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# Question regarding speed electricity Answered

Hi people!

I have another thing which i do not totally understand, so as usual i have asked in one of my favourite sites! :)

I always had a doubt on how "fast" is electricity. In our real world, i can observe that it is pretty fast. If i turned on a switch connected to a light bulb, i can see it turned on pretty much "instantaneously". So a question came to mind, how fast it really is? Is it near to the speed of light? So, i did some googling. According to what i have found and understood so far, it is the electric field that carries information and not the electrons themselves. And electric field travels near to the speed of light( of course, depending on the medium).

If that is the case, then from the "textbook" definition of electricity, it is the movement of electrons right? But according to what i have understood so far, electrons move pretty slowly. In DC a few millimeters per hour or so(not having the exact figure in mind, pardon me) and the average displacement in AC is zero. Then how exactly is electricity so fast?

I would appreciate it if someone explains it in a simple way, both on speed and what exactly is movement of electrons. :)

Update: Okay, so far my questions are answered, and it seems to take off in different directions(like science always does :D). So to stay true to the question posted here, i would like to say my doubts cleared! Many thanks to people who have answered my question, Specially Dr.Kelsey and Josehf , for the information provided! I will post other doubts in new topics.

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## 50 Replies

syang8 (author)2013-03-10

I can't help but want to chime in with our 'Pi-powered Experimenter's Bench(PEB)', a KickStarter project titled 'Data Acquisition System for Raspberry Pi', and our Lesson Plan example --Measuring Speed of Sound, see --https://docs.google.com/file/d/0BxuyQoc-vUJ-OXB0T1pKQWlhaVk/edit. We are working on our 'Optical Explorer' and Ras-Das-2 boards that will increase our sampling rate to 50 MSPS, A PEB with this set of boards will allow experimenation to measure speed of light, and indirectly speed of electricity through a medium, in your case a wire. Good Luck.

Orngrimm (author)2013-02-14

You have to distinguish between the speed of the impulse (Speed when you push the electron "Harry" into one end of the cable till another electron falls out the other end) and the speed of the electron itself (the time it takes electron "Harry" to travel the length of the cable).

Impulse: Around 95-97% of the speed of light in an unshieldes copper-cable. Around 66% of the speed of light in a typical coaxial-cable. Both those values are from wiki (http://en.wikipedia.org/wiki/Speed_of_electricity)

The electron itself is awfull slow in normal conductors! You can overtake it easy! Harry seems to be a 200 year old Methuselah...
In a 1mm diameter-copper-wire the speed is around -1.0 m/hour.
Yes! minus one meter per hour!
See, the electrons travel from - to the + pole. Not the other way around... :)
See http://en.wikipedia.org/wiki/Drift_velocity if you want to calculate that for yourself and with other metals (Other densities and other atomic weights).

Josehf Murchison (author)2013-01-29

Speed of light until you look at component charge times or transition times.

Basically it is only a theory as to how fast light is every year our ability to measure it more accurately improves.

Joe

kelseymh (author)2013-01-30

No, sorry. The speed of light is an exact constant: 299,792,458.0000000000 meters per second. What changes is how long the meter is :-)

Josehf Murchison (author)2013-01-30

The difference between stupidity and genius is that genius has its limits.
Albert Einstein

kelseymh (author)2013-01-30

No. It's my job to know stuff; I'm a research physicist. The speed of light was set as a fixed constant a decade or more ago, once the measurements had smaller uncertainties than the uncertainty in the length of the physical meter bar.

Josehf Murchison (author)2013-01-30

The speed of light in vacuum, known as C, is 299,792,458 meters per second.

The speed at which light propagates through transparent materials such as glass or air, is less than C about 200,000 km/s, sorry mixed my kilometers and my meters.

kelseymh (author)2013-01-30

Exactly so. The difference in speed depends on the index of refraction, and since the index of refraction may be wavelength dependent, the speed of light is not a unique value in materials.

