Think I worked it out once. Any know the density of free conduction electrons in copper? my copy of Kay and Laybey is downstairs and while theres a whole raft of books here, I am temporarily unsure which one might produce a quick answer
Explain the Neutral???
- Thread starter cross
- Start date
Sponsored Links
ok
Current=number of electronsN x charge on one electron e X velocity v x cross sectional area A.
so velocity= I/(NAe)
v= 10Amps/(8x10^28 x 10^-6 x 1.6x10^-19)
v=10^-3 m/s
travel in one direction is for 1/100 second. so distance travelled is 10^-5m.
0.01mm
Current=number of electronsN x charge on one electron e X velocity v x cross sectional area A.
so velocity= I/(NAe)
v= 10Amps/(8x10^28 x 10^-6 x 1.6x10^-19)
v=10^-3 m/s
travel in one direction is for 1/100 second. so distance travelled is 10^-5m.
0.01mm
[If bathjobby and uselessFCUK could stop reading, it would be appreciated, as they seem to think I mustn't disagree with what people write here.]
=============================================
I always thought that the "speed" of electricity was, if not actually the speed of light, at least of the same order of magnitude? (0.9C rings a bell)
C is approximately 300000km/s.
so 1/100th of a second is approximately 2700km, not .01mm.
One of us has gone dreadfully wrong somewhere. I don't think it's me, but I could be wrong...
If you're right, why isn't there a delay of nearly 6 days between exchanges on a telephone call over a 500km land-line?
=============================================
[/If bathjobby and uselessFCUK could stop reading, it would be appreciated, as they seem to think I mustn't disagree with what people write here.]
=============================================
I always thought that the "speed" of electricity was, if not actually the speed of light, at least of the same order of magnitude? (0.9C rings a bell)
C is approximately 300000km/s.
so 1/100th of a second is approximately 2700km, not .01mm.
One of us has gone dreadfully wrong somewhere. I don't think it's me, but I could be wrong...
If you're right, why isn't there a delay of nearly 6 days between exchanges on a telephone call over a 500km land-line?
=============================================
[/If bathjobby and uselessFCUK could stop reading, it would be appreciated, as they seem to think I mustn't disagree with what people write here.]
Sponsored Links
- Joined
- 3 Dec 2004
- Messages
- 287
- Reaction score
- 1
- Country
Gentlemen,
We are in danger of getting into Quantum Theory and the differing properties that electrons demonstrate depending on whether they are being waves or particles.
It's not that the electrons stream up and down the wire at the speed the electricity seems to get from one end to the other.
An interesting, and almost-but-not-quite relevant, analogy is a six inch nail. When you hit the head with a hammer, it is not the atoms you hit that go into the wood. The energy of impact is transferred, really quite fast, atom to atom from those you hit to those at the pointy bit.
Electricity works the same way.
We are in danger of getting into Quantum Theory and the differing properties that electrons demonstrate depending on whether they are being waves or particles.
It's not that the electrons stream up and down the wire at the speed the electricity seems to get from one end to the other.
An interesting, and almost-but-not-quite relevant, analogy is a six inch nail. When you hit the head with a hammer, it is not the atoms you hit that go into the wood. The energy of impact is transferred, really quite fast, atom to atom from those you hit to those at the pointy bit.
Electricity works the same way.
The electrical wavefront propagates at the speed of light in the medium concened, which is copper. Off hand I am not sure how fast that is, but something approaching c, yes. But as the last man said, the actual electrons do not move very fast. What is happening is that one electron moves a tiny bit, which increases the electric charge near the next one along, which pushes it along in turn, which increases the electric field acting on the next one, and so on.
The generator starts all this off by apllying a magnetic field to the electrons in its winding. The turning of the generator coils means that the field is always pushing against the electrons to get them started moving.
But the actual electrons you just used in your TV are exactly the same ones you used last night. Boy they must be sick of it.
The generator starts all this off by apllying a magnetic field to the electrons in its winding. The turning of the generator coils means that the field is always pushing against the electrons to get them started moving.
