- #36
sophiecentaur
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Which one?
Bassalisk said:Everything.
Bassalisk said:Well yes, when u put it that way.
Bassalisk said: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.
Kholdstare said:Its flow of electric charge.
sophiecentaur said:Nothing wrong with that but the term flux gets further away from the implication of speed which we need to avoid, perhaps.
sophiecentaur said:Nothing wrong with that but the term flux gets further away from the implication of speed which we need to avoid, perhaps.
Kholdstare said: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.
sophiecentaur said:Current is the Time derivative of Charge. NOT the speed of charge carriers. Do you not see the difference? This is so basic.
toneboy1 said:That seems completely different to:
I'm sure Bassalisk would have a fit...
DragonPetter said:They are not different, only different levels of describing the same concept. Kholdstare is explaining the physical description of how it comes about.
toneboy1 said: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
toneboy1 said: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 said:
...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.
sophiecentaur said:Furthermore -...No charges can flow between them at all. Charges flowing in one place can cause charges to flow in another place.
toneboy1 said: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 said: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.
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 said:Right, so the actual modulation of the inductance / capacitance on the transmission circuit antenna is how they send information for say FM?
sophiecentaur said: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.
sophiecentaur said: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 ). 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.
sophiecentaur said: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.
OH! The transmission line! So is that meant to have a constant voltage along it?sophiecentaur said: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 said: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 said: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.