What is electricity exactly?

[toneboy1]

I've always been given conflicting messages like that we only use conventional current because they were confused years and years ago. Anyway I'm wondering if 'electricity' is the flow of electrons or "electric charge" like I heard once, I assume they meant like a flow of empty valency shell holes.

Any thoughts as to which it is?


Thanks again!

[jambaugh]

It is the flow of electric charge. However this is typically the flow of electrons. The flow of empty valence shell holes in one direction corresponds to the flow of electrons (each filling the prior hole) in the opposite direction. It is just two ways of describing the same thing.

But if you have say, a radioactive source emitting alpha particles (helium nuclei with charge +2e) then this flow of particles is also an electric current.

[toneboy1]

Right, I see, so if you were to apply that to a battery in slow motion and super-vision and just to focus on the empty holes, would they move out of the positive terminal through the wire, through the circuit back into the negative terminal?

Cheers.

[Averagesupernova]

I don't think you have what people refer to as hole flow in conductors. Just semiconductors.

[toneboy1]

I don't think you have what people refer to as hole flow in conductors. Just semiconductors.

There must be, otherwise the shell of the conducting atoms would fill up.

[Bassalisk]

Electricity is not flow of electric charge. What we think of electricity is a impulse of energy. Yes electrons do move, but very very very slow. Like a half a centimeter a second. But they bump into each other and create this impulse, like a wave which travels with speed of roughly half speed of light. (depending of which material are we talking about)

[RJK]

I've often wondered just how far the electrons (or electric charge) travel per unit of time. Not as fast as light, right. But if the power generating station is say 100 miles away, would one power cycle (60Hz) or 17mSec travel to my homes and back to the generator?

[jambaugh]

I've often wondered just how far the electrons (or electric charge) travel per unit of time. Not as fast as light, right. But if the power generating station is say 100 miles away, would one power cycle (60Hz) or 17mSec travel to my homes and back to the generator?

As I recall, the actual electron drift speeds are suprisingly slow...(googling)... Here's a figure from http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html

Drift speed of 4.3mm per second.

[RJK]

4.3mm/sec? But when I flip on a light switch 100 yards away it comes on immediatly. Something isn't right with that.

[jambaugh]

4.3mm/sec? But when I flip on a light switch 100 yards away it comes on immediatly. Something isn't right with that.

What travels at near speed c, is the voltage which is the electromagnetic interaction between the electrons. Voltage = electromagnetic interaction which propagates at speed c. Note the electrons do vibrate around thermally at substantially faster than 4.3mm/sec.

Here's a partial analogue. If you turn on or off a faucet, the drop/jump in pressure along the pipe propagates at the speed of sound in water even though the actual flow rate of the water is much less than that speed... and a good thing to or you'd be cut to ribbons taking a shower :smile:

[Bassalisk]

Like jam said, they do move, but in that process of moving they bump into each other, because they are tightly packed. That bumping is also form of movement. And that movement propagates at the speed, roughly half the speed of light. (Why half? Material reasons)

If you really want to understand this movement, you will have to uptake solid state chemistry etc, thermal movement, drift speed, thermal speed etc.

http://phet.colorado.edu/en/simulation/signal-circuit

In this circuit you will see this bumping, if you watch carefully.

I repeat, current is NOT movement of electric charges, this is common misunderstanding. Current is a impulse of energy. And even that last one cannot be taken for granted.

Life sworn physicists deny even the concept of current. They say that only electric potential exists. Everything else is derived.

[I_am_learning]

I would like to put up my views. Electricity is invisible liquid that flows in wire/ Electricity is flow of electrons/Electricity is flow of charge/Actually elctrons flow // etc etc are all theories.
First decide which phenomenum you want to study, the choose thories. Theories are just means of explaining out things. There is not any theory that provides the 'actual fact' for it don't exist. However, there is a general trends that newer thoery are supersets of older thoery's so explain more things. So we are led to believe that the most recent theory explains the actual thing.
For example, its not that bad to assume that the sun revolves round the earth and design a solar panel that faces the sun. You don't need to know that its actually the earth that revolves round the sun. However, if you want to make a solar system voyager, then you may even need to consider that the earth don't rvolve round the sun in nice circle.

So, in nutsell, my view is that: Don't run after theories, for there is no ultimate theory. Choose just the sufficient theory that explains the phenomenum of your corncern.

