Currents through cables

[Chris]

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?

 

[whit3rd]

On Tuesday, January 5, 2016 at 10:58:07 AM UTC-8, Chris wrote:

Is it possible for two different AC currents to flow in opposite
directions through a single conductor at the *same* time?

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.

For example, the concept of VSWR seems to imply this is possible...

How come they don't collide with each other?

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.

 

[Kaz Kylheku]

On 2016-01-05, Chris <cbx@noreply.com> wrote:

Is it possible for two different AC currents to flow in opposite
directions through a single conductor at the *same* time?

Yes. This is how your land telephone line can work over a single pair of
wires.

> How come they don't collide with each other?

They do but in a very way; but see "principle of superposition".

 

[Phil Allison]

Chris wrote:

Is it possible for two different AC currents to flow in opposite
directions through a single conductor at the *same* time?

** 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.

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?

** 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

 

[Tim Wescott]

On Tue, 05 Jan 2016 18:55:25 +0000, Chris wrote:

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?

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

 

[Phil Hobbs]

On 01/05/2016 10:29 PM, Tim Wescott wrote:

On Tue, 05 Jan 2016 18:55:25 +0000, Chris wrote:

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?

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.

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

 

[Jasen Betts]

On 2016-01-05, Chris <cbx@noreply.com> wrote:

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?

they aren't currents they're waves.

--
\_(ツ)_

 

[Kaz Kylheku]

On 2016-01-07, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2016-01-05, Chris <cbx@noreply.com> wrote:
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?

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.

 

[szczepan bialek]

"Jasen Betts" <jasen@xnet.co.nz> napisał w wiadomości
news:n6kps6$f3s$1@gonzo.alcatraz...

On 2016-01-05, Chris <cbx@noreply.com> wrote:
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?

they aren't currents they're waves.

So in the conductors with the AC no currents.
S*



---
Ta wiadomosc zostala sprawdzona na obecnosc wirusow przez oprogramowanie antywirusowe Avast.
https://www.avast.com/antivirus

 

[szczepan bialek]

"Chris" <cbx@noreply.com> napisał w wiadomości
news:n6h3it$gq1$5@dont-email.me...

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?

They collide. It was discovered by O. Lodge (see Fig 2):
http://www.antiquewireless.org/uploads/1/6/1/2/16129770/48-oliver_lodge.pdf
"The electrical waves produced by the oscillations at A traveled along the
wires and
were reflected at the far ends. Lodge knew that the longer spark at B3 was
due to
what he called the "recoil impulse" or "recoil kick" at the
end of the wires where the waves were reflected.
[4]
At spark gap B3 both the incident wave and the reflected
wave had their maximum values and were in phase. This produced a voltage
twice
as large as the voltage at spark gap A"

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*



---
Ta wiadomosc zostala sprawdzona na obecnosc wirusow przez oprogramowanie antywirusowe Avast.
https://www.avast.com/antivirus

 

[Julian Barnes]

On Fri, 08 Jan 2016 11:00:06 +0100, szczepan bialek wrote:

They collide. It was discovered by O. Lodge (see Fig 2):
http://www.antiquewireless.org/uploads/1/6/1/2/16129770/48-
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!

 

[szczepan bialek]

"Julian Barnes" <jb9889@notformail.com> napisał w wiadomości
news:n6rtj0$s22$1@dont-email.me...

On Fri, 08 Jan 2016 11:00:06 +0100, szczepan bialek wrote:

They collide. It was discovered by O. Lodge (see Fig 2):
http://www.antiquewireless.org/uploads/1/6/1/2/16129770/48-
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:
https://en.wikipedia.org/wiki/Etheric_force

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*

 

[wbeaty]

On Tuesday, January 5, 2016 at 10:58:07 AM UTC-8, Chris wrote:

Is it possible for two different AC currents to flow in opposite
directions through a single conductor at the *same* time?

No, because two entirely separate things are flowing in a wire. "Watts" describes 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 mobile charges.

Watts are a measure of traveling wave-energy. And yes, we can have two different 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 inside a wire is only wiggling slightly back and forth, not rushing rapidly forward.

Less confusing: energy is the traveling WAVES, and charge is the MEDIUM through which the waves travel. Watts describes the moving waves, and Amperes describes the vibration of the medium. Can we have two separate waves going 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 flows of electrical energy are like sound waves, while electric currents are like water motion. We certainly can have two sound waves traveling in opposite 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 in opposite directions, they collide and just subtract from each other, leaving a single remainder. But if you send two energy-flows in opposite directions, they pass through each other, creating standing waves.