In air, with an index of refraction of 1.000293 (that's for dry air at 0 C and 1 atm pressure), the speed of light is about 299,078 km/s.

In borosilicate glass (Pyrex), with an index of refraction of 1.33, the speed of light is about 225,400 km/s.

In diamond, with an index of refraction of 2.42, the speed of light is about 123,900 km/s.

Non-transparent materials may also have an index of refraction, applicable to electromagnetic waves other than visible light. A coaxial cable, for example, with 50 ohm impedance, has an effective "index of refraction" for low frequencies around 3, so the signal propagation speed in coax is around 100,000 km/s.

Josehf Murchison (author)2013-01-30

Good then to answer the question proposed here, electricity is really fast and it exact speed is not that important unless you are building a particle accelerator or the like.

“Speed of light until you look at component charge times or transition times.” Is a good answer for his question considering the speed of light changes depending on what material it is traveling through?

kelseymh (author)2013-01-30

Right. But that is not the question posted. The question was basically, "why does electricity move so fast if the electrons move so slowly?" That has been answered. And "component charge times" isn't relevant to answering the question of why the electrons move slowly, or how the slow movement of electrons through metal can translate into the very rapid speed of "electricity."

charmquark (author)2013-01-31

But the property of refractive index is regarding to light isn't it? And light is not affected by external electric and magnetic fields. And electric field alone constitutes to electricity, as i understand from our discussion so far. But it is different from EM waves as it is affected by external EM fields. Then, is electric field too is affected by refractive index and such, similar to light?

And kelseymh, that is exactly my first question as you guesed. "how the slow movement of electrons through metal can translate insto the very rapid speed of "electricity.""

charmquark (author)2013-01-31

To put it this way, refraction is an optical phenomena, as i understand. It affects things like light( which is not affected by external Electric or magnetic fields). Is it applicable to the electric field? I remember only things like permittivity(or permeabilty for magnetic fields) affecting these fields.

P.S: i know it is not exactly related to the question, but i want to continue this discussion anyway since it is rather interesting and informative.

P.P.S: I hope that that kels and josehf are having a healthy argument, and not take it personally. :)

kelseymh (author)2013-01-31

Refraction is a general effect for all kinds of waves, not just visible light. It occurs at the boundary between two media, when the speed of propagation is different in those two media.

You can build an apparatus to demonstrate the effect with water -- surface waves in water propagate at different speeds depending on the depth of the water below them. If you build a tank with a deep part, and a sharp step to a much shallower part, then waves (ripples) which you start at the deep end will be bent (and their wavelengths will change) as they pass over the wall.

Similarly, sound waves will be refracted when crossing a boundary between materials of different density (and hence different sound speeds). For example, seismic waves refract when they pass from the overlying crust into the mantle, at a depth called the Mohorovic discontinuity.

Josehf Murchison (author)2013-01-31

As kelseymy said “Light is a form of electromagnetic radiation. All forms of EM radiation are reflected and refracted in materials.”
Light is affected by gravity magnetic fields and ion fields as in an oscilloscope or a picture tube of a TV set.

kelseymh (author)2013-01-31

The oscilloscope and CRT are not using their electric fields (voltage plates) to affect light at all! Those voltage plates cause the deflection of the beam of electrons which hits the glass face of the tube, exciting the layer of phosphors on that glass, and generating a spot of light as a secondary interaction.

charmquark (author)2013-02-01

Exactly, i was about to tell the same thing and you did. :)
I read that too last semester and it was pretty interesting. Most people think it is the light being deflected, where as it actually the electron beam, which in turn excites the phosphor screen. Although, the end result is the same, but as far as i know, light is not affected by Electric or magnetic fields.

But on the other hand, gravity DOES affect light. Take an example of black hole. According to what i have understood, it does affect the path of light.

