Does Electricity travel at the Speed of Light?

[Macavity]

How Fast does Electricity Travel? A "good question" for students?
YOU may already "know"! But a thought-provoking experiment... 😎
 

 

Edited January 30, 2022 by Macavity

[johninderby]

Electrons have mass , photons don’t. Hence light wins.

Edited January 30, 2022 by johninderby

[iantaylor2uk]

Electromagnetic waves travel at the speed of light (in a vacuum) but the electrons flowing through a wire will travel much more slowly

[saac]

Students are generally surprised to find that the rate of electron drift is quite low - our teaching model of current as a flow of  electrons (or holes for that matter) is so incomplete on many levels but still useful. I do like the Veratsium video on energy  transmission taking place outside of the conductor. 

Jim 

 

 

[City9Town0]

I = nAve     !!!!!!!!

[Alien 13]

13 minutes ago, saac said:

Students are generally surprised to find that the rate of electron drift is quite low - our teaching model of current as a flow of  electrons (or holes for that matter) is so incomplete on many levels but still useful. I do like the Veratsium video on energy  transmission taking place outside of the conductor. 

Jim 

 

 

That Veratsium video is a good watch, it sort of made sense why good audio cables are not just about the conductor type but also the insulator and overall construction.

Alan

[Macavity]

5 minutes ago, Alien 13 said:

That Veratsium video is a good watch...

I hadn't come across that (name) - I shall watch it. "Good stuff" whatever!

[Alien 13]

5 minutes ago, Macavity said:

I hadn't come across that (name) - I shall watch it. "Good stuff" whatever!

Here it is but lots of others in the series..

Alan

[saac]

3 minutes ago, Macavity said:

I hadn't come across that (name) - I shall watch it. "Good stuff" whatever!

They are generally well produced and entertaining Chris and cover a broad range of physics principles .  His  "energy in wires" video was good as it created a lot of challenge videos offering different explanations.  I guess in the end it's all heavily influenced by whatever model  is being used (same old classical or quantum physics routine) .  Anyway as a mechanical engineer I'm holding to my belief that electricity obeys Newton's laws of motion - the rest is magic  

Jim

[johninderby]

.

 

Edited January 30, 2022 by johninderby

[saac]

19 minutes ago, Alien 13 said:

That Veratsium video is a good watch, it sort of made sense why good audio cables are not just about the conductor type but also the insulator and overall construction.

Alan

I must admit Allan that prior to watching it I would never have really thought about it like that. Then again I suppose when we talk about "cross talk" among data cables (ethernet etc) then that is the same principle at work.  I guess also something similar at play when you hold say a TV aerial cable you can affect the signal - maybe !

I wonder if in your previous life in Avionics you must have come across the wonderfully named Time Domain Reflectometer (I think that was its name).  The avionics technicians at Kinloss were trying to educate me on how they could use it to chase down breaks in the Nimrod's wiring loom.  As they explained it, it all sounded oh so plausible and fine but walking away it just confirmed that the world of electronics, avionics in particular, was truly a black art to me !

Jim 

[iantaylor2uk]

14 minutes ago, saac said:

They are generally well produced and entertaining Chris and cover a broad range of physics principles .  His  "energy in wires" video was good as it created a lot of challenge videos offering different explanations.  I guess in the end it's all heavily influenced by whatever model  is being used (same old classical or quantum physics routine) .  Anyway as a mechanical engineer I'm holding to my belief that electricity obeys Newton's laws of motion - the rest is magic  

Jim

You have to be careful applying Newton's laws of motion to photons - if you say the force is proportional to the change of momentum you are OK (as photons have momentum) but you can't say force is mass times acceleration, since the mass of a photon is zero, and photons can only accelerate by changing direction (as their speed is constant). All this assumes the photon is in a vacuum. You can also use conservation of energy laws too, but clearly the standard equation for kinetic energy (as being equal to half*mass*speed squared) doesn't hold.  

[900SL]

53 minutes ago, saac said:

Students are generally surprised to find that the rate of electron drift is quite low - our teaching model of current as a flow of  electrons (or holes for that matter) is so incomplete on many levels but still useful. I do like the Veratsium video on energy  transmission taking place outside of the conductor. 

Jim 

 

 

Most of the 'models' taught in school and college are incorrect. I now just fall back on 'it's all fields, man, fields, everywhere..'

[Alien 13]

18 minutes ago, saac said:

I must admit Allan that prior to watching it I would never have really thought about it like that. Then again I suppose when we talk about "cross talk" among data cables (ethernet etc) then that is the same principle at work.  I guess also something similar at play when you hold say a TV aerial cable you can affect the signal - maybe !

I wonder if in your previous life in Avionics you must have come across the wonderfully named Time Domain Reflectometer (I think that was its name).  The avionics technicians at Kinloss were trying to educate me on how they could use it to chase down breaks in the Nimrod's wiring loom.  As they explained it, it all sounded oh so plausible and fine but walking away it just confirmed that the world of electronics, avionics in particular, was truly a black art to me !

Jim 

Cable design can be a black art and "cross talk" can exist along a single cable, reflections from an un terminated or incorrectly terminated connection are common too.... It gets spookier when dealing with microwave components 😀

Alan

[saac]

22 minutes ago, iantaylor2uk said:

You have to be careful applying Newton's laws of motion to photons - if you say the force is proportional to the change of momentum you are OK (as photons have momentum) but you can't say force is mass times acceleration, since the mass of a photon is zero, and photons can only accelerate by changing direction (as their speed is constant). All this assumes the photon is in a vacuum. You can also use conservation of energy laws too, but clearly the standard equation for kinetic energy (as being equal to half*mass*speed squared) doesn't hold.  

