Understanding the Nature of Electricity: Flow of Electrons or Electric Charge?

[sophiecentaur]

Ok if you say so.

It's not just I who says so!

 

[Bassalisk]

It is likely that one of the changes that will be made to the SI within the next few years is that the Ampere will be defined by fixing the value of e; this means that the base unit Ampere will be realized by counting the number of charges that passes through a circuit in a second. Hence, the Ampere (and therefore arguably current) will be defined to be flow of charge.

Note that there are already current generators out there that generate a current by pumping electrons one-by-one. The main problem with the ones we have is that the current they can generate is to small (pA) but that is a problem that will hopefully be solved.

My whole point here is: IF the current is defined as the directional flow of charge(in a wire), then by calculations and given parameters you would find that propagation of current is very slow, which we all know isn't.

 

[Studiot]

As to the question of what is electricity I commend the introductory chapter of Professor Hammond's book where he addresses exactly this question on pages 12 - 13.
He provides several explanations one of which is intriguingly

Every student of electricity is likely to be asked by his friends to explain what electricity is....

..."I do not know"
This will also be true and often gives a pleasing sensation to the questioner.

 

[sophiecentaur]

Current is the Time derivative of Charge. NOT the speed of charge carriers. Do you not see the difference? This is so basic.

 

[Bassalisk]

Current is the Time derivative of Charge. NOT the speed of charge carriers. Do you not see the difference? This is so basic.

Ok i rest my case.

 

[sophiecentaur]

Which one?

 

[Bassalisk]

Everything.

 

[sophiecentaur]

Everything.

Meaning that the "propagation of current" (whatever that means) is very fast? But we know the charge carriers in a metal move verrry slowly but in a CRT they go very fast. Perhaps you mean the Energy is transferred very fast. Yes - quite near the speed of light.
You could, at least, try to use the right terms if you want to change the course of Physics.

 

[Bassalisk]

Well yes, when u put it that way.

 

[toneboy1]

This was a great thread.

 

[sophiecentaur]

Well yes, when u put it that way.

 

[DragonPetter]

I strongly disagree with definition that current is MOVEMENT of electric charge. This is wrong, in my opinion, on some very important levels. Current is, for me impulse of energy. It is in some cases movement of charge, but in general theory where circuit is analysed, its not.

If that is definition, that means that charges in the wires move at speeds near speed of light, which we all know don't. They move very slow at drift velocity. Further this would mean that the wire would get super hot in very short time.
Current is a wave, similar to electromagnetic wave that propagates through medium.

Current can be defined as actual charge carrier flow. I think the mix up is thinking of flow rate as speed (displacement rate) - they are not the same thing. Is it safe to say current flow is more similar to a flux concept than a displacement concept? If you look at a certain cross section, current is the number of charge passing through that surface at a given time. You can have a lot of charge move really slowly (and so displace very little distance), and it will still be a high current because the number of charges is so high.

 

[sophiecentaur]

Just consider a CRT circuit. The same number of electrons per second going all the way round. The ones in the wires going at snail's pace and the ones in the tube going at a speed not much lower than c. Flux not speed.

 

[Kholdstare]

Its flow of electric charge.

 

[sophiecentaur]

Its flow of electric charge.

Nothing wrong with that but the term flux gets further away from the implication of speed which we need to avoid, perhaps.

 

[toneboy1]

Nothing wrong with that but the term flux gets further away from the implication of speed which we need to avoid, perhaps.

h'mm, ok that confuses me a bit, we agreed that I = time derivative of charge = charge/time
(I think)

So as the electrons are drifting IN THE WIRE quite slowly SOMETHING is moving near the 'speed' c depending on material. But we also said that current = flow of electric charge ??

*EDIT*
could we model the flow of charge like a sound wave, electrons pushing on each other but not really moving position?

 

[sophiecentaur]

Better. Whatever the 'amount' of current, the signal / energy / wave gets there at the same speed.

 

[Kholdstare]

Nothing wrong with that but the term flux gets further away from the implication of speed which we need to avoid, perhaps.

Well, basically flow of charge is what defines current. However, one can draw a relationship between drift velocity and current. I = q x n x A x vd. But saying that the current is all about speed and nothing else is false. The (average) speed of electron multiplied by electron concentration per unit length (nA) gives the number of electron passing a particular point (actually area) of the conductor per unit time.

 

[toneboy1]

Well, basically flow of charge is what defines current. However, one can draw a relationship between drift velocity and current. I = q x n x A x vd. But saying that the current is all about speed and nothing else is false. The (average) speed of electron multiplied by electron concentration per unit length (nA) gives the number of electron passing a particular point (actually area) of the conductor per unit time.

That seems completely different to:

Current is the Time derivative of Charge. NOT the speed of charge carriers. Do you not see the difference? This is so basic.

I'm sure Bassalisk would have a fit...

 

[DragonPetter]

That seems completely different to:
I'm sure Bassalisk would have a fit...

They are not different, only different levels of describing the same concept. Kholdstare is explaining the physical description of how it comes about which shows that you can derive the flux (what the definition of current flow hinges upon) as being dependent on the charge drift velocity. Because something is dependent on a variable does not make that variable the definition of what that something is - there are other factors in the context, being q, n, and A, that allows the inclusion of drift velocity to derive current flow.

 

[toneboy1]

They are not different, only different levels of describing the same concept. Kholdstare is explaining the physical description of how it comes about.