[Bassalisk]

I would like to put up my views. Electricity is invisible liquid that flows in wire/ Electricity is flow of electrons/Electricity is flow of charge/Actually elctrons flow // etc etc are all theories.
First decide which phenomenum you want to study, the choose thories. Theories are just means of explaining out things. There is not any theory that provides the 'actual fact' for it don't exist. However, there is a general trends that newer thoery are supersets of older thoery's so explain more things. So we are led to believe that the most recent theory explains the actual thing.
For example, its not that bad to assume that the sun revolves round the earth and design a solar panel that faces the sun. You don't need to know that its actually the earth that revolves round the sun. However, if you want to make a solar system voyager, then you may even need to consider that the earth don't rvolve round the sun in nice circle.

So, in nutsell, my view is that: Don't run after theories, for there is no ultimate theory. Choose just the sufficient theory that explains the phenomenum of your corncern.


I believe I know what are you talking about. But that only goes to the level when you are solving problems.
And I think you are wrong when you say that theories don't explain things. Explaining really goes down how much vocabulary you have to describe a certain event.

Classical physics was fine and dandy until you went to molecular level, and still quantum physics is made in such way that it applies to macroscopic ways too. How do you think people felt when Schrodinger derived almost all quantum laws from his equation?

Leibniz and Newton came to the same formula. One from physics one from mathematics. All things are connected. So theories do explain a lot of things. But some theories are not meant to explain, but rather to help in some problems.

Take an example of Maxwell's contour currents. He developed an system of equations to easily solve some complex circuits. But in a real theory, there is no such thing as contour current.

I believe that this fellow is asking what is happening when u put a light bulb connected with wires to the battery terminal. Why does it light up? Of course there is a explanation that is reasonable and correct.

Quantum physics, solid state can verify this easily. But right now we are trying to give some RIGHT directions, so a guy can go on his own and explore the concept.

Of course you cannot explain current and electricity in one forum thread. People spend their whole lives just studying that.



And like I said in one forum thread before, conventional way of current is used because Franklin put it that way. Why it didn't change? Because it would took too much effort and you would get not so much of a result. They didn't know back then what is going down on the microscopic level. He chose from + to - for the same reasons people chose left to be left and right to be right. There is no special reason for it, just you have to set some things. Later when other electronic elements where made, there was no conflict with it so, things stood like they are today.

[sophiecentaur]

I would like to put up my views. Electricity is invisible liquid that flows in wire/ Electricity is flow of electrons/Electricity is flow of charge/Actually elctrons flow // etc etc are all theories.
First decide which phenomenum you want to study, the choose thories. Theories are just means of explaining out things. There is not any theory that provides the 'actual fact' for it don't exist. However, there is a general trends that newer thoery are supersets of older thoery's so explain more things. So we are led to believe that the most recent theory explains the actual thing.
For example, its not that bad to assume that the sun revolves round the earth and design a solar panel that faces the sun. You don't need to know that its actually the earth that revolves round the sun. However, if you want to make a solar system voyager, then you may even need to consider that the earth don't rvolve round the sun in nice circle.

So, in nutsell, my view is that: Don't run after theories, for there is no ultimate theory. Choose just the sufficient theory that explains the phenomenum of your corncern.

There are good theories and there are poor theories. A good theory doesn't only explain your chosen phenomenon. it can accurately predict what will happen in brand new situations. Unless you can come up with a theory that explains significantly more than the conventional theories for 'electricity' then stick with them. They work well enough to make this computer talk to your computer, wherever you happen to be.

Only an eejit would call any theory an "ultimate" theory. But,also, no one but an eejit would say that all 'old' theories are wrong. They just have a limited scope.

[sophiecentaur]

btw, does this "invisible liquid" also flow across the plates of a Capacitor, through air or an insulator?

[I_am_learning]

I am sorry if I tried to mess up the learning process of the OP. But, I simply wanted to warn him about what thoeries really are. You guys of course have told much better than me on that subject.

[jambaugh]

...I repeat, current is NOT movement of electric charges, this is common misunderstanding. Current is a impulse of energy. And even that last one cannot be taken for granted.

Life sworn physicists deny even the concept of current. They say that only electric potential exists. Everything else is derived.

I take some exception to these statements. Current IS movement of electric charge BY DEFINITION. Signals on the other hand...

