# What is the speed of electricity?

#### nitinshetty

What is the speed of electricity?
How fast does electricity travel?

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#### Lucretius

I would think it travels at the speed of light, seeing as the electromagnetic force is carried by the photon,which is massless, and travels at the speed of light therefore.

#### whozum

nitinshetty said:
What is the speed of electricity?
How fast does electricity travel?
If you define electricity as the flow of current, then electricity travels much slower than the speed of light. What you are looking for is drift velocity of electrons through a medium. The speed depends on the amount of current and certain characteristics of the medium it is propogating through.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html" [Broken] is a nifty little explanation.

However, the electric field in the wire is established at close to the speed of light (this is the signal sent from your light switch to your light bulb, from example) and electrons closest to the bulb start flowing through the bulb as soon as this field is established.

I don't really think electricity has a speed

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#### Pengwuino

Gold Member
whozum said:
I don't really think electricity has a speed
Well i think you nailed it when you mentioned drift velocity. I asked one of my professors this last semester because i also figured electricity travels at the speed of light and he was like "nooooooooooo, way slower then that". I thought electricity is electron travel... which has mass...

Sounds like electricity may be one of those badly-named concepts in relation to electro-magnetic propogation but id unno

#### whozum

Pengwuino said:
Sounds like electricity may be one of those badly-named concepts in relation to electro-magnetic propogation but id unno
Yeah, I dont think "electricity" has a speed. Its like saying whats the speed of gravity.

#### Kazza_765

Hi.

I thought that both electricity and gravity have speeds. I think what nitinshetty is asking is how quickly electricity propogates through a material, in the same way that mechanical forces propogate at the speed of sound in the material and gravity propogates at the speed of light i think (I'm not sure if propogate is word I am looking for here).

For example, If I had a copper wire running from my living room light switch to the sun and I flicked the light switch off would it take 8 minutes before the sun turned off, or much longer, ignoring the time it takes the light to travel from the sun back to me.

#### Pengwuino

Gold Member
It would take quite a long time. It would take however long the signal takes to get to the sun (based on drift velocity i bet) plus the time it takes for light to travel to earth (if we're timing it from your house).

Whozum, from what im picking up in bits in pieces from around here, there seems to be theories to indicate that gravity is like EM waves or something that actually has a speed.

#### cscott

Is it wrong saying gravity (if that's the right word to use) travels at the speed of light as per GR?

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#### Lucretius

cscott said:
Is it wrong saying gravity (if that's the right word to use) travels at the speed of light as per GR?
I hope not, because that's what I thought it was…

#### chroot

Staff Emeritus
Gold Member
The term 'electricity' is ambiguous.

Changes in the electromagnetic field propagate at the speed of light in vacuum. On a printed circuit board wire, which is surrounded by dielectric material, changes in the electromagnetic field propagate at about six inches per nanosecond, or about half the speed of light.

Electrons themselves have thermal velocities on the order of a million m/s, less than one percent of the speed of light. In addition to their random thermal motion, they actually drift from one terminal of a battery to the other at a very slow speed, on the order of centimeters per hour.

- Warren

#### whozum

Pengwuino said:
Whozum, from what im picking up in bits in pieces from around here, there seems to be theories to indicate that gravity is like EM waves or something that actually has a speed.
I'm aware, but my point was you made the assumption that I was talking about gravity waves, whereas I could've been a misinformed person and referring to something else, like falling due to gravity, etc.

#### Art

Pengwuino said:
Whozum, from what im picking up in bits in pieces from around here, there seems to be theories to indicate that gravity is like EM waves or something that actually has a speed.
Per Einsteins GR theory gravitational waves do travel at the speed of light. If a mass such as the sun disappeared the resulting gravity waves would spread out at the speed of light taking 8 mins to reach the earth..

#### Pengwuino

Gold Member
whozum said:
I'm aware, but my point was you made the assumption that I was talking about gravity waves, whereas I could've been a misinformed person and referring to something else, like falling due to gravity, etc.
Well, i thought gravity was gravity waves.... i dont know anything :D

#### whozum

Pengwuino said:
Well, i thought gravity was gravity waves.... i dont know anything :D
Yeah no one knows what I'm talking about (which kinda proves my point).

I'm gonna shut up now.

#### VinnyCee

Electricity moves at the speed of light...

In a superconducting material, electricity is able to move at full speed, the speed of light.

Question: DOES THE ELECTRON TRAVEL AT THE SPEED OF LIGHT?
------------------------------------------------
Answer: No. If you calculate the instantaneous speed of electron
using the theoretical models, it comes out to be the velocity
of light. However, we cannot measure instantaneous speeds, but only
speed averaged over some time scale by measuring position at two
instants of time. When we do this, the speed is always less than
the velocity of light. Theoretical models also predict average
velocity less than speed of light.
The above discussion only applies to speed of light
in vacuum. Electrons can , and do, travel at
speeds faster than
speed of light in some media.
Jasjeet

Source

#### ZapperZ

Staff Emeritus
2018 Award
VinnyCee said:
In a superconducting material, electricity is able to move at full speed, the speed of light.
Since when?