So, yes Josehf is correct at mentioning gravity, while kelsey is correct to mention the CRT mechanism.:)

kelseymh (author)2013-02-01

Light is affected by magnetic fields when propagating through a medium (see my link to the Faraday effect, posted previously). The roughly analogous effect with electric fields is the Kerr effect, but the latter is really a change in the material's index of refraction due to the electric field (see, for example, Pockels cells in optical systems).

Gravity affects everything, including electromagnetic radiation, by causing geodesics to have curved paths through spacetime. You don't need a black hole, either: general relativity had its first experimental confirmation in 1919 when stars were observed during a solar eclipse. Stars were measured as being in positions farther away from the Sun than they should have been, because their light was bent as it went past the limb of the sun (compare the ray diagram for a magnifying lens).

charmquark (author)2013-02-01

Whoa! Pardon me for ignoring that link, i was not exactly paying attention. Wow, So light(Or in general the whole EM spectrum?) is affected by external electric or magnetic field? Here i was thinking that by definition, since EM waves are charge-less, it wouldn't be affected. Although from what i understand so far, i doesn't DEFLECT it, but yes, Joeshfh i think mentioned about deflection, so our points still stand valid, but i was wrong in mentioning that it is not affected, so pardon me people. Now i would sure want to know more, and hopefully, understand! :)

Josehf Murchison (author)2013-02-01

Don’t forget gravity.

You can't blame gravity for falling in love.
Albert Einstein

Even if it is a force of attraction.
Me

Forty years ago I wrote a poem called “Dawn”, and ever since I have been trying to photograph the sun on the horizon exactly as I remember it on the day I wrote the poem.

Now there are a hundred things I could say about photographing the sun however the one relevant here is the sun gets distorted by refraction when it is on the horizon. It gets flattened by gravity and magnified by refraction of the air.

There are some neat things you can see when you observe near objects.

kelseymh (author)2013-02-01

It's a very difficult picture to take, first because you need a heavy filter to reduce the sun's intensity, and second because there are almost always some clouds which partially obscure the sunset.

The "flattening" you describe is purely a refractive effect. As the sun gets close to the horizon, the lower limb is refracted through the atmosphere and appears higher than it really is, giving the sun a "squashed" appears, seeming to float on or just above the horizon.

There is no gravitational lensing involved -- the earth is not massive enough for such an effect to be visible to the naked eye.

Josehf Murchison (author)2013-02-01

I know what you ment but only a fool looks at the sun with the naked eye.

kelseymh (author)2013-02-01

Oh, yes! Reminds me of the classic sign from the MIT laser lab, "Do not look into laser with remaining eye." ;-)

Trying to photograph a clear-air sunset can damage your digital camera even if you don't look at the sun yourself. And even if there's no damage, the light intensity will saturate most of the CCD, unless you've got the right filters :-(

Josehf Murchison (author)2013-02-02

Full spectrum polarized lenses and don’t go cheap or your colors will be off.
I like that sign I have only one working eye.

kelseymh (author)2013-02-01

Well, as I said, this stuff does get complicated! If you're considering the usual physics idealization, of light travelling through a perfect vacuum, then you're correct -- static electric and magnetic fields do not affect it.

When light is travelling through a material, however, it is interacting with the atoms and electrons of that material. Electric and magnetic fields can affect those interactions, and thereby affect the light. I would encourage you to read a bit about both the Faraday effect (rotation of polarization angle due to a magnetic field) and the Kerr effect (change of index of refraction due to an electric field).

There's definitely nothing to apologize for! You're asking good questions, and working hard to fill in the gaps of what you learned (and overlooked :-) in your classes.

charmquark (author)2013-02-02

oh. Sure i would like to take a look at them. But first, let me cover some more basics because right now, it looks pretty intimidating to me! :D

charmquark (author)2013-02-01

Oh wait, i seem to remember something from the EM theory class about his stuff. Yes, i seem to remember some kind of derivations for cases where there are two different media(Or mediums, which one is it?) having different RI's..... boundary conditions? Oh well, darn me for ignoring those classes -_-. Wish i had payed some attention, maybe i would have looked less stupid :D

kelseymh (author)2013-02-01

The proper formulation of Snell's law is cos(theta1)/cos(theta2) = n2/n1, where n2 and n1 are the refractive indices of the two media (that is the correct plural). For most cases we deal with, one of the two media is air, and 1.000293 is close enough to 1.000 that we ignore the difference.

