Re: magnetic field

From: Sylvia Else (sylvia_at_not.at.this.address)
Date: 09/27/04 

Date: Mon, 27 Sep 2004 13:13:03 +1000

Kevin Kilzer wrote:

> On Sun, 26 Sep 2004 18:53:21 -0400, "Ken" <lera@sympatico.ca> wrote:
>
>
>>I would like to know what is a magnetic field. I mean what is it composed
>>of.
>>I searched google , asked people around me , no one seems to know.
>>Obviously everyone knows where how, but not what.
>>I thought it was electrons, but that cant be.
>
>
> Basically, both electric and magnetic fields store electromagnetic
> enegy. Electric fields can be created by something as simple as a
> battery, and magnetic fields come from magnets or from electrical
> coils while a current flows. Why magnets make magnetic fields is a
> question for quantum physics.
>
> With respect to Sylvia, James Clerk Maxwell expressed a relationship
> between electric and magnetic fields in the mid 1800's. Maxwell's
> contribution was to show that a magnetic field is created as an
> electric field changes, and an electric field is created as a magnetic
> field changes. The result is that if I switch an electric field off
> and on, I get a magnetic field that swells and collapses with each
> on-off transistion.
>
> If I flip the swith fast enough at some frequency, I will find that
> the magnetic field comes and goes with the same frequency. But, since
> a changing magnetic field will create its own electric field, the
> effect moves out to this new field, which makes another magnetic
> field, and so on, and that is radio.
>
> Kevin
>

I'm not disagreeing with Kevin, but there is a simple thought experiment
that shows how careful one must be about taking a theory, such as that
of James Clerk Maxwell, and attempting to use it as anything more than a
description.

Take two electrons, separated in space, stationary relative to some
observer. There's an electric field, obviously, but no magnetic field.
Now take another observer moving perpendicularly to the line joining the
electrons. This observe sees the electrons in motion. Electrons in
motion are an electric current, and an electric current produces a
magnetic field, so for that observer there is a magnetic field present.

So one observer finds a magnetic field present where another observer
finds none. The notion that a magnetic field has a concrete existence is
clearly problematic. This paradox doesn't appear in the theory itself,
because it simply tells you what will happen (or more exactly, what your
measurements will show). It doesn't say anything about what is "really"
there.

Sylvia.

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