Re: Energy Cancellation (or conversion of radiation to dark rest mass?)

On Jan 31, 3:26 am, Roy Lewallen <w...@xxxxxxxxx> wrote:
AI4QJwrote:
"Roy Lewallen" <w...@xxxxxxxxx> wrote in message
news:13pv49i42n10v46@xxxxxxxxxxxxxxxxxxxxx

Wow, what a lack of misunderstanding!

Of course anyone can assign a relative sign in front of a power quantity..
For example, it might be convenient to assign a minus sing to reflected
power to distinguish it from forward power. But in no way does assigning a
minus sign as a means to signify "reflected power" also mean that the power
is "negative". It simply means positive power moving in the negative
direction! This as opposed to negative voltage which truly is negative with
respect to an absolute reference point of earth ground.

No, negative power means that *energy* is moving in the direction we've
declared to be negative. This follows directly from the definition of
power as the time derivative of energy. No movement of power is necessary.

Here is a simple question: Real power = I**2/R

How can real power be negative?

The use of "real" indicates that you're referring to *average* power.
I'm not. If when I say "power" you read "average power", then
misunderstanding is certain to occur.



I assume you will not tell me to use negative resistance or jI.

How can the energy in the power ever be negative? I assume you will not tell
me to consider negative time.

Give me ONE example of how real power expressed this way can EVER be
negative. Every time you give me a v(t)*i(t) and assign a sign to indicate
direction of current flow -i(t), I will ask you to calculate i(t)**2/R and
tell you to make that a negative number.

This is straight from EE101.

You apparently took a different curriculum than I did. The references I
have (see the end of this posting) support what I learned, that power =
v(t) * i(t), which obviously (I hope it's obvious anyway) can be
negative as well as positive. Let's look at a simple circuit consisting
of a series resistor, capacitor, inductor, and voltage source. The
equation relating i(t) and v(t) looks like this (I don't recall the
course number, but the problem appears less than half way into my
elementary circuits text, so it was probably first semester circuit
analysis):

L*di/dt + Ri + 1/C * Integral(i dt) = v

If you multiply both sides by i, you have v * i on the right, which is
power, and on the left you have i^2 * R for the central term, but two
more terms to deal with. So it's hopefully obvious that power isn't i^2
* R. Or if not, it should at least show you that v(t) * i(t) isn't equal
to i^2(t)/R. And all this without having to introduce negative time.
Incidentally, the above equation is correct regardless of the nature of
v(t) -- it doesn't need to be sinusoidal or even periodic. So it's as
basic as they come.

If you plot the power (v * i) at some point in this circuit, you'll find
that it's positive for part of each cycle and negative for the remainder
of the cycle. If you do the integration of this over a cycle and divide
by the period in order to get the average power, you'll find that the
average power (what you've called "real power") is indeed I(RMS)^2 * R,
which is positive.

If a circuit has only resistance and no way to store energy, then v(t) =
i(t) * R, power is i^2 * R, and power is always positive. Negative power
occurs when energy is stored for part of a cycle, then returned during
the rest of the cycle, as in the series RLC circuit. Or a transmission
line which isn't perfectly terminated.

A recurring problem is confusion between power, which is v(t) * i(t),
and average power, which is the integral of this over one cycle divided
by the period. Anyone confused by this can easily find ample study
material in any elementary electrical circuits text book. The discussion
of RMS athttp://www.eznec.com/Amateur/RMS_Power.pdfmight also be
helpful in understanding the relationship between power and average
power, although it doesn't deal with energy storage.

Many people learn only about average power because it's adequate for
most everyday applications, and the math involved with calculating
instantaneous power is considerably more involved. But anyone trying to
understand the movement of energy in a system capable of storing energy
really needs to understand power on a more fundamental level, as the
time derivative of energy movement. And I hope it's still being taught
in engineering schools!

It's no wonder that people are having so much trouble understanding energy
movement, and why it's so easy to lead them astray with red herrings and
pseudo-science,

OMG, give me a break. Tell me about the law of conservation of negative
energy.

If you'll look in a basic physics text you'll find plenty of references
to negative potential energy. That concept, however, isn't necessary in
order to use and understand the directional property of power.

when there's so much misunderstanding about the underlying concepts and
terminology. If you start with concepts which are fundamentally erroneous,
the knowledge you build on them is bound to be wrong.

