Speed of Electricity

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QwertyKey
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Speed of Electricity

Postby QwertyKey » Tue Aug 04, 2009 5:08 pm UTC

I have been trying to research Electricity on the Internet and I have not really gotten any answers.
Also, I am still a student and I really do not know much about Electricity yet.

The Speed of Electricity.
How does "Electricity" travel?
Basically, I am asking if you had a battery and a light bulb and a wire, how does this "Electricity" travel to light up to bulb? I am not really concerned about the exact mathematics in practice but in theory of all the formulae. I think Electron Drift Speed is quite unrelated to my question here, too. I also have read about Velocity Factors, Dielectric Constants, etc but how does this Electromagnetic Wave(I assume its a wave that lights up the bulb, read my first/second statement if I'm wrong) travel? It propagates through the air or the conductor? How does the medium affect the speed?

Also, I myself know Electricity is a vague term.

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Re: Speed of Electricity

Postby BlackSails » Tue Aug 04, 2009 6:24 pm UTC

Electrons move from areas of high electric potential to low electric potential (the ground)

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Re: Speed of Electricity

Postby artifact » Tue Aug 04, 2009 6:46 pm UTC

Yeah electron drift speed isn't too important. Think of a battery hooked up to a very thin wire with a single file line of electrons along it. Its just a changing electric fields that carries the influence down the wire. One end of the battery is going to push away on an electron because it has a negative charge build up. When that electron gets pushed away its going to smack into an electron in front of it in the wire. So that second electron is going to get pushed away, and it will bump into the third electron and so on. On the other end of the battery the same thing is happening but in the other direction. That end has a positive charge build up so it is pulling electrons in. But as soon as an electron gets even a tiny tiny bit closer to the one in front of it is going to start pushing it. So that happens almost instantaneously. Almost though, because these pushing and pulling forces are caused by the electric fields that all the charges have. And the speed at which an electric field changes is going to be limited by the speed of light. So the "speed of electricity" in this sense is going to be the speed of light. Full out electromagnetic waves are not necessary. The speed these electrons themselves end up moving is the electron drift speed.

I assume that this speed could be less than the speed of light but I'm not sure why or how. I don't really like my explanation all that much, but its the best I can think of now.

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Re: Speed of Electricity

Postby Charlie! » Tue Aug 04, 2009 8:46 pm UTC

Imagine that instead of a wire you had a long trough of water, open on the top. The water is equivalent to voltage.

So now you pour a biiig bucket into one end of your trough (you turn on the lightswitch), and you get... a wave! The wave slooshes down to the end of the trough, raising the water level as it goes to a new, higher level.

That wave is essentially how electricity travels. When you pour some electrons into one end of the wire, they push the electrons in front of them, which push the electrons in front of THEM, etc. A wave of voltage travels down the wire, and eventually reaches the lightbulb. In university physics we actually measured the speed of the wave, and the wave moves pretty close to the speed of light, even though the individual electrons don't move very quickly.

It's not really an electromagnetic wave, since there's no "magnetic" part. It's just an electric wave. With AC you have a lot of those waves going in opposite directions, so repeated movement through the bulb makes it shine. With DC you end up with a more "static" situation after one wave, with individual electrons just moving at the drift speed (not very fast).
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Re: Speed of Electricity

Postby Averazul » Tue Aug 04, 2009 9:17 pm UTC

Electricity moves through a wire the same way water moves through a garden hose.

When the hose is full of water, if you put more water in one end of the hose, some different water (but the same ammount) comes out the other end. In truth, these are not simultaneous actions: The pressure of the increased water in one end of the hose travels down the hose at the speed of sound (in water) until the pressure reaches the other end and the water at that end comes out. The important idea is that it is the pressure which makes things happen in the hose, and the pressure moves down the hose much faster than the actual water.

Wires are like hoses, except that the water is actually electrons, and the wire is always full of electrons (whereas the hose can be empty at times). The pressure that moves through the hose as a result of adding more water is equivalent to connecting a battery to one end of the wire. The voltage at one end pushes excess electrons into one end of the wire, which causes the electrons at the other end to come out of the wire. Again, the same electrons you put into one end are not the same electrons coming out of the other. The voltage (potential) at one end of the wire moves down the wire at a much greater speed (maximum is c in perfect conductors, real wires come pretty close).

