Re: Questions (Space)

On Fri, 7 Sep 2007 22:05:00 GMT+1, Tina Hall wrote:

If there's a problem, it's translating. Not English, but how you
understand it into something I understand. Guessing the right analogies
can be difficult (with all the years of us posting here you still don't
have the direct experience of teaching me something and seeing the
result face to face), some hit home, others just confuse. So I try to
add my own asking whether they're ok.

Ok, let's try it. Here goes --

One of the first useful things people found out about electricity and
magnetism is that they are connected. The connection is that if one
changes, it creates the other, and vice versa.

If we have a particle with an electric charge, an electron for instance,
there is a zone around it in which we can find out that there's an
electron nearby because we can detect the electricity. That area is
called the "electric field". The usual way of depicting it is lines
going straight out from the electron, radially. In three dimensions,
much like a sea urchin or one of those fluffy balls some people use for
taking a bath. The lines aren't really there, mind you; the zone is
homogeneous, but thinking about it as lines is handy. I think of
electric field lines as being red. People often refer to it as just the
"E field."

Similarly, the zone around a magnet is a "magnetic field". "M" is used
for mass, so the magnetic field is called the "B field." (I don't know
why they used "B".) Like the electric field, it's commonly depicted as
lines; I think of them as blue. Magnetic field lines are always a loop
that goes back through the magnet, while electric field lines are radial
(important to know for some things, but it doesn't matter here.)

In either the electric field or the magnetic field, energy is stored. I
think of it as space being like a spring, or perhaps rubber -- it's
stretched or compressed, which stores energy.

Start with a magnetic field and move away from the magnet, so all you
can see is blue lines. Pick a vertical one for now, and snip off a bit
of one line. Now you have a little bit of isolated magnetic field, not
connected to anything. It has a direction, say from bottom to top (a
little arrow point at the top might help; it does for me.)

Can't be, you say -- and you're absolutely right; that isolated snippit
can't exist without the magnet to keep it there. So it goes away. The
line becomes shorter and shorter until it winks out at a point.

But the energy that was stored in that bit of magnetic field is still
there. It can't just vanish. It has to go into some other effect. Where
did it go?

When a magnetic field changes or moves, it creates an electric field
perpendicular to it. As the vertical blue line shrinks, a horizontal red
one grows; when the blue line disappears, the red one will be the same
length the blue one was originally -- that is, the bit of isolated
electric field now contains the energy that was originally in the bit of
magnetic field. Its arrow point goes to the right.

Of course the bit of electric field has the same problem the magnetic
field did; it can't exist without a particle nearby. So it, too, starts
to go away -- and, as it shrinks, it creates a perpendicular magnetic
field. This time, though, the direction, the arrow point, will be
opposite to the original one, top to bottom. The electric field shrinks
and the magnetic field grows, until the electric field is zero and the
magnetic field has all the energy. Then the magnetic field shrinks,
creating an electric field with its arrow to the left, then the electric
field shrinks, creating a magnetic field with its arrow up. Back where
we started -- one cycle.

That process, of magnetic field creating electric field creating
magnetic field, goes on forever once begun. It's one of the rules of the
Universe (nobody really knows why) that the place where it happens can't
stand still, so the whole process is moving through space at the speed
of light. In this case, it will be either toward you or away from you.
(The direction depends on Time. As Jonathan points out, there is no time
at the speed of light, so from the point of view of the bit of energy
we're looking at the effects of it goes both ways. Only you and I are
limited enough to choose a direction.)

The alternating strong-and-weak of the electric and magnetic fields is
what is spoken of as "waves". The rate at which the alternation happens
is the "frequency". The speed of light, divided by the rate of
alternation, is the "wavelength". The whole system, of electric field
and magnetic field swapping energy, and the locus of that exchange
moving through space, is "electromagnetic radiation".

If there is a lot of energy in the system, the alternation happens very
fast so the wavelength is very small. If there is little energy in the
system, the alternation happens slowly and the wavelength is large.
Radio doesn't have much energy, so the alternation only happens a few
times, up to a few million times, per second, and the wavelength comes
out to kilometers or meters. Light has a lot of energy, so the
alternation of B and E field happens billions of times per second and
the wavelength is in micrometers. But the same process happens in both
effects -- the energy is stored in electic field and magnetic field,
alternately, as it travels through space. That's why I said before that
radio is cool and slow, while light is hot and fast.

And of course you could just as well start with a snippit of electric
field, which would decay to produce a B field, which would decay to make
an E field... in fact, this is what a radio transmitter does. We make an
electric current flow in the antenna. This creates a bit of isolated B
field, which decays to form an E field, and so on.

There's much more, but is this bit useful and/or understandable?

Regards,
Ric

--
Posted via a free Usenet account from http://www.teranews.com

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