If you mean "electricity" and the supercurrent, it does NOT flow at the speed of light, thank you. The zero resisitivity is NOT due to it moving at that speed. Rather, it is due to long-range coherence of the condensed cooper-pairs. The gazillion cooper-pairs in this state are all "entangled" to each other, sharing the same state. Nowhere in the formulation - and certainly NOT in the BCS theory - is there anything moving at "full speed, the speed of light".

And your statement above has no correlation to what you cited from the ANL's Newton's site.

Question: DOES THE ELECTRON TRAVEL AT THE SPEED OF LIGHT?
------------------------------------------------
Answer: No. If you calculate the instantaneous speed of electron
using the theoretical models, it comes out to be the velocity
of light. However, we cannot measure instantaneous speeds, but only
speed averaged over some time scale by measuring position at two
instants of time. When we do this, the speed is always less than
the velocity of light. Theoretical models also predict average
velocity less than speed of light.
The above discussion only applies to speed of light
in vacuum. Electrons can , and do, travel at
speeds faster than
speed of light in some media.
Jasjeet

Source
Zz.

#### tony873004

Gold Member
Inside your computer, the manufacturer tries to keep the distances between various circuits (data buses?) as short as possible. This makes for a faster computer.

#### Chronos

Gold Member
This would make for an interesting experiment. You could wire some bulbs in series using a very long coil of thin wire and see if there is a measurable time interval between the first and last bulb lighting up.

#### kirovman

Both Gravity and Electromagnetic interaction have speed. Their speed is the speed of light - the speed of their virtual particles if you like (Graviton and photon respectively). However the speed of current flow is the drift velocity of the electron.

Pengwuino said:
Well, i thought gravity was gravity waves.... i dont know anything :D
I think gravity waves are real gravitons - where the gravitational force is mediated by virtual gravitons. Compare this with the electromagnetic force - it is mediated by virtual photons, but the real photons are the ones you actually see.

#### whozum

Chronos said:
This would make for an interesting experiment. You could wire some bulbs in series using a very long coil of thin wire and see if there is a measurable time interval between the first and last bulb lighting up.
Wouldn't the difference be the the time it takes for the E field to go from one to the other (speed of light / coil length).

#### pervect

Staff Emeritus
The first step approximate moel of the circuit would have it primarily be an inductor. If you put a large voltage V across it, dI/dt would be V/l.

However, there would also be a small capacitance between each turn. This small capacitance would cause a large step on the input to generate a small step on the output (depending on what else was connected to the output).

A really detailed model of the coil, assuming it was enclosed in a conductive tube, would probably be based on a "helical resonator". Most helical resonators are open at the end, this one would be a bit different.

Basically it's probably a mistake to think of the signal as travelling through the wire - you really need to think of fields, and solving Maxwell's equations.

#### leright

If by "electricity", you mean the flow of electrons through a conductive medium, then the electrons themselves flow at an extremely slow rate...something like 2 cm/min. However, the electromagnetic field around the wire propogates at nearly the speed of light.

#### Lyuokdea

The best explanation of this is provided by (I think) Feynman. He gives the example of a water hose connected to your house. The electrons move very slow, so you would think it works like a hose, where you turn on the water, and it takes a minute for the water to travel through the hose, and then come out the end.

But that's not correct. To continue the example, suppose water has been flowing out the hose for a while, and you go to the nozzle and turn turn off the water. Now go back and turn off the pump and reopen the nozzle (without letting the existing water out). Now, when you turn the hose back on at the pump, even though the water should take a couple seconds to travel through the hose and come out, the water comes out instantly. This is because although the water moves at a slow speed, the pressure effect of the water travels much faster (in this case, at the speed of sound in water, I believe).

The electron should be thought of as a full water hose, because the medium (the copper wire) is full of electrons. Although the electrons move quite slowly, the "pressure effect" of the electrons moving travels very quickly (at the speed of light in the copper)

Thus the electrons that come power your T.V. the second you turn it on were electrons that were probably already inside the TV very close to the motor when you turned the power on, they were not electrons all the way from the plug.

Hope that example helps clarify the situation.

~Lyuokdea

#### Danger

Gold Member
Chronos said:
This would make for an interesting experiment. You could wire some bulbs in series using a very long coil of thin wire and see if there is a measurable time interval between the first and last bulb lighting up.
Wouldn't they all light simultaneously because nothing will happen until the circuit is completed?

Lyuokdea said:
Thus the electrons that come power your T.V. the second you turn it on were electrons that were probably already inside the TV very close to the motor when you turned the power on, they were not electrons all the way from the plug.
How old are you, anyhow? I'm going to have to start calling you 'Grampa', since the last TV with a motor was built in 1935!

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