On the other, I don't know to which physicists you've been listening. Electric potential is a convention we can implement since electrical forces are conservative. Electromagnetic Force (the E and B fields) are what exist as they are empirically observable (by virtue of their effect on a test charge). Likewise charge is observable and thence too changes in charge and hence flow of charge a. k. a. currents. The physicist qua physicist believes in the empirical and understands theoretical constructs as speculation.

[Bassalisk]

I take some exception to these statements. Current IS movement of electric charge BY DEFINITION. Signals on the other hand...

On the other, I don't know to which physicists you've been listening. Electric potential is a convention we can implement since electrical forces are conservative. Electromagnetic Force (the E and B fields) are what exist as they are empirically observable (by virtue of their effect on a test charge). Likewise charge is observable and thence too changes in charge and hence flow of charge a. k. a. currents. The physicist qua physicist believes in the empirical and understands theoretical constructs as speculation.

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.

[Studiot]

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.

Perhaps if you had grown up in the age of vacuum tubes you would have had a different view?

[Bassalisk]

Perhaps if you had grown up in the age of vacuum tubes you would have had a different view?

What's your point mate?

[Studiot]

The anode current is a stream of electrons moving through space.
Exactly what you have rejected as a a current.

[Bassalisk]

The anode current is a stream of electrons moving through space.
Exactly what you have rejected as a a current.

Maybe definition and formulas say that current is charge/time, but here at my university, if you say that current is directional flow of charge you would be corrected. And I think i know what are you talking about.

It is in some cases movement of charge, but in general theory where circuit is analysed, its not.

Read my statements carefully.

Then we have to distinct what are we talking about, current in wire.

[Studiot]

Are you (and your university) denying that is how a vacuum tube (including old fashioned computer monitors so it was not so very long ago) works?

Why should this thread be limited to current in wire?

the title is 'What is electricity, exactly?'

[Bassalisk]

Are you (and your university) denying that is how a vacuum tube (including old fashioned computer monitors so it was not so very long ago) works?

Why should this thread be limited to curent in wire?

the title is 'What is electricity, exactly?'

Then well, I wandered off the subject, because i mainly study circuits. I deal with resistors and electronic elements and not vacuum tube. Electricity, for my needs, is what I said. Maybe in general physics is defined like you said.

I've always been given conflicting messages like that we only use conventional current because they were confused years and years ago.

He mentioned conventional current, so I figured he was talking about circuits.

[Studiot]

Don't forget, this is an electric universe.

The theory of man made circuits is one very small corner of electricity which runs everything from our minds and bodies to chemical reactions to planetary magnetism to....

[sophiecentaur]

. . . . 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.
. . . . . .


You need to be very careful when you make statements like that. The units of Current and energy are quite different so they just cannot be the same thing. If you are trying to tell people 'the way things are' then you must obey the basic rules! Current is the time differential of Charge - by definition. That charge may not be yer actual particles but it's still charge.

[Bassalisk]

You need to be very careful when you make statements like that. The units of Current and energy are quite different so they just cannot be the same thing. If you are trying to tell people 'the way things are' then you must obey the basic rules! Current is the time differential of Charge - by definition. That charge may not be yer actual particles but it's still charge.

If we are still talking about circuits, then I disagree. I will continue this discussion tomorrow.

[sophiecentaur]

I really don't think that you are in any position to argue with dimensional analysis. You would need to invent a whole new Science if you wanted to do that. The point is that Current and Energy exist as concepts inside and outside of circuit theory and you just have to be consistent about that.

[Bassalisk]

I really don't think that you are in any position to argue with dimensional analysis. You would need to invent a whole new Science if you wanted to do that. The point is that Current and Energy exist as concepts inside and outside of circuit theory and you just have to be consistent about that.

Ok if you say so.

[f95toli]

Maybe definition and formulas say that current is charge/time, but here at my university, if you say that current is directional flow of charge you would be corrected. And I think i know what are you talking about.


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.

[sophiecentaur]

Ok if you say so.

It's not just I who says so!:smile:

[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.
:biggrin:

[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 in to 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 in to 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]

[COLOR="Blue"]
...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.

[toneboy1]

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.

A couple dosen steps too far apparently!
Well how about Standing wave for reception but not for transmission? (all the energy being reflected back otherwise) Was I right there?

