"Sloshing" EM Energy
 

It has been said that the energy stored in the standing waves
of a transmission line just "sloshes" around.

We can demonstrate standing waves using a laser beam normal to
a perfect mirror. There are points of maximum irradiance and
points of minimum irradiance in the standing waves. So does
the EM energy in the standing waves of light in free space
"slosh" around like the energy in the standing waves in a
transmission line? If so, where does the inductance and
capacitance in free space come from to generate that 377 ohms
of characteristic impedance? If not, then why do the EM waves
in a transmission line behave differently than the EM waves
in free space? What different laws of physics do photonic waves
in transmission lines obey than do photonic waves in free
space? Of the E-field and H-fields rules for EM waves in free
space, which of those rules are violated by EM waves in a
transmission line? Is there one set of Maxwell's equations for
free space and a separate set for transmission lines? Did
Maxwell ever mention the scientific concept of "sloshing"?
--
73, Cecil http://www.qsl.net/w5dxp

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Cecil Moore wrote:

It has been said that the energy stored in the standing waves
of a transmission line just "sloshes" around.

We can demonstrate standing waves using a laser beam normal to
a perfect mirror. There are points of maximum irradiance and
points of minimum irradiance in the standing waves. So does
the EM energy in the standing waves of light in free space
"slosh" around like the energy in the standing waves in a
transmission line?

Yes -- there's energy actively bouncing around in that there beam; if
you could reduce it down to one laser burst that was shorter than the
distance between the laser and the mirror you'd (in theory at least) be
able to see it.

If so, where does the inductance and
capacitance in free space come from to generate that 377 ohms
of characteristic impedance?

They don't. The behavior of EM radiation in free space is described by
Maxwell's laws. The 377 ohms of characteristic impedance comes from the
permittivity and permiability of free space but inductance and
capacitance are only meaningful concepts if you have conductors in your
model.

If not, then why do the EM waves
in a transmission line behave differently than the EM waves
in free space?

Because they're bounded by conductors.

What different laws of physics do photonic waves
in transmission lines obey than do photonic waves in free
space?

None. They obey Maxwell's laws.

Of the E-field and H-fields rules for EM waves in free
space, which of those rules are violated by EM waves in a
transmission line?

None.

Is there one set of Maxwell's equations for
free space and a separate set for transmission lines?

No, just different boundary conditions to start.

All this is covered in a good college E&M course. I wish I had an E&M
book that I could recommend for self-study, but I don't. Mine is
"Elements of Engineering Electromagnetics", but I took a course. I
don't think I would have been able to just pick up the book and learn it
from there.

Did
Maxwell ever mention the scientific concept of "sloshing"?

Who knows? And was he talking about light waves or a wee dram of
whiskey at the end of the day?

As hard as it may be to believe for anyone who's gone through an E&M
course the original form of Maxwell's equations were more difficult to
comprehend than the way there're usually presented now -- the vector
notation that is currently used either wasn't around then or wasn't in
widespread use.

-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Fri, 17 Jun 2005 15:48:08 -0500, Cecil Moore
wrote:
We can demonstrate standing waves using a laser beam normal to a perfect mirror.
"We?"

Let's see, a hypothetical argument, involving
a hypothetical "We," performing
a hypothetical analysis that contains 0 places of precision, yielding
a hypothetical answer that will be
hypothetically true and
hypothetically false
hypothetically

"It was the best of times, it was the worst of times."

All this is covered in a good college E&M course. I wish I had an E&M
book that I could recommend for self-study, but I don't. Mine is
"Elements of Engineering Electromagnetics", but I took a course. I don't
think I would have been able to just pick up the book and learn it from
there.

I would recommend:

Introduction to Electromagnetic Fields, by Paul and Nasar, 3rd edition.
ISBN: 0070460833. Available from www.bn.com, used, from $59. The review of
vector calculus in the first two chapters is excellent. The text covers
plane waves incident on material boundaries (and the resultant standing
waves). It also covers transmission lines.

Regards,

Frank

Tim Wescott wrote:
Cecil Moore wrote:

It has been said that the energy stored in the standing waves
of a transmission line just "sloshes" around.

We can demonstrate standing waves using a laser beam normal to
a perfect mirror. There are points of maximum irradiance and
points of minimum irradiance in the standing waves. So does
the EM energy in the standing waves of light in free space
"slosh" around like the energy in the standing waves in a
transmission line?


