On Jan 14, 10:42*pm, Cecil Moore wrote:
On Jan 14, 3:40 pm, Keith Dysart wrote:

So energy does move within the line, though no energy crosses a point
where the voltage or current is always 0.

Zero *NET* energy moves in a standing wave pattern whether it be at a
node or anywhere else. Forward energy and reflected energy moves
across a node and everywhere else because there are no reflections in
a homogeneous medium.

From your posts I have not been able to determine exactly what
YOU mean by "*NET* energy moving". Several possibilities come to mind:
1) the time average of the instantenous power, P(x,t), at a
particular
point on the line.
2) the difference between the time average power in the forward and
reflected waves. This would be equivalent to subtracting the
forward and reflected power indicated by a Bird wattmeter.
3) the difference between the instantaneous forward and reflected
power at a particular time and point on the line.
4) the time average of the difference between the instaneous forward
and reflected power at a point on the line.
5) something completely different?

It would be valuable if you could indicate which of the possible
definitions of "*NET* energy moving" you mean.

It would be even more illuminating if you could describe what
measurements would need to be made on a line and exactly what
calculations should be performed to compute your definition
of "*NET* energy moving".

A rigorous definition might reveal that there is actually
agreement.

...Keith

Cecil Moore wrote:
On Jan 14, 4:22 pm, Jim Kelley wrote:

A standing wave, yet presumably not an electromagnetic wave! ;-)


EM waves necessarily travel at the speed of light in a medium.

Does a
standing wave travel at the speed of light?

If your answer is 'yes',
where does it go? If your answer is 'no', it is not an EM wave.

You didn't click on the link, did you. :-)

ac6xg

On Tue, 15 Jan 2008 06:44:05 -0800 (PST)
Keith Dysart wrote:

On Jan 14, 7:19 pm, Roger Sparks wrote:
On Mon, 14 Jan 2008 12:40:39 -0800 (PST)

Keith Dysart wrote:

snip...............

As you say, the energy moves between the E-field and the H-field,
but the locations of maximum energy along the line for each of these
fields is different, so the energy changes position on the line with
each cycle. The energy at any point on the line is not constant.

E-field energy will peak at the voltage maximums.
H-field energy will peak at the current maximums.
These are at different places (90 degrees apart).
So energy does move within the line, though no energy crosses a point
where the voltage or current is always 0.

I can understand no energy crossing a zero current point, but how do you justify no energy crossing a zero voltage point when current IS observed?

Well I start with P = V * I, so whenever the current or the voltage
is zero, there is no power. Specifically, V and I are measured at
the terminals of a network and P will be the power flowing into or
out of the network.

I can see why you find "no power" at the zero voltage point, but does that imply that there is no energy flow and no power from every perspective? As I write, I am struggling how to clearly differentiate between "power" as "work done" and energy as "capacity to do work", and what "network" are we defining.

Let's begin with the network. Drawing from your words below, we have two networks, one to the left and one to the right of our zero voltage point. When we test voltages on either side of the zero point, we find voltage. The question now is: "Which network did we join when we measured voltage?". The answer is: "We joined the network that we measured.". When we measure exactly in the center between networks, we join neither network.

For another way of looking at the two networks, let us place our voltage probes on each side of the zero voltage point on a/the wire connecting the two networks. We will detect a voltage and a current for any of the standing wave systems we are discussing. By changing our points of reference, we find that power is applied to the zero voltage zone during the instant of time the measurement is made.

I personally define power as a state/condition where "work 'is being' done", . Power must act over time and have a physical movement component. Voltage by itself does not fulfill this definition because no movement is observed. Current is movement, voltage is only an indication of where a concentration of charges is found.


In the case under discussion, there are two networks, one to
the left of the point on the line and one to the right and
we are measuring the power flowing between these two networks.

For an example of current without power, consider a loop of
superconductor with a current flowing in it. No voltage, no
power, but there is current.