[sophiecentaur]

So you buy this sexy expensive new receiver and then you have to climb up onto your chimney to connect it up to the aerial? This is killing me!!!
The antenna installation needs to be entirely separate from the the transmitter (except in the case of an MF receiver with a ferrite rod, where the coil / rod antenna is an integral part of the first stage of the receiver and is tuned as you turn the tuning knob - so your not as daft as you may look :wink:). For HF, VHF or UHF, however you may want to connect several receivers to an elevated antenna and they may each want to receive entirely separate channels.
Is this helping? I'm beginning to enjoy it.

[toneboy1]

So you buy this sexy expensive new receiver and then you have to climb up onto your chimney to connect it up to the aerial? This is killing me!!!
The antenna installation needs to be entirely separate from the the transmitter (except in the case of an MF receiver with a ferrite rod, where the coil / rod antenna is an integral part of the first stage of the receiver and is tuned as you turn the tuning knob - so your not as daft as you may look :wink:). For HF, VHF or UHF, however you may want to connect several receivers to an elevated antenna and they may each want to receive entirely separate channels.
Is this helping? I'm beginning to enjoy it.

I appreciate your help and I am amazed at your patience but I'm more confused now than when I started :P
I'll understand if you want to give up. God knows it's 4:15am here and I don't think I'm ever going to understand how you can get multiple signals off a piece of non resonating metal.

[sophiecentaur]

Resonance only means you get a bit more power out. Any passing em wave will induce a current into anything that conducts. Proper engineering just maximises the amount.
Two RF signals will produce two outputs from the device. (It's all linear.)
Don't give up.

[toneboy1]

Resonance only means you get a bit more power out. Any passing em wave will induce a current into anything that conducts. Proper engineering just maximises the amount.
Two RF signals will produce two outputs from the device. (It's all linear.)
Don't give up.

Ok, well I suppose could we PLEASE go over:

-If most don't resonate how is there a standing wave?
-Aren't standing waves bad? (because all the energy is being reflected back, or is that not true for receivers?)
-So if you've just got a conductor in the air, there is all manner of current in it (crazy mess) from all the different EM waves passing through it?

Edit- This picture is the type of thing I'm thinking of, what's going on inside the antenna (receiver) as far as voltages and currents?
How does the inductance and capacitance play a role?
Thanks

[sophiecentaur]

You don't need resonance for a standing wave. If a wave is reflected at a barrier, the reflected wave will interfere with the incident wave and the resultant will be a standing wave as the relative phases change over the distance from the reflection. You are right to pick this up because most times you are told about standing waves, you are dealing with strings at resonance - when there are reflections at both ends and, when the spacing is right, the energy builds up in the interference pattern and you get resonance (But for friction losses, the amplitude would go infinite). If you take a short string, fixed at the other end, and waggle it from side to side. you will get a standing wave, even for a very slow oscillation but it isn't exactly obvious - except for the fact that the displacement of the string goes from max at your hand and zero at the other end. The shape of the string will be triangular for very frequencies but it will become more and more sinusoidal as you go faster, until you reach the resonant frequency, then the string shape will be a sinusoid.
This ties in with the images you have seen. Think of the wave along a short dipole as an interference pattern - min current at the end because the 'I' can't go anywhere!. If the dipole happens to be half wavelength long then the current will be in just the right phase to slosh up and down in time with the input signal and you will get resonance. Because the current is high, then this will couple best into a radiated wave. The antenna is 'matched' into free space and appears to be just a resistor of about 70Ω at the feed point aamof. The amplitude of the resonance is, of course, limited by the loss of energy into space.

You will, by now, be suffering from indigestion, I'm sure. There's an AWFUL LOT of this to take in.

[sophiecentaur]

OH yes - "reflection is bad" when it occurs in a transmission line because some of the power you want to be transmitted will be reflected and cause high voltages on the line - compromising the electronics and also introducing echos and non flat frequency response. Reflection IN an antenna is part of the way it works; you could say that the currents flowing due to the resonance will mean more power is radiated. Not all antennas are resonant, however - you can launch a wave from a very long wire and, by the end of it, most of the energy has been radiated.

[toneboy1]

OH yes - "reflection is bad" when it occurs in a transmission line because some of the power you want to be transmitted will be reflected and cause high voltages on the line - compromising the electronics and also introducing echos and non flat frequency response. Reflection IN an antenna is part of the way it works; you could say that the currents flowing due to the resonance will mean more power is radiated. Not all antennas are resonant, however - you can launch a wave from a very long wire and, by the end of it, most of the energy has been radiated.