 

[wbeaty]

On Friday, January 8, 2016 at 1:59:45 AM UTC-8, szczepan bialek wrote:

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.

No, you mean the large *wattage* in the forward direction is 10x stronger than 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 wiggle slightly. They do not travel along the wire to the antenna. It's the joules which do the traveling, not the coulombs.

Never forget that "AC" is truly alternating equally back and forth, with zero 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 outward by the antenna.

 

[wbeaty]

On Tuesday, January 5, 2016 at 6:17:15 PM UTC-8, Phil Allison wrote:

> Each can acts as both receiver and sender while a taught string carries vibrations travelling in both directions.

Yes, the traveling vibrations on the string are like the rf WATTAGE on an antenna 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 one tin can to the other. We can't have two "string currents" in opposite directions in the string, since there's only one string. If we try to pull the string in two opposite directions, it just sits without moving.

But we certainly can have two sound-waves going in opposite directions through the string. And sound waves are the analogy for watts, for rf power waves propagating along a transmission line.

 

[Phil Allison]

wbeaty wrote:

Is it possible for two different AC currents to flow in opposite
directions through a single conductor at the *same* time?

No, because two entirely separate things are flowing in a wire.
"Watts" describes the energy flow, and "Amperes" describes the
charge flow.


** Arrant nonsense.

Wires have simply current flowing in them.

Watts are a measure of traveling wave-energy.

** Shame that wires carry only current - not wave energy.

You must be from the twilight zone of physics ?



.... Phil

 

[Phil Allison]

wbeaty wrote:

Phil Allison wrote:

Each can acts as both receiver and sender while a taut string
carries vibrations travelling in both directions.

Yes, the traveling vibrations on the string are like the rf WATTAGE on an antenna cable. But the OP was asking about electric current, not about the watts of energy flow. Energy currents aren't electric currents.

** Oh really ??


> Each small piece of string just wiggles slightly, and does not zoom from one tin can to the other.

** Exactly like electrons in a wire.


> We can't have two "string currents" in opposite directions in the string,

** Meaningless drivel.

But we certainly can have two sound-waves going in opposite directions
through the string.

** Which is the analogy I was pointing out.

Doodle doodle, doodle doodle ...


.... Phil

 

[Kaz Kylheku]

On 2016-01-14, wbeaty <billb@eskimo.com> wrote:

Less confusing: energy is the traveling WAVES, and charge is the
MEDIUM through which the waves travel.

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.

> Watts describes the moving > waves.

Shere nonsense. DC producing heat through a resistor is still measured
in Watts.

Sure. But we never have two portions of charge going in opposite
directions in the same bit of wire.

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.

The good old water analogy: if a wire is like a pipe full of water,
then flows of electrical energy are like sound waves, while electric

You fucked 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.

currents are like water motion. We certainly can have two sound waves
traveling in opposite 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.

Bzzt: H2O molecules are moving randomly in every direcion, like
charge carriers.

If
you tried to create two currents in opposite directions, they collide
and just subtract from each other

.... 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?

> leaving a single remainder.

A remainder is something which emerges from an integer division,
not from subtraction.

 

[whit3rd]

On Thursday, January 14, 2016 at 8:46:32 AM UTC-8, Kaz Kylheku wrote:

On 2016-01-14, wbeaty <billb@eskimo.com> wrote:
Less confusing: energy is the traveling WAVES, and charge is the
MEDIUM through which the waves travel.

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".

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.

 

[szczepan bialek]

"wbeaty" <billb@eskimo.com> napisal w wiadomosci
news:a6f85ec9-997c-4723-9ae6-48944d92a003@googlegroups.com...
On Friday, January 8, 2016 at 1:59:45 AM UTC-8, szczepan bialek wrote:

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.

No, you mean the large *wattage* in the forward direction is 10x stronger
than a smaller *wattage* returning. SWR is a measure of reflected
*power* in watts, >not amperes.

See the Fig of circuit.
https://pl.wikipedia.org/wiki/SWR_meter
The SWR meter measure the two currents. To and fro.

"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
wiggle slightly. They do not travel along the wire to the antenna. It's
the joules which do the >traveling, not the coulombs.

Never forget that "AC" is truly alternating equally back and forth, with
zero 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 outward by the antenna.

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:

http://mpec.sc.mahidol.ac.th/radok/physmath/PHYSICS/i2.htm

"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:
http://www.tfcbooks.com/tesla/1929-09-22.htm

Tesla Coil generate very high voltage which is enough to bursts the
electrons from the end of the antenna.
Was not Tesla right?
S*