I agree. We could place voltage probes between any two points on the superconducting loop and not find voltage. Power is not being applied nor extracted from the superconducting loop system. I think we would all agree that energy is stored in the superconducting loop with current flowing.


Current is defined as movement of charges, and charges have energy by definition (how can they be charges without energy?).

Consider an object flying through space. No work is being done
(and therefore there is no power), but the object still has
kinetic energy.

Another point, the current is observed to change directions during the cycles, polarity also changes on each side of the zero voltage point. Where might the polarized energy come from if it does not cross the zero voltage point?

A thought experiment I have found useful is to consider a simple
resonant circuit made of an ideal capacitor and inductor. Charge
the capacitor to 10 volts and then connect the inductor. A sinusoidal
voltage and current will appear in the circuit.

Just as the inductor is connected:
- all the energy is stored in the capacitor
- the voltage on the capacitor is maximum
- there is no current in the inductor
After connecting the inductor:
- energy starts to transfer to the inductor
- the voltage on the capacitor is dropping
- the current in the inductor is increasing
Some time later:
- the voltage on the capacitor is 0
- the current in the inductor is maximum
- there is no energy stored in the capacitor
- all the energy is stored in the inductor
- no energy is moving from the capacitor to
the inductor
But the inductor insists that current continue
to flow:
- the capacitor begins to charge with a negarive
voltage
- energy begins to transfer from the inductor
back to the capacitor (note the change in the
direction of energy flow)
- the voltage on the capacitor is increasing
negatively
- the current in the inductor is dropping
Sometime later:
- the current in the inductor has dropped to
zero
- the capacitor has a maximum negative voltage
- all the energy is in the capacitor
And this continues forever at the resonant
frequency of the capacitor and inductor circuit.

But no energy is moving from the capacitor to
the inductor when the voltage on the capacitor
is zero and the current in the inductor is
maximum. It is at these times that the direction
of energy flow is changing, as well as when the
voltage in the capacitor is maximum and the
current is zero.

When the voltage on the capacitor is zero, the voltage on the entire system is zero, no matter our reference point. The system energy is completely contained in the moving current with a direction of energy flow completely defined. For an instant, the inductor is like a superconducting loop.

From a traveling wave standpoint, the resonant capacitor/inductor system contains a positive wave and a negative wave, equally balanced energy wise. When the capacitor is completely charged, the positive and negative waves are at the reversal/mid point of the cycle where each wave is maximally displaced from center (which is at the electrical center of the inductor). When the capacitor is completely discharged, the two waves superimpose and both reside in the inductor at identical times. The energy of both waves is completely contained in the electromagnetic field that exists outside the wires containing the two traveling waves. Do the waves exist on the wire at this instant, or have they completely desolved into a space field we observe as magnetic force? The current seems to be flowing so I would say the waves both continue to exist.


I can kinda see how like charges could repell so that waves of like polarity might "bounce" but I can't see how waves of opposite polarity might "bounce". If waves of opposite polarity "bounced", why would the polarity change during the cycle on each side of the "bounce" point?

An excellent counter-example. I may have fallen into
the trap of looking at the examples that support the
argument rather than looking for the ones that don't.
This will take some cogitating. Maybe its the end
of the line for the "bounce hypothesis".

To me, it is much more rewarding to work with traveling waves that pass through one another, interacting to create standing waves.

I don't object to this view, as long as the waves are
viewed as having voltages or currents but no power.

Have you considered how energy is transfered between elements of the transmission line over time if we do not have an ongoing application of power? Doesn't one section of line apply power to the next successive section of line an instant of time later after it received applied power? We agree that a transmitter applies power at the input of a transmission line. Isn't the first section of transmission line just the power source for the second piece of line?

I think of the traveling waves as transporting power and energy through time and physical distance. The highest voltage points physically "move" (found in a new location) as time passes, as do the highest current points, and always together in phase.