Ian I was joking buddy    But hey I haven't met an electron yet that does not tremble when students try to apply F = ma to their voltage divider circuits  

Jim 

[saac]

28 minutes ago, 900SL said:

Most of the 'models' taught in school and college are incorrect. I now just fall back on 'it's all fields, man, fields, everywhere..'

No I wouldn't say that they are incorrect , they are after all models. They are appropriate for the level of prior knowledge and level of understanding sought.  It is often said that "science is a series of ever reducing lies." 

Jim   

[Carbon Brush]

An interesting watch. Staying off transmission lines, resonance, crosstalk, etc. that complicate the issue 🥴🤨 (he touched on these) and keeping it basic...

The energy transmission down a wire is a bit like a newtons cradle. Where each ball represents an electron in a metal atom.

https://commons.wikimedia.org/wiki/File:Newtons_cradle.gif

The energy in a ball at one end is quickly transferred to the ball at the other end, but there is little movement in the constrained balls.
For copper, the speed of movement of energy from one end of a wire to the other is about 2/3 light speed.
Yes it varies with assorted factors, but 2/3 is keeping it simple.

Electrons do drift from one atom to the next if you leave the switch closed for a long time. But very very very slowly when compared to the 2/3 light speed of energy transmission in a copper wire.
An idea of electron drift speed I remember being told, that is easy to understand......
If you have a torch with filament lamp and battery. It is unlikely an electron will travel from one side of the bulb to the other in the life of battery.
Obviously a calculated figure as you can't paint or otherwise label an electron😁

If you want complexity, you can push electrons into a wire, then pull them back. When you do this very quickly and the wire is a certain size/shape related to the rate of push/pull, then carefully measure energy, you discover some energy is lost.
Well we all know energy is not 'lost' it always goes somewhere. In this case the wire has become a radio transmitting aerial.
But I said keeping it basic in the first paragraph 🤐.

 

[Macavity]

20 minutes ago, Alien 13 said:

Cable design can be a black art and "cross talk" can exist along a single cable, reflections from an un terminated or incorrectly terminated connection are common too.... It gets spookier when dealing with microwave components 😀

Electromagnetism is quite a "challenging" [Uni/college etc.] course... Questions to avoid? 😁
A bit off topic: I have e.g. yet to visit: https://en.wikipedia.org/wiki/Near_and_far_field 
But such things DO crop up in the REAL world - Perhaps more often these days?

[cajen2]

Little question: if photons don't have mass (which of course they don't, or they couldn't travel at the speed of light), (1) why are they 'lensed' by gravity and (2) what causes light pressure, as in solar sails?

I've always wanted to know, but nobody's been able to tell me.

[saac]

6 minutes ago, Macavity said:

Electromagnetism is quite a "challenging" [Uni/college etc.] course... Questions to avoid? 😁
A bit off topic: I have e.g. yet to visit: https://en.wikipedia.org/wiki/Near_and_far_field 
But such things DO crop up in the REAL world - Perhaps more often these days?

I'm immediately thinking about things like RIFID tags and maybe even bluetooth  - I may well be wrong though  - well outside my comfort zone now.  What always makes me smile though is to reflect on the body of knowledge we now have with such easy access. Do you remember what it was like in the bad old days  

Jim

[iantaylor2uk]

Photons don't have mass but they do have energy and momentum. A photon can lose energy by "falling" in a gravitational field, and will become red shifted. 

[saac]

12 minutes ago, cajen2 said:

Little question: if photons don't have mass (which of course they don't, or they couldn't travel at the speed of light), (1) why are they 'lensed' by gravity and (2) what causes light pressure, as in solar sails?

I've always wanted to know, but nobody's been able to tell me.

The gravitational lensing of photons arises because mass (gravity) deforms spacetime through which the photons travel.  So the photons are feeling the secondhand effects of gravity in a way.

 

The pressure on the solar sail comes from a change in momentum experienced by the photon - photons do have mass (more correctly momentum) just not a rest mass (the rest mass of a photon is = 0) .  When the photon strikes the solar sail it experiences a change in momentum having imparted some of its momentum to the sail.

 

Jim 

Edited January 30, 2022 by saac

[Macavity]

19 minutes ago, saac said:

I'm immediately thinking about things like RIFID tags and maybe even bluetooth  - I may well be wrong though  - well outside my comfort zone now.  What always makes me smile though is to reflect on the body of knowledge we now have with such easy access. Do you remember what it was like in the bad old days  

Jim

I suppose the difficulty is (as often with Physics) to decide which regime applies?
Naively, when folk string wires across fields, I might think of "transmission lines"...
Or even (perhaps here) a Folded Dipole. But, radio waves were ONCE measured in
TENS of metres. Is this the "Zero Hz" limit? Transients & Oscillations? <waffles> 🥳

Edited January 30, 2022 by Macavity

[cajen2]

44 minutes ago, saac said:

The gravitational lensing of photons arises because mass (gravity) deforms spacetime through which the photons travel.  So the photons are feeling the secondhand effects of gravity in a way.

 

The pressure on the solar sail comes from a change in momentum experienced by the photon - photons do have mass (more correctly momentum) just not a rest mass (the rest mass of a photon is = 0) .  When the photon strikes the solar sail it experiences a change in momentum having imparted some of its momentum to the sail.

 

Jim 

I think the problem I have with this is how a particle can have momentum without  mass....but physics was never my strong suit. 😉

If you want to know the difference between a gerund and a present participle, I'm your man! 😄

Edited January 30, 2022 by cajen2

[johninderby]

20 minutes ago, cajen2 said:

I think the problem I have with this is how a particle can have momentum without  mass....but physics was never my strong suit. 😉

Just skimmed through it and more info than I want really. 😁

https://courses.lumenlearning.com/physics/chapter/29-4-photon-momentum/

Edited January 30, 2022 by johninderby