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