The difficulty is that some waves definitely transport
energy while others do not and I do not see a good
explanation for what turns the former into the latter,
as happens, for example, when the pulses collide.

Some waves transport energy, and some do not! That distinction bothers me less now that I have participated in this thread for a while. For me, the traveling wave always has current and voltage in phase, and always carries power. If I can not find power, then we must have a standing wave. For me, traveling waves is all that we really have, they are primary. All other waves flow/result from the traveling waves.

Another observation that has helped me is the recognition that "waves" are just statistical groupings of repetitious events. It is very convenient to treat waves as physical objects but they are really groupings of much, much smaller events. How small is the smallest event, no one seems to know, but the highest frequencies still seem to be electromagnetic events. If the "wave" is composed of a group of much smaller events (such as movement of electrons, or smaller), then it is not so hard to accept that we might detect passage of two waves as having a different voltage and energy level from what we would expect if the voltage and energy were static.


Would it help your visualization process to observe that when two waves of SAME POLARITY but traveling in opposite directions cross, the currents accompaning the waves are moving in opposite directions both before and after crossing? When two waves of OPPOSITE POLARITY but traveling in opposite directions cross, the currents are moving in the same direction both before and after crossing.

I do understand (at least I think I do) the methodology
for superposing the waves of voltage and current,
computing the results and deriving the power from the
result.

I just am not happy that this results in waves sometimes
transporting energy and sometimes not, without a good
explanation of the transition.

If we were talking about water, water behind a dam is like voltage, with the height of the water the potential energy, measured in head (feet), or PSI if measured at the bottom of the dam. A pipe to the bottom of the dam will squirt water at a high velocity but no head or PSI. The potential energy of the water behind the dam has been converted to kinetic energy measued in velocity of a moving mass. The moving water can be stopped, and if carefully done, the static head reached by stopping the water will nearly reach the original water level behind the dam. It would reach the same level if it were not for friction losses. Electrical current is something like that moving water.

Agreed. I have used this analogy as an aid to understaning
though it becomes challenging at RF.

...Keith

This discussion helps clarify things in my mind. I hope it helps you as well.

73, Roger, W7WKB

Cecil Moore wrote:

I've always known that a graph drawn with a pencil on a piece of paper
is not an EM standing wave but, for awhile, you seem to have forgotten
that simple fact.

A plot is simply the graphical representation of an equation. Since I
was the one pointing it out to you, it seems you are the one who had
"forgotten that simple fact".

The envelope is also not the wave.

It all comes from the same equation, Cecil. If the thing which
amplitude modulates a wave cannot be called a wave, then what should
it be called?

ac6xg

On 15 Jan, 11:27, Roger Sparks wrote:
On Tue, 15 Jan 2008 06:44:05 -0800 (PST)

Keith Dysart wrote:
On Jan 14, 7:19 pm, Roger Sparks wrote:
On Mon, 14 Jan 2008 12:40:39 -0800 (PST)

Keith Dysart wrote:

snip...............

As you say, the energy moves between the E-field and the H-field,
but the locations of maximum energy along the line for each of these
fields is different, so the energy changes position on the line with
each cycle. The energy at any point on the line is not constant.

E-field energy will peak at the voltage maximums.
H-field energy will peak at the current maximums.
These are at different places (90 degrees apart).
So energy does move within the line, though no energy crosses a point
where the voltage or current is always 0.

I can understand no energy crossing a zero current point, but how do you justify no energy crossing a zero voltage point when current IS observed?

Well I start with P = V * I, so whenever the current or the voltage
is zero, there is no power. Specifically, V and I are measured at
the terminals of a network and P will be the power flowing into or
out of the network.

I can see why you find "no power" at the zero voltage point, but does that imply that there is no energy flow and no power from every perspective? *As I write, I am struggling how to clearly differentiate between "power" as "work done" and energy as "capacity to do work", and what "network" are we defining.

