Does electricity travel at the speed of light?

[Desmostylus]

None whatsoever to the speed question.

[zut]

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]

My head hurts.

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]

*Originally posted by Desmostylus *
**None whatsoever to the speed question. **

My mistake. Sorry for confusing things.

[andy_fl]

*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 ® 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]

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]

*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]

*Originally posted by Algernon *
**My head hurts.

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]

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]

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]

*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]

Thanks all. I am now less ignorant than I was an hour ago.
I appreciate your patience.

[Popup]

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[sup]-8[/sup] Ohm m)

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

[andy_fl]

*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

[Popup]

And I did preview it at least three times…
Aluminium, Aluminium, Aluminium.

(Or Aluminum, if you’re thus inclined.Both spellings have been around for a long time.)

[bobk2]

*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]

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]

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.”

[bbeaty]

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]

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.