OH! The transmission line! So is that meant to have a constant voltage along it?
(maybe if we were looking at an instant in time)

If you've just got a conductor (an unused antenna) sitting in the air, with multiple EM waves going through it, are there multiple standing waves due to these EM passing?

[sophiecentaur]

OH! The transmission line! So is that meant to have a constant voltage along it?
(maybe if we were looking at an instant in time)

If you've just got a conductor (an unused antenna) sitting in the air, with multiple EM waves going through it, are there multiple standing waves due to these EM passing?

The Peak to Peak voltage variations should be the same at all points along the line.

In pretty well every situation we are dealing with a linear system. The currents / waves along the antenna are a superposition of the effects of all the different passing EM waves. Multiple standing waves, if you like.

[toneboy1]

In pretty well every situation we are dealing with a linear system. The currents / waves along the antenna are a[COLOR="Magenta"] superposition of the effects of all the different passing EM waves. Multiple standing waves, if you like.

THANK YOU I've been looking for that answer for ages!
So they can sort of filter out different frequencies of this one, superposition-ed wave, with different circuits to use the same antenna for multiple signals at once?

[sophiecentaur]

Of course. The same way that an antenna can grab a range of signals out of the air from a whole range of EM waves, then a selective receiver can filter out the narrow band of frequencies it wants to look at from a single antenna downlead. Just think of the many instances where a single antenna feeds a large number of receivers (blocks of flats with a distribution amp on the roof) and where a number of transmitters can feed into a single transmitting antenna (UHF TV transmitting stations).
Superposition's the thing.

[toneboy1]

Of course. The same way that an antenna can grab a range of signals out of the air from a whole range of EM waves, then a selective receiver can filter out the narrow band of frequencies it wants to look at from a single antenna downlead. Just think of the many instances where a single antenna feeds a large number of receivers (blocks of flats with a distribution amp on the roof) and where a number of transmitters can feed into a single transmitting antenna (UHF TV transmitting stations).
Superposition's the thing.

Great!
How can an antenna have standing wave voltages on it, yet the charges don't 'resonate'?
So they're moving with the erratic signal, could you clarify what you mean? Maybe its the terminology 'resonate' that is throwing me.

Thanks

[sophiecentaur]

Great!
How can an antenna have standing wave voltages on it, yet the charges don't 'resonate'?
So they're moving with the erratic signal, could you clarify what you mean? Maybe its the terminology 'resonate' that is throwing me.

Thanks
Current can flow without there needing to be any resonance. An electric field can cause charge to flow in an antenna just as it can anywhere else. There is also the Magnetic field that will be causing currents. Alternating in direction isn't the same as resonance.

Are you trying to link this into QM, or something?

[toneboy1]

Are you trying to link this into QM, or something?

I assure you I am not, lol.

I'm just trying to get my mind around what is taking place in the antenna to create these standing wave voltages and currents.

[sophiecentaur]

You don't need a standing wave to pick up some energy from a passing EM wave. It's only a design detail to make it more effective.

Try this view of a dipole. If you have a two wire transmission line then a progressive EM wave can pass along it, bound to the two wires (this is how AC 'electricity' travels along wires). If you terminate the line with the appropriate resistor (the load) then the power is all dissipated in the load. If you terminate the line incorrectly (like an open circuit) then the wave is all reflected back (a nuisance, sometimes) and you will have a standing wave in it.
Now split the last metre or so of the line and separate the wires to form a straight line at right angles to the line (a dipole). You still have a transmission line and it's still terminated at an open circuit and you will still get a standing wave with NO CURRENT flowing at the tips. But there is still SPACE for some of the energy to be radiated into. This radiated energy 'looks' to the line like a resistance because Power is going somewhere. You can actually measure a resistance (an RF resistance, that is) if you connect an analyser / bridge / gizmo to the line and it's called (not surprisingly) the Radiation Resistance. But there is still a standing wave of current on the dipole and you will also measure a reactance across the line. By choosing the dipole to be a half wave long, the reactance goes to zero (that's your resonance). It isn't essential to resonate but you get a good 'match' if you do. All that remains to do is to choose your transmission line dimensions to have the same characteristic impedance as the dipole presents and all the power gets radiated. The dipole looks like about 70 Ohms so if the line is also 70 Ohms, Bob's your Uncle.
Most feeder is coaxial and not two wire (better isolation for the feeder) and you can commonly get 50 Ohm and 75 Ohm - not 70 Ohm. This is because of the way that real dipoles and real antennae are connected etc. etc. and you don't need to worry too much first time round about the discrepancy.
But the basic thing is that a dipole behaves very much like a continuation of the transmission line that feeds it.