Let's begin with the network. *Drawing from your words below, we have two networks, one to the left and one to the right of our zero voltage point. *When we test voltages on either side of the zero point, we find voltage. *The question now is: "Which network did we join when we measured voltage?". *The answer is: "We joined the network that we measured.". *When we measure exactly in the center between networks, we join neither network. *

For another way of looking at the two networks, *let us place our voltage probes on each side of the zero voltage point on a/the wire connecting the two networks. *We will detect a voltage and a current for any of the standing wave systems we are discussing. *By changing our points of reference, we find that power is applied to the zero voltage zone during the instant of time the measurement is made.

I personally define power as a state/condition where "work 'is being' done", . *Power must act over time and have a physical movement component. *Voltage by itself does not fulfill this definition because no movement is observed. *Current is movement, voltage is only an indication of where a concentration of charges is found.



In the case under discussion, there are two networks, one to
the left of the point on the line and one to the right and
we are measuring the power flowing between these two networks.

For an example of current without power, consider a loop of
superconductor with a current flowing in it. No voltage, no
power, but there is current.

I agree. *We could place voltage probes between any two points on the superconducting loop and not find voltage. *Power is not being applied nor extracted from the superconducting loop system. *I think we would all agree that energy is stored in the superconducting loop with current flowing.



Current is defined as movement of charges, and charges have energy by definition (how can they be charges without energy?).

Consider an object flying through space. No work is being done
(and therefore there is no power), but the object still has
kinetic energy.

Another point, the current is observed to change directions during the cycles, polarity also changes on each side of the zero voltage point. *Where might the polarized energy come from if it does not cross the zero voltage point?

A thought experiment I have found useful is to consider a simple
resonant circuit made of an ideal capacitor and inductor. Charge
the capacitor to 10 volts and then connect the inductor. A sinusoidal
voltage and current will appear in the circuit.

Just as the inductor is connected:
- all the energy is stored in the capacitor
- the voltage on the capacitor is maximum
- there is no current in the inductor
After connecting the inductor:
- energy starts to transfer to the inductor
- the voltage on the capacitor is dropping
- the current in the inductor is increasing
Some time later:
- the voltage on the capacitor is 0
- the current in the inductor is maximum
- there is no energy stored in the capacitor
- all the energy is stored in the inductor
- no energy is moving from the capacitor to
* the inductor
But the inductor insists that current continue
to flow:
- the capacitor begins to charge with a negarive
* voltage
- energy begins to transfer from the inductor
* back to the capacitor (note the change in the
* direction of energy flow)
- the voltage on the capacitor is increasing
* negatively
- the current in the inductor is dropping
Sometime later:
- the current in the inductor has dropped to
* zero
- the capacitor has a maximum negative voltage
- all the energy is in the capacitor
And this continues forever at the resonant
frequency of the capacitor and inductor circuit.

But no energy is moving from the capacitor to
the inductor when the voltage on the capacitor
is zero and the current in the inductor is
maximum. It is at these times that the direction
of energy flow is changing, as well as when the
voltage in the capacitor is maximum and the
current is zero.

When the voltage on the capacitor is zero, the voltage on the entire system is zero, no matter our reference point. *The system energy is completely contained in the moving current with a direction of energy flow completely defined. For an instant, the inductor is like a superconducting loop.

From a traveling wave standpoint, the resonant capacitor/inductor system contains a positive wave and a negative wave, equally balanced energy wise. *When the capacitor is completely charged, the positive and negative waves are at the reversal/mid point of the cycle where each wave is maximally displaced from center (which is at the electrical center of the inductor). *When the capacitor is completely discharged, the two waves superimpose and both reside in the inductor at identical times. *The energy of both waves is completely contained in the electromagnetic field that exists outside the wires containing the two traveling waves. *Do the waves exist on the wire at this instant, or have they completely desolved into a space field we observe as magnetic force? *The current seems to be flowing so I would say the waves both continue to exist.