You can actually see the effect clearly if you take a magnifying glass with you the next time you go snorkeling. You will notice that the glass does not do a very good job of magnifying underwater, because the refractive indices of glass and water are quite similar.

charmquark (author)2013-02-01

Oh yes, i did observe that a long time ago, and my professor told me the same answer when questioned. :)

Josehf Murchison (author)2013-02-01

It is not east to do but if you want to see something neat look though an air bubble while under water.

Josehf Murchison (author)2013-01-31

To quote Albert Einstein
If you have two men that agree all you have is two men that agree, however if you have two men that argue you have learning and learning leads to knowledge.

charmquark (author)2013-02-01

That is a good one! :)

kelseymh (author)2013-01-31

Hear, hear!

kelseymh (author)2013-01-31

Light is a form of electromagnetic radiation. All forms of EM radiation are reflected and refracted in materials, and therefore an index of refraction is well defined for all of them.

EM radiation can be affected by external fields. In particular, the Faraday effect describes the rotation of the plane of polarization in an external magnetic field.

What is true is that to first (linear) order, EM waves do not interact with other EM waves. That is not strictly true, as non-linear (higher order) effects, and quantum effects, do cause self-interactions.

Josehf Murchison (author)2013-01-30

“both on speed and what exactly is movement of electrons. :)”
As to the movement of free electrons and such there is no short answer.
A good example would be a toy you see on an office desk I can’t remember its name but it has the rocking balls. In it the balls in the center don’t move.

charmquark (author)2013-01-31

I guess you are referring to Newton's Cradle :)

Josehf Murchison (author)2013-01-31

Newton's Cradle thanks an explanation works better with the right name I kept thinking Gauss as in a gauss gun and I knew it was both right and wrong at the same time.

And now for a simple explanation.

What happens inside a conductive material is very much like a tiny newton’s cradle.

A free electron enters an atom’s outer electron shell where the molecular bond is at its weakest and it kicks out an electron making it a free electron just like the balls in a newton’s cradle. This movement is very small and needs to happen many times just to move an electron 1nm.

Now think of a wire as doing this on mass and you have electrons movement in a wire.

The speed of this effect in a length of wire is just like a very long newton’s cradle at the speed of light.

The speed of light in vacuum, known as C, is 299,792,458 meters per second.

The speed, at which light propagates through transparent materials such as glass or air, is less than C about 200,000,000 meters per second.

Without knowing the material the speed of light in a specific material is expressed as A=CK

Where A is acceleration, C is the speed of light in a vacuum and K is the constant at witch light travels through a specific material.

kelseymh (author)2013-01-31

Could you point us toward a reference for your last two paragraphs? There is no "acceleration" involved in the index of refraction. The expression I think you are looking for is n = c/v where n is the index of refraction of the material (at the wavelength you're considering), c == 299,792,458 m/s is the speed of light in vacuum, and v is the speed of light at the particular wavelength through the material you're considering.

Josehf Murchison (author)2013-01-31

And I thought you might pick on molecular bond, I just picked a term people might easier understand.

The answer to your question is 42 and yes I calculated it carefully numerous times even in binary it is 10101.

kelseymh (author)2013-01-31

Yup. You were describing the motion of electrons in terms of individual atoms; in that case it is always the outermost electrons which are involved (in making bonds, or in being freed by ionization or voltage, or whatever).

In metals, the situation is slightly more complicated. The individual atomic orbitals overlap so much that you really have to talk about "bands" which encompass the whole material. The "outermost" electrons are the ones in the so-called "valence band". They move around, but they're more or less bound to the metal crystal.