[Averagesupernova]

An excellent post sophie!

[sophiecentaur]

Tvm.

[toneboy1]

You don't need a standing wave to pick up some energy from a passing EM wave. It's only a design detail to make it more effective.

Try this view of a dipole. If you have a two wire transmission line then a progressive EM wave can pass along it, bound to the two wires (this is how AC 'electricity' travels along wires). If you terminate the line with the appropriate resistor (the load) then the power is all dissipated in the load. If you terminate the line incorrectly (like an open circuit) then the wave is all reflected back (a nuisance, sometimes) and you will have a standing wave in it.
Now split the last metre or so of the line and separate the wires to form a straight line at right angles to the line (a dipole). You still have a transmission line and it's still terminated at an open circuit and you will still get a standing wave with NO CURRENT flowing at the tips. But there is still SPACE for some of the energy to be radiated into. This radiated energy 'looks' to the line like a resistance because Power is going somewhere. You can actually measure a resistance (an RF resistance, that is) if you connect an analyser / bridge / gizmo to the line and it's called (not surprisingly) the Radiation Resistance. But there is still a standing wave of current on the dipole and you will also measure a reactance across the line. By choosing the dipole to be a half wave long, the reactance goes to zero (that's your resonance). It isn't essential to resonate but you get a good 'match' if you do. All that remains to do is to choose your transmission line dimensions to have the same characteristic impedance as the dipole presents and all the power gets radiated. The dipole looks like about 70 Ohms so if the line is also 70 Ohms, Bob's your Uncle.
Most feeder is coaxial and not two wire (better isolation for the feeder) and you can commonly get 50 Ohm and 75 Ohm - not 70 Ohm. This is because of the way that real dipoles and real antennae are connected etc. etc. and you don't need to worry too much first time round about the discrepancy.
But the basic thing is that a dipole behaves very much like a continuation of the transmission line that feeds it.



So this transmission line for say, a block of flats. A coaxial cable coming off the antenna on the roof, terminated open-circuit with separate 'wires' coming off it at right angles to each unit's individual receiver? (it won't be 'matched'?)

ALSO, I assume the matter of the antenna DOESN'T operate from 'dipole moments', thats a separate concept isn't it?

THANKS

[sophiecentaur]

So this transmission line for say, a block of flats. A coaxial cable coming off the antenna on the roof, terminated open-circuit with separate 'wires' coming off it at right angles to each unit's individual receiver? (it won't be 'matched'?)

ALSO, I assume the matter of the antenna DOESN'T operate from 'dipole moments', thats a separate concept isn't it?

THANKS

What do you mean by that? The antenna will, ideally, be matched to the 50 Ohm feeder.
If you know about transformers at 50Hz AC, you will know that you can transform a 230V supply to suit a low resistance bulb (designed to operate at 12V, say). If the bulb is a 100W rating, it will draw about 8A. It will have a resistance of 1.5Ω. But the mains will only be supplying 100W, so it must be supplying less than 0.5A so it will be 'seeing' a resistance of more than 500Ω. You can ignore those actual details but you can see that the transformer is also transforming the load resistance. RF matching networks do the same sort of thing and it is easy to split a 50Ω line to feed two 50Ω loads with a suitable loss-less splitting network. (They would look like 25Ω if you just put them in parallel). The system can remain matched throughout and they get 50% each of the power. You are right to say that 'just hanging' random feeders across a feeder will spoil the match and will lose a lot of signal power.
In practice, there will, normally, be a distribution amplifier (on the roof) so that everyone gets a high level signal but the down-leads are isolated from each other.


BTW Dipole Moment is not part of this stuff.

[toneboy1]

What do you mean by that? The antenna will, ideally, be matched to the 50 Ohm feeder.
If you know about transformers at 50Hz AC, you will know that you can transform a 230V supply to suit a low resistance bulb (designed to operate at 12V, say). If the bulb is a 100W rating, it will draw about 8A. It will have a resistance of 1.5Ω. But the mains will only be supplying 100W, so it must be supplying less than 0.5A so it will be 'seeing' a resistance of more than 500Ω. You can ignore those actual details but you can see that the transformer is also transforming the load resistance. RF matching networks do the same sort of thing and it is easy to split a 50Ω line to feed two 50Ω loads with a suitable loss-less splitting network. (They would look like 25Ω if you just put them in parallel). The system can remain matched throughout and they get 50% each of the power. You are right to say that 'just hanging' random feeders across a feeder will spoil the match and will lose a lot of signal power.
In practice, there will, normally, be a distribution amplifier (on the roof) so that everyone gets a high level signal but the down-leads are isolated from each other.

