What is electricity exactly?

DragonPetter

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

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

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

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

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

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

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

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 ). 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

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

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

Attached Thumbnails

 

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! 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?