Tell me how to make I**2/R a negative number that generates negative heat
and which is not fundamentally erroneous.

I^2*R is one expression of average power, which has never been what I've
been discussing. So I'll gladly leave this proof to you.

Yes, thuis truly is fundamental. I am rather surprized at this diatribe and
how it exposed *your* lack of understanding of fundamental concepts.

AI4QJ

I plead guilty to not knowing everything, and to always striving to
learn more. I am comforted, however, by the number of respected
professors and textbook writers who share my lack of understanding of
these concepts. For example,

1. Pearson and Maler, _Introductory Circuit Analysis_, p. 251:

"For our conventions the power to the right is p = vi. . ."
"The capital letter P is used to designate average power, whereas the
lower case letter p is a function of time. Figure 5.14 shows voltage,
current, and power as a function of time. At those instants of time at
which power is negative, energy is flowing to the left in Fig. 5.13."
The little program TLVis1 I introduced a while ago generates waveforms
for voltage, current, and power very much like fig. 5.14.

2. Van Valkenburg, _Network Analysis_, 3rd Edition, p. 420:

"The rate at which energy is being absorbed [by a one port network] is
the power given by p = dw/dt = v(t)i(t) W (watts)" Figures 14-4 and 14-5
on the following pages show graphs of the power for an inductor and
capacitor respectively which are sinusoidal and centered around zero. On
p. 424: "Energy is the integral of power. Using the convention for sign
illustrated by Fig. 14-2, we see that, for positive p, energy is being
supplied to the inductor and capacitor for storage. When p is negative,
energy is being returned to the source. We know that no more energy can
be returned than supplied, of course, and the fact that w [the energy]
returns to zero value for every cycle of the voltage and current implies
that the energy supplied ins returned entirely every cycle for the
inductor and capacitor."

I'm curious. What text did you use for your EE101 course which says
otherwise?

Roy Lewallen, W7EL

Roy, your references and your own statements are only saying that,
yes, it can be or usually is an engineering conevntion to put a minus
sign in front of P to indicate direction. I violently agree! As you
know, quoting from my preceding post:

"Of course anyone can assign a relative sign in front of a power
quantity.
For example, it might be convenient to assign a minus sing to
reflected
power to distinguish it from forward power. But in no way does
assigning a
minus sign as a means to signify "reflected power" also mean that the
power
is "negative". It simply means positive power moving in the negative
direction!"

Then you proceeded to go through all the trouble to cite beginner
engineering textbooks (I guess that's an insult or something) in which
this was, in fact, being done. Fine. Obviously I agree because I said
the same thing in the preceding post.

What you originally disagreed with me on was my statement that there
is no such thing as "negative power". On a relative scale, there
obviously is such a thing as positive power going in a direction that
we define as positive and also the opposite direction that we define
as negative. But in terms of an absolute scale or reference there is
*no such thing* as negative power. We cannot remove heat by passing
current through a resistor. Why is this important? Because in this
discussion people were getting confused and started adding and
subtracting powers as if they were conservative field vectors (such as
voltage). Power can only be vectored by it's direction of movement. It
is not a field vector. The point is (pardon the pun), to superimpose
waves you must do so only with field vectors, not directional vectors,
or in the very least you cannot mix the two. There is no power
"vector" which can reduce to zero by adding a positive power to a
negative power. At the point where you add the positive and negative
powers, does it cancel to zero? No, it quadruples! I can assign a
negative sign to all money that is no longer in my bank account; that
does not mean it is negative money, it is still positive money but it
exists elsewhere. This is extremely fundamental. It is like I am
saying there is no such thing as negative temperature, that is, below
the reference of absolute zero. Then Roy comes back and says "Oh yes
there is, the temperature outside was -15F last night. My Textbook
says that, for the purposes of this book, all references to degrees
shall be in Fahrenheit. There obviously is a temperature below 0F
which is negative, thus negative temperatures must exist." If you can
just bridge the gap between relative to absolute, then you will
partially understand why you cannot add power "vectors" like you can
add vectors from a conservative field.

AI4QJ

.

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