The similarities end at the end of the wire/hose that the electrons/water come out. Whereas water is happy to leave the hose and fall to the earth via gravity, this is not an acceptable path for the electrons. The electrons would rather stay all bunched up at the end of the wire than be spit out into the air, so ya have to givem a good path. That path can be: lightbulb, speaker, motor, heater, etc. The electrons will go through whatever path you give them that leads to a lower voltage (potential). The most convenient path is Battery --> wire1 --> bulb --> wire2 --> battery.

The reason that the voltage exists within the battery is that there are too many electrons in there at once, trying to get away from each other. If you let some electrons out of the high energy end without putting any more back in, then that potential which arose from an excess of electrons disappears. By ending the circuit at the other end of the battery, you can take some high energy electrons from the positive terminal, run them through your bulb (turning them into low energy electrons) and put them back into the negative termal of the battery. You've still got as many electrons as you started with in the battery, so your potential isn't lost, and you've extracted energy from the electrons by running them through the bulb.

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Re: Speed of Electricity

Postby Charlie! » Tue Aug 04, 2009 9:22 pm UTC

Averazul wrote:The reason that the voltage exists within the battery is that there are too many electrons in there at once, trying to get away from each other.

Not actually true :P

What happens is that the positive and negative sides of the battery work together in a chemical reaction. One is a chemical reaction that wants to give up electrons, and the other wants to take electrons. There's always a "normal" number of electrons in the battery, it's just that the electrons are shifted from (usually) one metal to another metal.
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Re: Speed of Electricity

Postby brakos82 » Tue Aug 04, 2009 9:29 pm UTC

Averazul wrote:The reason that the voltage exists within the battery is that there are too many electrons in there at once, trying to get away from each other.


Mental picture: a bunch of people dressed as large yellow spheres stuck in a small room trying to shove each other out of the way, eventually bouncing off each other and knocking them over.

And I'm pretty sure nothing kinky would happen from this. :|
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Re: Speed of Electricity

Postby PM 2Ring » Wed Aug 05, 2009 2:12 am UTC

Charlie! wrote:
Averazul wrote:The reason that the voltage exists within the battery is that there are too many electrons in there at once, trying to get away from each other.

Not actually true :P
Charlie! speaks wise words. Voltage is a lot like pressure. A voltage difference causes current to flow, the same way that water flows in the hose due to a pressure difference. To be more accurate, voltage is a measure of electrical potential energy, and electrical current naturally flows from a region of high potential to one of low potential.

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Re: Speed of Electricity

Postby phlip » Wed Aug 05, 2009 3:10 am UTC

Since noone seems to have given any numbers, I'll say that a common rule of thumb is that the propagation speed in copper wire is about 2/3 of the speed of light (much faster than the velocity of the actual electrons themselves). I don't really know how precise that approximation is, though... I imagine it would change a fair bit with the material involved, and the shape and layout of the cable... like, if you move the wires closer together, you'd increase the capacitance between them (and the crosstalk), which I think would probably increase the amount of time it'd take a signal to propagate from one end to the other.

[edit]
Wikipedia wrote:In contrast [to the drift velocity], electromagnetic wave propagation is much faster, and depends on the dielectric constant of the material. In a vacuum the wave travels at the speed of light and almost that fast in air. Propagation speed is affected by insulation, such that in an unshielded copper conductor it is about 96% of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.

I guess I was misremembering the coax figure...

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Re: Speed of Electricity

Postby mr-mitch » Wed Aug 05, 2009 4:44 am UTC

Electrons themselves travel very slowly with drifts speeds of order 10^-1 (if I'm remembering that correctly).

When the electrons (charge) move, they generate an electric+magnetic field which propels other electrons, thus, the information travels very fast albeit the actual electricity/electron flow is slow.

