Wasn't going to weigh in on this one, but there are a few inaccuracies here which require correction. No criticism implied, I can't remember where I placed my keys five minutes ago; I will not criticize a lapse of memory over 40 years.
Energy or power density from a point source spreading out into a sphere drops as the square of the distance traveled, since the surface area of the sphere is 4*pi*r^2, and energy is conserved. Note this has nothing to do with light per se. It just is. Electromagnetic energy can travel in a complete vacuum, devoid of any matter whatsoever. The electric and magnetic fields associated with a traveling electromagnetic wave are displacements in the very fabric of space itself. This was demonstrated by Michelson-Morley either very late in the nineteenth or very early in the twentieth century. Ken Javor ---------- >From: geor...@lexmark.com >To: george_t...@dell.com >Cc: brian_ku...@leco.com, emc-p...@majordomo.ieee.org >Subject: RE: How does RF travel through outer space? >Date: Thu, Nov 30, 2000, 4:10 PM > > > Geez, this question makes me feel like I'm on "Who want to be a millionaire"! > Brain should have provided multiple choices for his two questions. > > George, good anaolgy re the pumpkin. Here is some other stuff I remember > from my hated electric field classes some 40 years ago. If the radiator is a > point > source in space, and the energy is equally distributed in a spherical pattern, > the > energy at any point in space drops off as the cube of the distance from the > source. > If the radiator is an infinite line, the energy drops off as the square of the > distance > from the source. If the source is an infinite plane, the energy remains the > same > regardless of the distance from the plane. Are these right? > > Now to the tough part about RF travelling in space. Note that all energy we > receive from the sun comes in some form of electromagnetic energy, ranging > from frequncies which give us light, to infrared which gives us heat, in > addition > to all kinds of non-structered RF signals. > > Doesn't electromagnetic energy depend on the sequential exitation of electrons > from one adjacent molecule to another to "travel" anywhere? It is my > understanding > that space is not a vacuum, but merely a lower pressure than that here on earth. > The > molecules may be further apart (less dense), but are still out there. The only > TRUE > vacuum I know of is what they call a "black hole" in space, which literally > sucks all > nearby matter into it......but this is still a theory. > > Brain, the RF energy, or any other part of the EM spectrum, travels through > space > just as in the water you mentioned, only with different molecules, and spacing > of > same. > > These are my FINAL answers. Did I win anything, or must I go home a loser? > > George > > > > > george_tang%dell....@interlock.lexmark.com on 11/30/2000 03:56:59 PM > > Please respond to george_tang%dell....@interlock.lexmark.com > > To: brian_kunde%leco....@interlock.lexmark.com, > emc-pstc%majordomo.ieee....@interlock.lexmark.com > cc: (bcc: George Alspaugh/Lex/Lexmark) > Subject: RE: How does RF travel through outer space? > > > > > We all just carved pumpkins not too long ago. We can use pumpkins to > explain one of these questions. If you put a 5 watt light bulb at the > center of your carved pumpkin, then each square inch of the internal pumpkin > surface gets the amount of light energy given by the expression: > > 5W / (internal surface area of pumpkin) = light energy per square inch > > Now you move your light bulb to a bigger pumpkin and do the same > calculation. You find that each square inch of your bigger pumpkin gets > less light energy due to a bigger surface area. This is why RF signals drop > off at the rate of 1/distance squared, since the pumpkin surface area is > proportional to the square of the radius. Light is simply a higher > frequency emission than RF, but the same concept applies. > > As far as "How RF travel through vacuum," you can think of it this way: RF > is composed of electric field and magnetic field. Electric field is simply > the attraction force between the positive charges and the negative charges. > And magnetic field is the interaction between 2 current loops. It is not > hard to imagine that refrigerator magnets will work in vacuum or protons and > electrons will attract in outer space. RF is simply the electric and > magnetic fields changing polarity at a very rapid rate. > > George > > > > -----Original Message----- > From: brian_kunde [mailto:brian_ku...@leco.com] > Sent: Thursday, November 30, 2000 12:51 PM > To: emc-pstc > Subject: How does RF travel through outer space? > > > > > > Hello, > > I'm sorry if this is too simple of question... "How does RF travel through > outer > space?". > > I will be teaching a class in which this question will come up. I want to be > prepared with all the basic science behind this principal. I need an > explaination that is simple and easy to understand. > > People seem to have no problem understanding how waves can travel through > mass > such as a body of water but can not understand how it can travel where there > is > no mass. I also understand that there is a lot of debate over how Light > travels > through space (photons and all). > > Also, I understand that RF signals degrade at a rate of 1/distance(squared). > What force is causing this attenuation? > > Try to keep it simple for my audience it not all that technical. Appreciate > the > help. Please forgive any improper punctuation or word misuse. > Brian > > > > > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > > > > > > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