But the actual electrons you just used in your TV are exactly the same ones you used last night. Boy they must be sick of it.
- Joined
- 3 Dec 2004
- Messages
- 287
- Reaction score
- 1
- Country
Sorry, I did really mean to get round to the point about the neutral. So let me introduce another ridiculous and flawed analogy.
The electrons in the copper are like a long piece of string. Getting power from the power source to your electrical device is like using that long piece of string to turn something far away.
So with your long piece of string you can pull hard at A and make something happen at B.
Note that the piece of string you pull doesn't have to move all that fast, but that the thing happening at B happens as soon as you pull the string at A.
If you keep on pulling the string in the same direction you have DC. If you pull it backwards and forwards you have AC.
And of course you can see that to keep on transferring power from A to B you need a loop of string, not just one piece - so you got to have two bits joining A&B - hence live AND neutral.
The electrons in the copper are like a long piece of string. Getting power from the power source to your electrical device is like using that long piece of string to turn something far away.
So with your long piece of string you can pull hard at A and make something happen at B.
Note that the piece of string you pull doesn't have to move all that fast, but that the thing happening at B happens as soon as you pull the string at A.
If you keep on pulling the string in the same direction you have DC. If you pull it backwards and forwards you have AC.
And of course you can see that to keep on transferring power from A to B you need a loop of string, not just one piece - so you got to have two bits joining A&B - hence live AND neutral.
Sorry, I did mean to get round to the point about quantum theory. In general, classical electrodynamics or any classical force theories are the limiting case of quantum theories. Quantum theories tend to be used for single particles. Which can behave very strangely. We could consider the probability of an electron suddenly hopping from one 'state' to another all by itself. This happens, and is used in some semiconductors. But when we are considering the actions of many particles we are effectively talking about the average behaviour of all the particles together. On average they are very reliable in their behaviour and behave just like little marbles.
- Joined
- 3 Dec 2004
- Messages
- 287
- Reaction score
- 1
- Country
...or a piece of string.Damocles said:
- Joined
- 3 Dec 2004
- Messages
- 287
- Reaction score
- 1
- Country
Prefer the string theory myself.
The electrons in a wire are driven along by a wave OUTSIDE the wire. You cannot get an E/M wave to travel very far through a conductor. The speed of this wave is its speed in the medium surrounding the wire. For overhead power cables in free air it's as near to c as makes no difference. In a PVC insulated cable it depends on the relative permittivity of the PVC and this varies with frequency. It's about 2.5 at radio frequencies but I don't know about 50Hz.
In any medium, speed of E/M wave = c / sqrt(epsilon-r x mu-r). This raises an interesting possibility because diamagnetic materials have mu-r < 1. Find something for which epsilon-r x mu-r < 1 and you can break the light barrier. Maybe there's a proof somewhere that no such material can ever exist!
In any medium, speed of E/M wave = c / sqrt(epsilon-r x mu-r). This raises an interesting possibility because diamagnetic materials have mu-r < 1. Find something for which epsilon-r x mu-r < 1 and you can break the light barrier. Maybe there's a proof somewhere that no such material can ever exist!
Indeed, I have it that the skin depth of a copper wire carrying 50 hz is 1cm. So the electric field will not penetrate more than 1cm into my 10mm^2 cable. Oops, that means it will go right through to the other side. Mind, at 50Mhz that would be 0.01mm. Which is why coax is thin.
Dear me, I never was that good at electromagnetism. The incomprehension is coming back to me....
Dear me, I never was that good at electromagnetism. The incomprehension is coming back to me....
I
Igorian
Now we know why it take so long to qualify as a sparky 
- Joined
- 3 Dec 2004
- Messages
- 287
- Reaction score
- 1
- Country
Oh no they're not. The wave outside the wire is caused by the activity of the electrons inside the wire.felix said:
Discuss.
On the other hand, does the original poster feel that any of the replies to date actually give a helpful answer to what seemed like a very simple question?
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