When you apply a voltage, some of those electrons can gain energy and move up to the "conduction band." Those electrons move through the metal much more freely, and are the ones which participate in carrying current. But it's not an individual electron which zips from one end of a wire to the other. Rather, just as with your and Kiteman's analogies, electrons join the big mass of electrons from one end, and some of the electrons at the other end get pushed off the wire. That collective pushing is much faster than the individual electrons bobbling about.

Hey, I just thought of yet another analogy! Remember the old children's song "there were ten in the bed..."?

charmquark (author)2013-02-01

Good explanation! Now it finally looks clear to me.

Josehf Murchison (author)2013-01-31

I love children’s songs.

Yep the same but different like a gauss gun and a newton’s cradle.

People ask for a simple explanation when in truth a simple explanation is like the five year old on his way to the store trying to decide how much time he spends investigating each nook and cranny in the neighborhood along the way.

kelseymh (author)2013-01-29

The flow of electric current is not the bulk motion of individual electrons through a wire, as you're taught in elementary school.

As you have guessed and described above, the bulk electron motion (which is called drift) is extremely slow, a few mm/s (not mm/hour!). So how can the electric current travel so fast? Because it isn't the electrons themselves carrying current, but rather electromagnetic waves which are induced by the accelerated (back and forth) motion of the electrons. Those waves travel at the speed of light, and travel outside the metal wire(!), carrying the energy of the current.

iceng (author)2013-01-29

Then DC is slower ?

charmquark (author)2013-01-30

It depends on what you mean. As i understand now, electricity, DC or AC, is the electric field carrying the energy and not the electrons. And DC or AC current will flow pretty much at the same speed. It's the electron's movement that is slow, i.e. it will slowly drift across the conductor. AC, by definition, having alternating polarities will move the electrons in opposite directions on each reversal, thus making a net displacement of zero. DC on the other hand, move electrons in one direction causing a net displacement but, not very quick though.

iceng (author)2013-01-30

I was referring to kelseymh's

"electromagnetic waves which are induced by the accelerated (back and forth) motion of the electrons. Those waves travel at the speed of light, and travel outside the metal wire(!), carrying the energy of the current."

There is NO back and forth accelerating motion of the electrons in DC.

And by that definition Direct Current only depends on bulk electron flow drift.
Maybe that is why Edison failed in promoting his DC electrical network
which required a substation every  ten miles.

kelseymh (author)2013-01-30

The electrons get accelerated by the voltage difference, and then encounter "resistance" in the form of scattering off the metal atoms. There's still acceleration going on, and an EM field is developed (the current induces a magnetic field). That field carries energy, and can propagate signals from one end of the wire to the other.

The question of "how fast does electricity travel" for DC is generally a question of how quickly do transients propagate. For example, when you flip a wall switch, how long does it take for the lamp to turn on (neglecting electronic effects or capacitors charging)? The answer is basically the speed of light, so in your house it's probably about 10 ns or so (assuming 3 meters of wiring from the switch through the wall to the outlet).

charmquark (author)2013-01-31

Yes, i think it's called "Bremsstrahlung"

http://en.wikipedia.org/wiki/Bremsstrahlung

kelseymh (author)2013-01-31

Bremsstrahlung ("braking radiation") is a consequence of high energy (millions of electron volts or more) electrons being deflected as they pass near atoms/nuclei in a material, or being deflected by a focussing magnetic field. The conduction electrons in a metal do not have sufficient energy to emit bremsstrahlung.

charmquark (author)2013-02-01

I see. Here it thought that any electron with sufficient acceleration would emit a photon on deceleration. Now that you mentioned that it requires more than that(about million eV), i guess i will have to understand it better this time. :)

kelseymh (author)2013-02-01

Any acceleration does cause the electron to emit, but the energy of what is emitted depends on the available kinetic energy of the electron (that's just simple energy conservation :-). For conduction electrons, the available energy is a tiny fraction of an meV, so the frequency of emitted photons would be down in the long-wave radio, and undetectable.