Yes -- there's energy actively bouncing around in that there beam; if
you could reduce it down to one laser burst that was shorter than the
distance between the laser and the mirror you'd (in theory at least) be
able to see it.

If so, where does the inductance and
capacitance in free space come from to generate that 377 ohms
of characteristic impedance?


They don't. The behavior of EM radiation in free space is described by
Maxwell's laws. The 377 ohms of characteristic impedance comes from the
permittivity and permiability of free space but inductance and
capacitance are only meaningful concepts if you have conductors in your
model.

If not, then why do the EM waves
in a transmission line behave differently than the EM waves
in free space?


Because they're bounded by conductors.

What different laws of physics do photonic waves
in transmission lines obey than do photonic waves in free
space?


None. They obey Maxwell's laws.

Of the E-field and H-fields rules for EM waves in free
space, which of those rules are violated by EM waves in a
transmission line?


None.

Is there one set of Maxwell's equations for
free space and a separate set for transmission lines?


No, just different boundary conditions to start.

All this is covered in a good college E&M course. I wish I had an E&M
book that I could recommend for self-study, but I don't. Mine is
"Elements of Engineering Electromagnetics", but I took a course. I
don't think I would have been able to just pick up the book and learn it
from there.

Did
Maxwell ever mention the scientific concept of "sloshing"?


Who knows? And was he talking about light waves or a wee dram of
whiskey at the end of the day?

As hard as it may be to believe for anyone who's gone through an E&M
course the original form of Maxwell's equations were more difficult to
comprehend than the way there're usually presented now -- the vector
notation that is currently used either wasn't around then or wasn't in
widespread use.

-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Hi Tim,
Cecil is just trying to crowd Roy into slapping leather
(figuratively speaking). Cecil thinks he already knows the
answer to all these questions, so there's no
point in answering him. He'll be at it for awhile, until he
realizes rhetorical confrontation won't work.
73,
Tom Donaly, KA6RUH

"Richard Clark" wrote -
"It was the best of times, it was the worst of times."

=============================

Richard, now you're plagiarising Charles Dickens.
----
Reg.

Tim Wescott wrote:
. . .
All this is covered in a good college E&M course. I wish I had an E&M
book that I could recommend for self-study, but I don't. Mine is
"Elements of Engineering Electromagnetics", but I took a course. I
don't think I would have been able to just pick up the book and learn it
from there.
. . .

A few months ago, I came upon a book that really looks like it might
fill the bill: _Engineering Electromagnetics_ by Nathan Ida. The text is
clear but doesn't skimp on math or theory. At the end of each section,
there are numerous examples showing how the concept is applied in the
solution of real problems -- something sorely missing in most other
texts and, for that matter, in a lot of college courses. For example,
after the "Inductance and Inductance" section in the "Magnetic Materials
and Properties" chapter are the following fully worked and explained
examples:

Application: Self-inductance of a toroidal coil
Application: Self-inductance of a long solenoid - Inductance per unit
length
Application: Inductance per unit length of coaxial cables
Application: Mutual inductance between a wire and a toroidal core -
core memory
Mutual inductance between straight wire and loop
Self- and mutual inductances in multiple coils

It's sort of like a Shaum's Outlines and textbook combined, but in a way
that you can see the transition from the theory to practice. It's also a
good reference to use later on.

And the answers to all the problems (but no details about how they were
solved) are at the back of the book.

I was lucky and found a used one at Powell's while browsing in their
technical bookstore, but even new it's a bargain.

Roy Lewallen, W7EL

Did
Maxwell ever mention the scientific concept of "sloshing"?

No. The electron had not yet been discovered.

Tim Wescott wrote:
All this is covered in a good college E&M course.

Uhhhh Tim, those were rhetorical questions aimed at people
who believe that the energy in EM waves can slosh around
at sub-light speeds. Where the heck did that idea come
from anyway?
--
73, Cecil http://www.qsl.net/w5dxp

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"Reg Edwards" wrote in message
...

"Richard Clark" wrote -
"It was the best of times, it was the worst of times."

=============================

Richard, now you're plagiarising Charles Dickens.
----
Reg.

Cecil, define 'sloshing.'

Walt,W2DU