidly along the columns of electrons. The energy is carried by the circuit as a whole, not by the individual charged particles.

In discussing this misconception with teachers, I find that they see nothing wrong with teaching it to their students! After all, the kids instantly grasp the "freight cars with coal" story since it is very visible and it offers a sensible explanation. What more can we ask? Yet there is a serious problem here: electrons flow slowly, and in AC circuits they don't flow at all, instead they wiggle. In order to really understand electric circuits in the more advanced classes, a student must UNLEARN the seductive freight-cars analogy. "Unlearning" rarely happens, and so the analogy forms a learning barrier which can forever prevent any further progress.

There are TWO main things that flow along wires:
- Electric Charge
- Electric Energy
There are several other things that flow as well, but to keep it simple, we'll ignore them.
Because there are TWO things flowing, we cannot call them by the name "electricity." For this reason, we cannot ask "what is electricity?" Instead we have to ask more specific questions like these:
1. What is the stuff that flows through a light bulb and comes back out again through the other wire?
2. What is the stuff that flows into a light bulb and gets changed entirely into light and heat?

The answer to question #1 is ELECTRIC CHARGE. Charge is a "stuff" that flows through lightbulbs, and it flows around a circuit. Normally no charge is lost during the operation of a circuit, and no charge is gained. Also, charge flows very slowly, and it can even stop flowing and just sit there inside the wires. In an AC circuit, charge does not flow forwards at all, instead it sits in one place and wiggles forwards and back.

The answer to question #2 is ELECTRICAL ENERGY. It's also called "electromagnetic energy". This energy is also like a "stuff" and it can flow from place to place. It always flows very fast; almost at the speed of light. It can be gained and lost from circuits, such as when a light bulb changes the flow of electrical energy into a flow of light and heat. Thanks for the information. I snipped the above because...
a) it precisely explains the roots my own confusion, and my own mis-education at the hands of what I thought were very competent teachers; and
b) FWIW, it highlights the passages that were particularly instructional to me.