Is it possible for two different AC currents to flow in opposite directions through a single conductor at the *same* time?
For example, the concept of VSWR seems to imply this is possible, with a large current in the forward direction flowing towards an antenna and a smaller one returning in the opposite direction back to the transmitter (whenever there's an impedance mismatch.)
How come they don't collide with each other?
Alternating current means that current flows both ways; you can get POWER to flow in opposite directions, 'to' at frequency A and 'fro' at frequency B.
They do, but wire is linear, so the waves just occupy the same space... similarly, radio waves can deliver power southward from one transmitter, and northward from a different transmitter, to your location. You can tune in either, with no interference.
Yes. This is how your land telephone line can work over a single pair of wires.
They do but in a very way; but see "principle of superposition".
** At any point in time, there is only one current in one direction. But that current can be the sum of two or more AC sources located at various points along the wire. ** Ever see or use a "tin can telephone" ?
Each can acts as both receiver and sender while a taught string carries vibrations travelling in both directions.
... Phil
What everyone else has said, plus this:
This sort of stuff is taught as if it's just absolute fact. But the truth is that it's just one valid explanation out of many. It happens to be an explanation that covers a whole lot of ground, so if you only have one mental model to go by, it'll help you the most.
So on the one hand you should understand it thoroughly and know what it all means. On the other, you shouldn't assume that the actual underlying physical reality either is or is not exactly what you've been taught.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
It works because the system is linear. It certainly isn't obvious otherwise.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
they aren't currents they're waves.
-- \_(?)_
Maybe we should have a hashup here to cover the bases.
In a conductor, we can discuss various abstractions:
- Instantaneous current and voltage: what the aggregate of electrons is doing.
- AC: an abstraction describing a sinusoidal, periodic voltage or current, and combinations of such, independently of time. (Yes, though the "C" in AC is "current", it really just means "electricity", and not specifically current, which is why we can say "120 VAC" with a straight face).
- DC: a situation in which the instantaneous current and voltage exist over a span of time.
- Signal transmission: wave-like propagation of a changing voltage/current over distance through a conductor. Closely related to AC.
All these abstractions are susceptible to linear superposition. However, when we are looking at AC, we cannot blindly apply superposition. If we consider the combination of two AC voltages or currents, we have to keep in mind their frequency and phase. Generally speaking, those have to be the same.
When we analyze a circuit using "AC analysis", the signal in all the nodes is derived from the same AC source. The frequency is the same, as is the phase (modulo the effects of the reactance of the circuit components on phase: to take care of which, we use complex impedances and complex math: this math maintains the relationship between the AC view and the correct instantaneous current/voltage view.)
If multiple AC sources with different characteristics drive some input, we can still analyze things separately for those sources and superimpose. We just don't superimpose the AC values. For a given node of interest, we can reduce from the AC domain to the instantaneous domain, and superimpose the instantaneous current and voltage.
For instance, suppose a 60Hz, 12V signal is driving the top of some resistor (ground is on the opposite end). Suppose another 12V signal at precisely the same 60 Hz frequency is driving the same resistor also. (Assume the sources have zero impedance; they are unaffected by each other, etc).
We cannot just add together these 12V quantities. For instance, suppose that they are precisely 180 degrees out of phase? Then the sum is zero: no current flows through the resistor, the voltage at the top is always
0V, and no power is dissipated.What we can do is, for each source, reduce the AC voltage and current to a function of time, which gives us the instantaneous picture. These functions can then be superimposed and the result analyzed.
So in the conductors with the AC no currents. S*
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They collide. It was discovered by O. Lodge (see Fig 2):
In such way works the long wire antenna and the loop antenna. They radiate from the "collide" places. In normal mast the large current in the forward direction flowing towards an antenna is 10 times stronger than "a smaller one returning in the opposite direction back to the transmitter". The mast radiate from the end. S*
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oliver_lodge.pdf
Thanks for posting that. I'd always believed that lightning was oscillatory in nature as Lodge originally did. Nice to have that little misconception cleared up!
The oscillations may be continuous or damped. The spark is the damped oscillations.
But was the queston if the spark transport net electric charge in one direction. "Edison believed that the spark transmitted electricity without carrying any charge." Next was proved that: " Elihu Thomson and Edwin Houston, a high school teacher with whom Thomson had studied. Thomson and Houston conducted a series of careful experiments where they discovered that the sparks actually carried a charge." From:
So it should be obvious that the damped oscillations are always stronger in one direction than in the opposite. So the lightning transport the electric charge from a cloud to the soil. Do you agree? S*
No, because two entirely separate things are flowing in a wire. "Watts" de scribes the energy flow, and "Amperes" describes the charge flow.
In your antenna wire, it's the wattage which flows along. The amperes are very different; a measure of slight microscopic vibration of the wire's mob ile charges.
Watts are a measure of traveling wave-energy. And yes, we can have two dif ferent energy-flows going in opposite directions in the same wire. (That's what VSWR is about; opposite-traveling RF wattage. Not colliding amperes. ) There can only be one amperes, because any small hunk of electrons insi de a wire is only wiggling slightly back and forth, not rushing rapidly for ward.