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QwertyKey
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Re: Speed of Electricity

Postby QwertyKey » Wed Aug 05, 2009 2:42 pm UTC

So, just to see if I see this correctly:

This Electric Wave travels near c and the exact speed depends on the conductor as it travels through the conductor. So if you had like a really long and coiled wire but the actual distance between the bulb and battery is small, the bulb should, in theory, light up later than another circuit with lower wire distance overall? I'm asking this as I was shown a picture and it showed a wave travelling through the air so I was like "?"...

However, I seriously have to ask this: how do Insulators get Dielectric Constants? I mean, you don't get a current if you use a wire made of insulators... so, in effect wouldn't this dielectric constant be at least Infinity, or not-applicable?

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Re: Speed of Electricity

Postby Charlie! » Wed Aug 05, 2009 3:53 pm UTC

QwertyKey wrote:So, just to see if I see this correctly:

This Electric Wave travels near c and the exact speed depends on the conductor as it travels through the conductor. So if you had like a really long and coiled wire but the actual distance between the bulb and battery is small, the bulb should, in theory, light up later than another circuit with lower wire distance overall? I'm asking this as I was shown a picture and it showed a wave travelling through the air so I was like "?"...
Yup, goes through the wire, so longer wire means longer time.

However, I seriously have to ask this: how do Insulators get Dielectric Constants? I mean, you don't get a current if you use a wire made of insulators... so, in effect wouldn't this dielectric constant be at least Infinity, or not-applicable?

You measure the dielectric constants by sticking them in the middle of a capacitor and turning it on. Although there's charge flowing into and out of the capacitor (at the start at least), there's no charge flowing through the dielectric (dielectric meaning "insulator that you stick in an electric field").

What dielectrics do is, as the charge builds up on either side of them, their molecules get slightly polarized. There's no current going into or out of them, but one side of them becomes slightly positive and the other end becomes slightly negative. If you put a bunch of negative charge against one side of the dielectric, that side will become just a little positive, making the charge more stable (lower potential energy and less force pushing it apart). That effect is useful in capacitors since they're big concentrations of charges that need to be kept stable.
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Re: Speed of Electricity

Postby rflrob » Fri Aug 07, 2009 11:58 pm UTC

QwertyKey wrote:However, I seriously have to ask this: how do Insulators get Dielectric Constants? I mean, you don't get a current if you use a wire made of insulators... so, in effect wouldn't this dielectric constant be at least Infinity, or not-applicable?


Depending on the exact amount of resistance in your dielectric, you might be able to get a high enough voltage to pump some measurable amount of electrons through. Terms like "resistor" and "insulator" and "conductor" are somewhat relative. Normally, wood is pretty insulative, but compared to the air, it's resistance is tiny. This is why you shouldn't stand near trees in lightning storms.
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Re: Speed of Electricity

Postby ian » Sat Aug 08, 2009 4:53 pm UTC

Hey, couldn't see it mentioned in the thread, what is the front of the wave/movement of electrons actually called? I called it the electon wavefront the other day but it didn't sound right.

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Re: Speed of Electricity

Postby bigglesworth » Sat Aug 08, 2009 4:57 pm UTC

I wonder what the speed is in a superconductor?
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Re: Speed of Electricity

Postby PM 2Ring » Sat Aug 08, 2009 5:20 pm UTC

bigglesworth wrote:I wonder what the speed is in a superconductor?

It's complicated, because charge in a superconductor is carried by Cooper pairs, not solitary electrons.

http://en.wikipedia.org/wiki/Cooper_pair
a Cooper pair is the name given to electrons that are bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper.[1] Cooper showed that an arbitrarily small attraction between electrons in a metal can cause a paired state of electrons to have a lower energy than the Fermi energy, which implies that the pair is bound.

[...]

The electrons in a pair are not necessarily close together; because the interaction is long range, paired electrons may still be many hundreds of nanometers apart. This distance is usually greater than the average interelectron distance, so many Cooper pairs can occupy the same space.[4] Electrons have spin-1⁄2, so they are fermions, but a Cooper pair is a composite boson as its total spin is integer (0 or 1). This means the wave functions are symmetric under particle interchange, and they are allowed to be in the same state. The tendency for all the Cooper pairs in a body to 'condense' into the same ground quantum state, or zero point is thought to be responsible for the peculiar properties of superconductivity.


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