You are a legend.
I feel like I've just taken a short course over the past couple days!

I didn't think so, out of interest, do you know what part of the EM wave causes the dipole moment (electron and proton to re-orientate) when a wave passes a molecule?
(maybe now I am "trying to link this into QM" :P )

[sophiecentaur]

You are a legend.
I feel like I've just taken a short course over the past couple days!

I didn't think so, out of interest, do you know what part of the EM wave causes the dipole moment (electron and proton to re-orientate) when a wave passes a molecule?
(maybe now I am "trying to link this into QM" :P )

Intense short course, definitely!
"Which part"??? There is an E field and an H field. They are both changing, why should only one "part" be responsible. In any case, if you are talking about interaction with a single molecule then you can identify one photon that will be responsible - so that's QM :tongue2: back to you!

[DrZoidberg]

Like jam said, they do move, but in that process of moving they bump into each other, because they are tightly packed. That bumping is also form of movement. And that movement propagates at the speed, roughly half the speed of light. (Why half? Material reasons)


The thermal velocity of electrons in a wire is around 100,000 m/s. That's very fast but still 3000 times slower than light. If you close a switch the resulting change in electric potential propagates close to the speed of light. If an electric signal (i.e. a propagating change in potential) moved because electrons are "bumping" into each other, it would only move at 100,000 m/s. Just like a sound wave in air. The thermal velocity of air molecules is 340 m/s, that means that pressure waves (sound waves) move at 340 m/s. So electric signals are no pressure waves. They have nothing to do with electrons bumping into each other. An electric signal is mediated by electric fields. Let's say you have a battery and short it with a copper wire. The chemical reactions inside the battery create a charge imbalance which in turn creates an electric field which then pushes electrons through the wire. btw. as long as a current is flowing an electric field can exist inside the wire.


I repeat, current is NOT movement of electric charges, this is common misunderstanding. Current is a impulse of energy. And even that last one cannot be taken for granted.

"impulse of energy"? Did you take that expression from a Star Trek episode? It's certainly not a proper physics term.

[toneboy1]

Intense short course, definitely!
"Which part"??? There is an E field and an H field. They are both changing, why should only one "part" be responsible. In any case, if you are talking about interaction with a single molecule then you can identify one photon that will be responsible - so that's QM :tongue2: back to you!

H'mm, ok well if there is a single particle of some charge, and a photon comes buy, would the particle move on one direction for half the wavelength then in the opposite for the other half? (and not get very far?)

Thanks

[toneboy1]

The thermal velocity of electrons in a wire is around 100,000 m/s. That's very fast but still 3000 times slower than light. If you close a switch the resulting change in electric potential propagates close to the speed of light. If an electric signal (i.e. a propagating change in potential) moved because electrons are "bumping" into each other, it would only move at 100,000 m/s. Just like a sound wave in air. The thermal velocity of air molecules is 340 m/s, that means that pressure waves (sound waves) move at 340 m/s. So electric signals are no pressure waves. They have nothing to do with electrons bumping into each other. An electric signal is mediated by electric fields. Let's say you have a battery and short it with a copper wire. The chemical reactions inside the battery create a charge imbalance which in turn creates an electric field which then pushes electrons through the wire. btw. as long as a current is flowing an electric field can exist inside the wire.

"impulse of energy"? Did you take that expression from a Star Trek episode? It's certainly not a proper physics term.


I wonder what Sophie will say, I recall sophiecentaur saying movement of electrons was in the mm/s, if there's a distinction I don't see it...

[truesearch]

I would agree with dr zoidberg's last comment.
I have read in the rules of these forums that content of posts should be traceable to accepted text books.
I have not met a text book that does not recognise that current is a flow of charge.
If there is a book that suggests otherwise I would like the reference, I want a copy of that book.

[toneboy1]

studiot gave a reference to....some sort of text book.
Anyway, I don't think the conclusion thus far was disputing that current was flow of charge/change in time
Just discussing the mechanics of it.