Less confusing: energy is the traveling WAVES, and charge is the MEDIUM th rough which the waves travel. Watts describes the moving waves, and Ampere s describes the vibration of the medium. Can we have two separate waves go ing in opposite directions at the same time? Sure. But we never have two portions of charge going in opposite directions in the same bit of wire.
The good old water analogy: if a wire is like a pipe full of water, then f lows of electrical energy are like sound waves, while electric currents are like water motion. We certainly can have two sound waves traveling in opp osite directions inside a pipe full of water. But this idea doesn't apply to water itself. In every small bit of pipe, there can only be one hunk of water moving in just one direction. If you tried to create two currents i n opposite directions, they collide and just subtract from each other, leav ing a single remainder. But if you send two energy-flows in opposite direc tions, they pass through each other, creating standing waves.
an
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No, you mean the large *wattage* in the forward direction is 10x stronger t han a smaller *wattage* returning. SWR is a measure of reflected *power* in watts, not amperes.
"Energy currents" are measured in watts, and are called "Power." "Charge currents" are measured in amperes, and are called "Electric Current ."
"Current" describes the vibration of mobile charges of the wire. They wigg le slightly. They do not travel along the wire to the antenna. It's the j oules which do the traveling, not the coulombs.
Never forget that "AC" is truly alternating equally back and forth, with ze ro forward motion on average. That's how charge-carriers behave. But it 's not at all how electromagnetic energy behaves. The energy, the joules, comes out of the transmitter, flows along the cable, then is radiated outw ard by the antenna.
ibrations travelling in both directions.
Yes, the traveling vibrations on the string are like the rf WATTAGE on an a ntenna cable. But the OP was asking about electric current, not about the watts of energy flow. Energy currents aren't electric currents.
Each small piece of string just wiggles slightly, and does not zoom from on e tin can to the other. We can't have two "string currents" in opposite di rections in the string, since there's only one string. If we try to pull t he string in two opposite directions, it just sits without moving.
But we certainly can have two sound-waves going in opposite directions thro ugh the string. And sound waves are the analogy for watts, for rf power wa ves propagating along a transmission line.
** Arrant nonsense.
Wires have simply current flowing in them.
** Shame that wires carry only current - not wave energy.You must be from the twilight zone of physics ?
.... Phil
** Oh really ?? ** Exactly like electrons in a wire. ** Meaningless drivel. ** Which is the analogy I was pointing out.
Doodle doodle, doodle doodle ...
.... Phil
Utter nonsense. The wave picture only emerges at high frequencies. As a rule of thumb, the length of a conductor has to be at least 1/20th of the wavelength before we consider "transmission line effects".
Within the audio frequency range, that distance frequencies, the wavelength of audio within unshielded copper (96% speed of light) is
15 kilometers or more, and so the wave-like transmission line effects like delays and reflection don't have to be considered until your wire is the better part of a kilometer long.Shere nonsense. DC producing heat through a resistor is still measured in Watts.
Yes we can; in fact in some situations we can visualize a "hole current" going in the opposite convention to regular current.
Current is just a vector quantity, and any vector can be decomposed into two or more vectors which sum to it.
Current is itself an aggregate, macroscopic phenomenon. The individual charge carriers in a conductor are moving around randomly in every direction: each one is a tiny piece of current. When a voltage is applied across the conductor, it causes a net drift of these carriers in one direction. Individually they still move around in every direction, but there is now a net drift. That net drift is what we call current.
You f***ed this up too. If wire is like a pipe full of water, then current is like ... water flowing through the wire under pressure.
Thanks for playing.
If the pressure variations are at a sufficiently high frequency, such that the length of the pipe is at least around 1/20th of the wavelength, then we have to start considering the acoustic effects.
Up until that point, we can assume that the pressure is the same everywhere in the section of an (ideal) pipe.
Bzzt: H2O molecules are moving randomly in every direcion, like charge carriers.
... contradicting the claim that there cannot be two or more currents.
If there cannot be two or more currents, where the hell did you get the two inputs to your subtraction?
A remainder is something which emerges from an integer division, not from subtraction.
See the Fig of circuit.
I the perfect mast no AC. There are the pulsating current. At the end of mast appears the "kinetic voltage". This voltage cause the field electron emission. In portions. Next is like in the Seebeck sirena:
"The air exits in bursts and becomes at equal time intervals a sequence of generating impulses of longitudinal waves and, if they are frequent enough, a tone is generated".
The radio waves are the same phenomena: "According to Mr. Tesla, the present broadcasting station does not propagate Hertzian waves, as has always been supposed, but acts more like an "ether whistle" - transmitting waves through the ether similar to the waves transmitted by an ordinary whistle through the air." From:
Tesla Coil generate very high voltage which is enough to bursts the electrons from the end of the antenna. Was not Tesla right? S*
No, not nonsense, just confusing. The transmission line/wave picture is not invalid at low frequencies. One can get by with single-series-inductor and single-parallel-capacitor models of the wire, though, which is a simpler model.
In the wave picture, DC power transmission comes from the electric field in the wire and the magnetic field around the wire. It's just a zero-frequency wave, not an exception.
The reason the wave picture is useful here, is that one can decompose the power transmission due to multiple frequencies, and sensibly measure power at high frequency going one direction, and power at low frequency going the other. One can, for example, have a house consuming
60 Hz power, but a transmitter sending your power-meter reports the other direction, from the house to the power company.ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.