I can kinda see how like charges could repell so that waves of like polarity might "bounce" but I can't see how waves of opposite polarity might "bounce". *If waves of opposite polarity "bounced", why would the polarity change during the cycle on each side of the "bounce" point?

An excellent counter-example. I may have fallen into
the trap of looking at the examples that support the
argument rather than looking for the ones that don't.
This will take some cogitating. Maybe its the end
of the line for the "bounce hypothesis".

To me, it is much more rewarding to work with traveling waves that pass through one another, *interacting to create standing waves.

I don't object to this view, as long as the waves are
viewed as having voltages or currents but no power.

Have you considered how energy is transfered between elements of the transmission line over time if we do not have an ongoing application of power? *Doesn't one section of line apply power to the next successive section of line an instant of time later after it received applied power? We agree that a transmitter applies power at the input of a transmission line. *Isn't the first section of transmission line just the power source for the second piece of line?

I think of the traveling waves as transporting power and energy through time and physical distance. *The highest voltage points physically "move" (found in a new location) as time passes, as do the highest current points, and always together in phase. *



The difficulty is that some waves definitely transport
energy while others do not and I do not see a good
explanation for what turns the former into the latter,
as happens, for example, when the pulses collide.

Some waves transport energy, and some do not! *That distinction bothers me less now that I have participated in this thread for a while. *For me, the traveling wave always has current and voltage in phase, and always carries power. *If I can not find power, then we must have a standing wave. *For me, traveling waves is all that we really have, they are primary. *All other waves flow/result from the traveling waves. *

Another observation that has helped me is the recognition that "waves" are just statistical groupings of repetitious events. *It is very convenient to treat waves as physical objects but they are really groupings of much, much smaller events. *How small is the smallest event, no one seems to know, but the highest frequencies still seem to be electromagnetic events. *If the "wave" is composed of a group of much smaller events (such as movement of electrons, or smaller), then it is not so hard to accept that we might detect passage of two waves as having a different voltage and energy level from what we would expect if the voltage and energy were static. *



Would it help your visualization process to observe that when two waves of SAME POLARITY but traveling in opposite directions cross, the currents accompaning the waves are moving in opposite directions both before and after crossing? *When two waves of OPPOSITE POLARITY but traveling in opposite directions cross, the currents are moving in the same direction both before and after crossing.

I do understand (at least I think I do) the methodology
for superposing the waves of voltage and current,
computing the results and deriving the power from the
result.

I just am not happy that this results in waves sometimes
transporting energy and sometimes not, without a good
explanation of the transition.

If we were talking about water, water behind a dam is like voltage, with the height of the water the potential energy, measured in head (feet), or PSI if measured at the bottom of the dam. *A pipe to the bottom of the dam will squirt water at a high velocity but no head or PSI. *The potential energy of the water behind the dam has been converted to kinetic energy measued in velocity of a moving mass. *The moving water can be stopped, and if carefully done, the static head reached by stopping the water will nearly reach the original water level behind the dam. *It would reach the same level if it were not for friction losses. *Electrical current is something like that moving water.

Agreed. I have used this analogy as an aid to understaning
though it becomes challenging at RF.

...Keith

This discussion helps clarify things in my mind. *I hope it helps you as well.

73, Roger, W7WKB

Roger you would be better to start off with first principles. Current
is by definition
a constant at all points where the imagianary vector is the angle or
phase of
a molecular dipole at the point of reference. Nothing is moving
forward. The only movement
of the molecular dipole is on of rotational movement about a static
pont of reference.
One you understand this you can then go on to magnetic fields created
by multiple
inherrant dipoles with respect to their phase angle which magnifies or
nullifies
the created magnetic field
Best regards
Art

Roger Sparks wrote:

I can see why you find "no power" at the zero voltage point, but does that imply that there is no energy flow and no power from every perspective? As I write, I am struggling how to clearly differentiate between "power" as "work done" and energy as "capacity to do work", and what "network" are we defining.

