Re: Questions (Space)
- From: "David M. Palmer" <dmpalmer@xxxxxxxxx>
- Date: Sun, 09 Sep 2007 11:56:15 -0600
In article <MSGID_2=3A240=2F2199.13=40fidonet_4c46a08e@xxxxxxxxxxx>,
Tina Hall <Tina_Hall@xxxxxxxxxxx> wrote:
Logan Kearsley <chrono.surfer@xxxxxxxxxxx> wrote:
Magnetic field lines bend charged particles (electrons, say) around
them. If the electron isn't moving through the field,
I thought that caused the field (see Rik's description). Else where is
the field coming from?
nothing happens to it, but as soon as it starts to move the magnetic
field tries to bend its path so that it will go around a field line
(which, of course, is entirely imaginary- no matter where the electron
is circling around, we'll just imagine a line to go through there, and
call it a a magnetic field line).
That would be ok if I knew where you get the field from.
Moving electrons are an electric current.
A current causes a magnetic field.
If you take a coil of wire and run current through it, you can get a
nice big magnetic field, which extends though empty space near the
coil. You now have a magnetic field in empty space.
You know where the magnetic field came from. But you don't have to
care. In the empty space, it doesn't matter (for the sake of the next
part of the argument) whether the magnetic field is from a coil of
wire, or a bar magnet, or the magnetic field fairy came and waved her
magic wand.
You have some empty space with a magnetic field in it. At any point in
the space it has a strength and a direction. (Take a magnetic compass
and the needle will point in the direction of the field, and the
stronger the field the harder it is to point the needle somewhere
else.) A magnetic field 'line' is the path you take when you start
somewhere and just follow the direction the compass is pointing.
Place an unmoving electron in that empty space. It will just sit
there, unaffected by the magnetic field.
Throw an electron in that empty space, across the field lines. Its
path will bend around the field lines. (If your compass needle wants
to point up, the electron will curve to the left. If your compass
needle wants to point to the left, the electron will curve down, etc.)
If you throw the electron along the field line, in the direction the
compass points (or the opposite direction) it will just go straight,
along the field line. If you throw it at an angle, it will spiral (in
a helix, not a flat spiral) around and along the field lines.
If you had a positively charged particle, instead of a
negatively-charged electron, it would spiral the other way.
So, when you have empty space that makes charged particles do what I
have just described, you say that it has a magnetic field in it. And
you don't have to care where the field came from.
As a digression, a compass needle has electrons whizzing around inside
it. The magnetic field tries to curve their paths, the rest of the
compass needle tries to push the electrons back, and to balance the
torques, the whole compass needle pushes against its bearing, in a way
that makes it 'want' to point North. These whizzing electrons are also
why the compass needle has a magnetic field of its own. That's why you
can use a compass to measure the magnetic field, instead of needing a
less convenient device that throws electrons through space and measures
how much they curve.
In contrast, an electric field is something that pulls on an electron
even if it is not moving. At each point it pulls in a certain
direction with a certain strength, and those are considered the
direction and the strength of the electric field. (Because the
electron is negatively charged, the direction is 180 degrees away from
what I just told you, but that's a detail we can ignore for now.)
So, I might say 'now you know what electric and magnetic fields are'.
Instead I'll be more accurate and say, 'now you know what electric and
magnetic fields do.' (I.e. electric fields pull on charged particles,
magnetic fields deflect moving charged particles.)
The trick is that there is no difference between moving and non-moving
particles. Build a train track through an area with a magnetic field
but no electric field. Load an electron onto the train. You will
notice that the electron just sits there, because there is no electric
field. Start the train running, and the electron will be pulled to the
right. That is because the electron is moving through a magnetic
field.
Now suppose you are riding the train. You are sitting down in your
seat and the electron is stationary in front of you. After a while,
you notice that it is being pulled to one side of the train. Since
there is something that causes a force on a stationary electron, you
know that you are in an electric field.
So you yell out the window to your assistant and tell him that there is
an electric field on the train, in addition to the magnetic field.
Your assistant replies that there is no electric field, there is only a
magnetic field that you are moving through.
The answer (apart from firing your assistant for having the gall to
contradict you) is that there is something called an 'electromagnetic'
field. This field contains both electric and magnetic components, and
it has rules that tell you what happens when you measure it in
different reference frames: a pure magnetic field measured in the frame
of the ground has an electric component when measured in the train's
reference frame. You can use this field to tell what happens to moving
and stationary electric charges as seen in any reference frame.
So now you know what the electromagnetic field 'does'. What it 'is',
(apart from "it 'is' the thing that 'does' that") is not likely to be a
fruitful topic for discussion in this particular thread.
--
David M. Palmer dmpalmer@xxxxxxxxx (formerly @clark.net, @ematic.com)
.
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