[Kholdstare]

I praise sophie's patience. @toneboy1 - read text books. Try to understand everything written there. That will save you a lot of time of what you're asking here.

[toneboy1]

I praise sophie's patience. @toneboy1 - read text books. Try to understand everything written there. That will save you a lot of time of what you're asking here.

As do I sophie's patience, as I have said. In fact at one point I had given up on a concept but sophie exclaimed not to.
Two things, first being that I can barely afford rent or three meals a day let alone text-books, I read everything relevant I can find on the internet not just forums, I spend a lot of as much time as I can learning about the natural world via the means available (khan academy, anything).
What I think is unfair Kholdstare is that you imply I could simply find things out elsewhere, as a matter of fact some of the time I want to cross-check that my model is actually the case, but it just so happens that I'm usually wrong :tongue2:

Second thing, If my questions (and the answers that follow) are of no interest or help to anyone else who finds the thread than I'll stop asking, I don't wish to selfishly consume people's time.

[Kholdstare]

@toneboy1 - I did not know about your economic problems. I am sorry to know that. But nothing can beat a well written textbook. It is typical among people to avoid reading textbooks and find quick answers and explanations. Sometimes this produces lengthy threads when the OP could read a little more and clear his doubts.

However, I deeply sympathize for your problem and I am sorry to suspect you as a lazy person. You are not selfishly consuming any people's time and everybody would love to help you.

[toneboy1]

@toneboy1 - I did not know about your economic problems. I am sorry to know that. But nothing can beat a well written textbook. It is typical among people to avoid reading textbooks and find quick answers and explanations. Sometimes this produces lengthy threads when the OP could read a little more and clear his doubts.

However, I deeply sympathize for your problem and I am sorry to suspect you as a lazy person. You are not selfishly consuming any people's time and everybody would love to help you.

I completely agree, I much preferred being able to bookmark things and it felt better on the eyes when I was at an institution that had a decent library.
Thank you for saying that. One problem I do find is that when you learn something small, that's wrong, early on, this little red herring ruins future models of how you see the world based on it, though I find when you are talking to another person they can easily say 'hey what you said is wrong or doesn't make sense' but when you're researching on your lonesome sometimes you just sit around getting more confused and you can't reason why.
(I hope people will still tell me If I am wasting someones time though in future)

[truesearch]

You are not wasting anybodies time!
No one is forced to post here! It is very satisfying to be able to help and there is so much expertise here that think we are all still learning.
We are all here for the same reason.

[sophiecentaur]

H'mm, ok well if there is a single particle of some charge, and a photon comes buy, would the particle move on one direction for half the wavelength then in the opposite for the other half? (and not get very far?)

Thanks
I think you mean half the period? But yes, basically.
This is the grey zone between QM and Classical.
There is a situation in the Ionosphere where there are free electrons that can be 'seen' to 'vibrate' as a radio wave passes. Again, this is because the energy gaps involved for a free electron in a very low density plasma are very small and a classical approach works fine by treating the plasma as a conductor with the electrons moving one way and the much heavier ions moving (a smaller distance) the other way, as the fields vary around them. Because the electrons are not in a metal, they actually do move a significant distance in the time period of a 1MHz radio wave.

If you don't have access to text books then trawl around the net for .org and .edu sites for more reliable opinions. Beware, there are some dreadful, cranky and harmful sites that may read as gospel. Look for a majority opinion if you get confused.

[toneboy1]

I think you mean half the period? But yes, basically.
This is the grey zone between QM and Classical.
There is a situation in the Ionosphere where there are free electrons that can be 'seen' to 'vibrate' as a radio wave passes. Again, this is because the energy gaps involved for a free electron in a very low density plasma are very small and a classical approach works fine by treating the plasma as a conductor with the electrons moving one way and the much heavier ions moving (a smaller distance) the other way, as the fields vary around them. Because the electrons are not in a metal, they actually do move a significant distance in the time period of a 1MHz radio wave.

If you don't have access to text books then trawl around the net for .org and .edu sites for more reliable opinions. Beware, there are some dreadful, cranky and harmful sites that may read as gospel. Look for a majority opinion if you get confused.


Thanks for the advice! There is an art to 'trawling', yes I've seen a lot of contradictory information on subjects where you would expect only people who know what they are talking about would contribute to.

I realise in hind-sight that I phrased my question poorly, intuitively I was thinking that only the E part of the photon would stimulate a dipole moment or electrons. I think you answered my queries never-the-less.

TYVM!