A member of this newsgroup has been sending me very sarcastic and insulting
messages about the "new theory of physics" implicit in a message I recently
posted about the difference between fields and waves. (This was in answer
to someone asking about emissions from computer terminals, which is why it’s
in sci.med.)
What follows is an explanation of the relevant physics in more detail.
If you’re already clear on the difference between fields and radiation,
you can stop here.
—
My "new theory" is merely that of James Clerk Maxwell, circa 1890.
The crucial point is that electromagnetic radiation (radio waves, light
waves, etc.) is more than just an oscillating electric or magnetic field.
This point is often misunderstood even by people who have been through
an introductory physics course; it’s easy to manipulate equations without
being aware of what they mean.
Maxwell’s equations state, among other things, that:
(1) A moving electric charge creates a magnetic field;
(2) A moving magnetic field creates an electric field (charge gradient).
The first of these is how a motor works; the second is how a generator works.
Maxwell then asked himself if a moving electric field and a moving
magnetic field could create _each other_, i.e., propel each other along
through space. The answer is yes, _if_ they are both oscillating and
moving precisely at the speed of light, with electric and magnetic vectors
at right angles to each other.
That’s electromagnetic radiation. Examples are light, X-rays, and radio
waves. The term "radiation" refers to the fact that it carries energy
off into space.
A magnetic or electric _field_ is not, generally, the same kind of thing.
A field, even an oscillating one, does not carry energy away from the
source unless something enters the field and intercepts the energy.
If it did, transformers (for example) would be impossible; a transformer
relies on the fact that an electric field can be converted into a
magnetic field and then back again efficiently, and many transformers
work quite well with no shielding.
What does this have to do with VDTs? Simple. The main emission from a
VDT is an oscillating magnetic field, which is not radiation and should
not be confused with radiation. (Whether it is hazardous is a separate
question; the point is that it’s not the *same* hazard as electromagnetic
radiation.)
–
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Michael A. Covington | Artificial Intelligence Programs
The University of Georgia | Athens, GA 30602 U.S.A.
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In article <1991Apr23.151327.9…@athena.cs.uga.edu> mcovi…@athena.cs.uga.edu (Michael A. Covington) writes:
[A lot of half-informed stuff, including:]
>A magnetic or electric _field_ is not, generally, the same kind of thing.
This is confusing near-field and far-field effects.
>A field, even an oscillating one, does not carry energy away from the
>source unless something enters the field and intercepts the energy.
>If it did, transformers (for example) would be impossible; a transformer
>relies on the fact that an electric field can be converted into a
>magnetic field and then back again efficiently, and many transformers
>work quite well with no shielding.
But transformers do leak magnetic flux, and this flux can induce
currents in other objects at some distance. Energy is definitely
"radiated"; however, the near-field effects fall off much faster
than inverse-square, and due to the tiny size of the transformer
compared to the wavelength of 60 Hz current (3000 miles!), the
far-field radiation is minuscule. However, most people spend lots
of time in the near-field region of the line/ground loop of feeder
power lines and such.
>What does this have to do with VDTs? Simple. The main emission from a
>VDT is an oscillating magnetic field, which is not radiation and should
>not be confused with radiation.
This is a false dichotomy. Someone working with magnetic-deflection
VDT’s (as I do) is close to the near-field region; their distance
from the deflection coils is only a few times the dimensions of those
coils. The near-absence of far-field radiation is irrelevant.
Incidentally, electrostatic-deflection CRT’s have negligible
time-varying magnetic fields, and it is much easier to shield
an electric field (one uses conductive coatings such as metal
foils). Any VDT hazard could be eliminated with a small change
in CRT technology.
Disclaimer: I don’t build VDT’s or CRT’s for a living. I do
have a degree in Electrical Engineering.
Cheers, Ron.