In that case I'm intrigued and confused. So it is kind of like a 'compression wave' of charge/time on a grand level but, so on a small level where does Kholdstare's physical description incorporate this?
thanks

 

[DragonPetter]

In that case I'm intrigued and confused. So it is kind of like a 'compression wave' of charge/time on a grand level but, so on a small level where does Kholdstare's physical description incorporate this?
thanks

Simply work out the units of the equation kholdstare provided and see that it gives the units in the definition sophiecentaur gave. That is a good first step to seeing how they are related.

I don't know if I would use phrases like grand level to describe it, but current can be given by kholdstare's equation within the context that it is derived, which are materials with charge carriers.

 

[toneboy1]

Ok, I still don't see how the model of 'slow moving charge carriers' (electrons), moving in some way fits into what is apparently the same model.

As far as I can see from following your instructions:
I=∆Q/∆t=qnAvd = (units) q . N/m^3 . m^2 . m/s = qN/s

this seems to imply that IT IS the slow moving charge carriers responsible for the fast speed of current...?

 

[sophiecentaur]

Ok, I still don't see how the model of 'slow moving charge carriers' (electrons), moving in some way fits into what is apparently the same model.

As far as I can see from following your instructions:
I=∆Q/∆t=qnAvd = (units) q . N/m^3 . m^2 . m/s = qN/s

this seems to imply that IT IS the slow moving charge carriers responsible for the fast speed of current...?

This would seem to be your problem. You are assuming that the current 'moves fast'. Why? Consider the water in the middle of a lake, with two fast-moving streams - one in one out. How fast is it moving in the middle? How many litres per second are going through the lake, though? Is it speed or volume flow that counts?
What DOES move fast is the effect, in an electric circuit, of turning it on. A pulse of EM travels through / along it and will cause current to flow into the light bulb at the end in a matter of a few nanoseconds. That doesn't mean that the 'current flowed all the way' to the bulb from the battery in order to light it up.
When you stamp on a bicycle pedal, the bike moves forward 'instantly'; you don't wait for the chain to move round from the chain wheel to the sprocket before you move off. Also, by choosing an appropriate gear, you can deliver different amounts of power with the same pedalling speed. Chain speed / electron speed are not the relevant quantities here.
I think some people here are trying to 'bend' reality to fit their own personal models of 'Electricity'. It would be better to start with the very basics and move towards a fuller understanding. The accepted model really does work well and I think you need to believe it's the best way to look at things.

 

[toneboy1]

...I think some people here are trying to 'bend' reality to fit their own personal models of 'Electricity'. It would be better to start with the very basics and move towards a fuller understanding. The accepted model really does work well and I think you need to believe it's the best way to look at things.

Ok, making sense again. So some EM field propagates through the wire making them all drift, first in (from battery) last out (to circulate back around).

Thanks!

 

[sophiecentaur]

Furthermore - you don't actually need a connection. The same thing happens between a transmitting antenna and a distant receive antenna. No charges can flow between them at all. Charges flowing in one place can cause charges to flow in another place.

 

[toneboy1]

Furthermore -...No charges can flow between them at all. Charges flowing in one place can cause charges to flow in another place.

Great answer before btw.

So trying not to use my own terms but the model I gather from what you've told me, there is this EM wave propagating through the antenna (at say 10MHz) and the charge throughout the antenna is sort of just oscillating where it is, because it moves too slowly to really go anywhere. (?) We are really seeing VOLTAGES along the antenna as the discernible change. (?)

Given I=∆Q/∆t=qnAvd What sort of current can we expect to see in the inductor if there is a bandpass filter connected to the antenna?
Would it be like a regular 2μA AC?

THANKS!

 

[sophiecentaur]

What sort of current can we expect to see in the inductor if there is a bandpass filter connected to the antenna?
Would it be like a regular 2μA AC?

THANKS!

If the bandpass filter presents the right impedance (matched) to the feed point of the antenna then you would expect an AC signal just like was transmitted. Mostly, of course, the signal would have modulation (information) on it (the only reason for transmitting in the first place, usually!)

As I mentioned before, most antennae (except for some AM broadcast transmitting antennae) do not have resonant circuits in them because they are required to transmit and receive over a range of frequencies.

If you look at the volts and current at different parts of a simple dipole, they follow the pattern of a standing wave on the antenna - Zero current at the ends, for instance and maximum volts at the ends for dipoles around half a wavelength long. Google imnages of current distribution on a dipole.

 

[toneboy1]

If the bandpass filter presents the right impedance (matched) to the feed point of the antenna then you would expect an AC signal just like was transmitted. Mostly, of course, the signal would have modulation (information) on it (the only reason for transmitting in the first place, usually!)

As I mentioned before, most antennae (except for some AM broadcast transmitting antennae) do not have resonant circuits in them because they are required to transmit and receive over a range of frequencies.

If you look at the volts and current at different parts of a simple dipole, they follow the pattern of a standing wave on the antenna - Zero current at the ends, for instance and maximum volts at the ends for dipoles around half a wavelength long. Google imnages of current distribution on a dipole.

Right, so the actual modulation of the inductance / capacitance on the transmission circuit antenna is how they send information for say FM?

(Google imaging λ/2...was helpful)

Standing wave for reception but not for transmission? (all the energy being reflected back otherwise)

If unless you have any objections to that I think I may have 'got it' ;D

As always, Thanks!

 

[sophiecentaur]

Right, so the actual modulation of the inductance / capacitance on the transmission circuit antenna is how they send information for say FM?

Owch. That's a step too far, I'm afraid. You're inventing things in your head! The FM signal is produced and amplified and it's then fed to the antenna (which is probably up at the top of a mast, tens of metres away). The matching network and antenna have to be designed to be broad band enough to take several signals at once so you can't do the modulation at the antenna.