# COMMUNITY : FORUMS : SCIENCE

## Question regarding speed electricity

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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syang84 years ago
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.
Orngrimm4 years ago

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 Murchison4 years ago
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
4 years ago
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 :-)
4 years ago

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

4 years ago
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.
4 years ago
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.
4 years ago
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.
4 years ago
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?
4 years ago
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."
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