Power at a particular point on the line is the rate of energy flow past
that point. It does no imply that any work is done anywhere, since any
energy flowing past the point can be stored. That is, in fact, exactly
what happens with the open circuited line in my analyses and illustrated
with TLVis1. You can see from the TLVis1 demo 4 that power is present at
all times and places along the line except a few select points. No work
is being done; energy is simply moving back and forth along the line and
between the E and H fields.

. . .

I personally define power as a state/condition where "work 'is being' done", . Power must act over time and have a physical movement component. Voltage by itself does not fulfill this definition because no movement is observed. Current is movement, voltage is only an indication of where a concentration of charges is found.

Of course you're free to define anything in any way you choose. But
you've chosen a definition that's different from the one accepted in all
of electrical circuit analysis and all textbooks. So you can expect to
have a good deal of difficulty communicating with people who are
acquainted with the universally understood definition and assume that's
what you mean, rather than your own personal definition. To them, power
is the time rate of energy flow, dE/dt, period.

. . .

Roy Lewallen, W7EL

Gene Fuller wrote:
This is truly bizarre. The E-field and the H-field are always at right
angles to each other in the plane wave case. They are also perpendicular
to the propagation direction. The same is true for the TEM mode in a
coaxial waveguide, which is the usual case discussed on RRAA. There is
no 0 or 180 degree involvement. I don't know what you are so confused
about, but you really need to rethink what you are saying.

What you are saying is 100% correct for plane EM traveling waves.
What you are saying is 100% incorrect for standing waves. It just
shows that Hecht was right when he said standing waves probably
don't deserve to be called waves. Maybe you should try to understand
why Hecht would say such a thing.

There is no need to sketch or calculate anything. A diagram showing the
relationship of the E-field vector and the H-field vector is in every
E&M and optics book I have ever seen.

Yes, and that diagram is for a *TRAVELING WAVE*, not for a standing
wave. Please find a reference with the E-fields and H-fields diagrammed
for a standing wave and get back to us. Better yet, consider there is
a need to sketch the fields if for no other reason, just to prove me
wrong.

This is not rocket science, Gene. When the magnitude of the Poynting
vector for the forward wave equals the magnitude of the Poynting vector
for the reflected wave, the net Poynting vector is obviously zero.
Since the Poynting vector equals ExH, the only way that the standing
wave Poynting vector can be zero everywhere is for the cross product
of ExH to be zero. The cross product is E*H*sin(A) and we know that
E and H are not zero, so sin(A) must necessarily be zero. So A must
necessarily be 0 or 180 degrees. Nothing else is possible.
--
73, Cecil http://www.w5dxp.com

Jim Kelley wrote:
You didn't click on the link, did you. :-)

Yes, I did, and Firefox said it couldn't find the page.
Why do you post URLs with ... at the end?
--
73, Cecil http://www.w5dxp.com

Jim Kelley wrote:


Cecil Moore wrote:

I've always known that a graph drawn with a pencil on a piece of paper
is not an EM standing wave but, for awhile, you seem to have forgotten
that simple fact.

A plot is simply the graphical representation of an equation.

Yet you said it was "the wave". It is not the wave. It is
simply some carbon rubbed on a piece of paper.

Jim, you requested that I be more precise and that's exactly
what I am being.
--
73, Cecil http://www.w5dxp.com

Keith Dysart wrote:
It would be valuable if you could indicate which of the possible
definitions of "*NET* energy moving" you mean.

Net energy is the difference between the average forward
energy and the average reflected energy. Please forget
instantaneous values. Instantaneous voltage and current
are obviously valuable concepts. For the context of this
discussion, seems to me instantaneous energy or power are
just diversions away from the real issues.

The real question is: Can an EM wave exist without energy?
Roy implies that it can. I challenge him to produce an
EM wave containing no energy.
--
73, Cecil http://www.w5dxp.com