magnetic field

[Perion]

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.

And so with any motion related phenomena. Person A is on a flatbed rail car.
Just as the train passes a road crossing A tosses a tennis ball straight up to a
height h and catches it as it comes back down. Person A calculates the distance
the ball has traveled as 2h. Person B is stopped at the crossing watching the
train go by and sees the ball as moving through a parabolic arc. He calculates
that the ball has moved a distance through space equal to the length of the
parabola, greater than 2h. The two observers arrive at completely different
values and describe completely different trajectory geometries. So, how far did
the ball REALLY move through space and what was its ACTUAL trajectory? What was
the ball's ACTUAL kinetic energy in the direction of the train's motion? A says
zero, B says it's half the mass of the ball times the velocity of the train
squared. Either the ball has energy or doesn't - who's right. The problem is
that even notions as simple as "trajectory" or "kinetic energy" aren't entities
in themselves but arise as part of a relationship between the motion of observer
with observed.

Perion

 

[Tom Del Rosso]

It is only assymmetrical because there are no magnetic charges.
Magnetic charge does not appear Maxwell's equations. The symmetries
or lack of them show up more strongly in the four-vector relativistic
formulations.

Kevin said, "a magnetic field is created as an electric field changes,
and an electric field is created as a magnetic field changes".

I thought a magnetic field is proportional to the current, not the
change in current. Whereas an induced current is proportional to the
rate of change of magnetic field. That's the assymetry I refered to,
but I'm seeking clarification. Am I wrong or are we both right?

 

[Sylvia Else]

Kevin said, "a magnetic field is created as an electric field changes,
and an electric field is created as a magnetic field changes".

I thought a magnetic field is proportional to the current, not the
change in current. Whereas an induced current is proportional to the
rate of change of magnetic field. That's the assymetry I refered to,
but I'm seeking clarification. Am I wrong or are we both right?

If you think about it, you'll see that a constant current is equivalent
to a constant RATE OF CHANGE of charge at two places.

Sylvia.

 

[operator jay]

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.

I was told that Einstein showed that there is not actually any magnetic
field and that what we see happen in motors etc. can be explained by
relativistic effects. Maybe this is related to this thought experiment? I
didn't get any further explanation, and I never did look into it any. Hang
on...




[from a google on:
einstein "no magnetic field"

http://www.analogzone.com/col_1028b.htm

Nobody Loves A Transformer, Part 2: Wire Again
by Brian McGinty


....it took Albert Einstein to figure them out. Stationary electrons don't
have a magnetic field. A single electron moving through empty space doesn't
have a magnetic field. Two electrons moving together, still no magnetic
field.

When electrons move relative to each other - towards each other, away from
each other, past each other - a magnetic field appears out of nowhere. In
fact, Einstein discovered the magnetic field is just the electric field as
viewed through changing frames of reference. Considered by many to be quite
a sharp guy, ...




j

 

[Sylvia Else]

I was told that Einstein showed that there is not actually any magnetic
field and that what we see happen in motors etc. can be explained by
relativistic effects. Maybe this is related to this thought experiment?

For sure, since it is exactly that relativistic model that prompted my
use of the idea in the thought experiment.

However, I wouldn't want anyone to get that idea that the field, and
resulting forces, are in some way produced by a relativistic "effect". I
make again my point that the theories only describe - they don't
explain - and they certainly don't produce effects.

Sylvia.

 

[Repeating Rifle]

And so with any motion related phenomena. Person A is on a flatbed rail car.
Just as the train passes a road crossing A tosses a tennis ball straight up to
a
height h and catches it as it comes back down. Person A calculates the
distance
the ball has traveled as 2h. Person B is stopped at the crossing watching the
train go by and sees the ball as moving through a parabolic arc. He
calculates
that the ball has moved a distance through space equal to the length of the
parabola, greater than 2h. The two observers arrive at completely different
values and describe completely different trajectory geometries. So, how far
did
the ball REALLY move through space and what was its ACTUAL trajectory? What
was
the ball's ACTUAL kinetic energy in the direction of the train's motion? A
says
zero, B says it's half the mass of the ball times the velocity of the train
squared. Either the ball has energy or doesn't - who's right. The problem is
that even notions as simple as "trajectory" or "kinetic energy" aren't
entities
in themselves but arise as part of a relationship between the motion of
observer
with observed.

Perion

Both are observing the same invariant phenomenon. Both descriptions are
correct. The Lorentz transformation can be derived by modifying Newtonian
theory the make both descriptions correct.

Bill

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

look at this one http://www.who.int/peh-emf/en/

What I see at that URL is basically no information at all about magnetic
fields.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

A field is a region of influence- hence a magnetic field is a region of
magnetic influence- i.e. there are effects due to the magnetism. Beyond that
one gets into "what is magnetism"

A magnetic field is like porn, I can't describe it, but I know it when I
see it! ;-)

You think magnetic fields are bad! Try describing a gravitational
field! It's *everywhere!* And you can't get away from it!

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

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.

First off, electrons aren't 'stationary'. They're always moving. And
then you can't 'observe' them because the act of observing them disturbs
them. It's the old Heisenberg's Uncertainty Principle.

If you ask me, we're getting into the "Blind Men and the Elephant"
territory.

 

[Sylvia Else]

First off, electrons aren't 'stationary'. They're always moving. And
then you can't 'observe' them because the act of observing them disturbs
them. It's the old Heisenberg's Uncertainty Principle.

The old "disturbance model" of quantum mechanics? That's been know to be
wrong almost since the dawn of the subject.

Sylvia.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

It isn't really composed of anything. It's a region in which a magnetic
force can be detected. Ever seen a police car on the motorway? Everyone
within 100 yards drives exactly at the speed limit. the police call this a
bubble or zone of legality. The zone isn't composed of anythig but you can
"feel the force".

I walked into Circuit City this afternoon and they were playing Star
Wars on the big screen TVs, so I stopped and watched it for a few
minutes. Luke was trying to resist Green Teeth's demand that he come
over to the Dark Side; and when he didn't, old Green Teeth started
zapping him with his hands. Old Darth finally couldn't take seeing his
son being killed, so he threw old Green Teeth over the rail into the
power reactor. And Darth ended up dying in Luke's arms.

Good special FX from Lucasfilm. I can't remember what year that was,
but it's been decades since I saw that movie. And then Han finds out
that Princes Leia is really Luke's sister, and they smooch and kiss, and
all the little gremlins cheer them on. And they blow up the Death Star,
and everyone lives happily everafter...

So what were we saying about Zone of Legality? Bubbles? Whatever
happened to Bubble Memory? There's some neat magnetic fields doing some
neat things. But that's now extinct.

 

[Sylvia Else]

So what were we saying about Zone of Legality? Bubbles? Whatever
happened to Bubble Memory? There's some neat magnetic fields doing some
neat things. But that's now extinct.

Didn't live up to its promise, and was overtaken in performance (speed
and density) by drams. MB memory was non-volatile, I seem to remember,
but that's not such a big issue.

Sylvia.

 

[Dick Alvarez]

<<a magnetic field is created as an electric field changes,
and an electric field is created as a magnetic field
changes".>>

thought a magnetic field is proportional to the current, not
the change in current. Whereas an induced current is
proportional to the rate of change of magnetic field.
That's the assymetry I refered to, but I'm seeking
clarification. Am I wrong or are we both right?>>

Both are right. The confusion arises because we are
talking about two different forms of electric "current".

One form of current is flow of charged particles (e.g.,
electrons) in a wire or other conducting medium, or even as
a beam of charged particles through a vacuum. A charged
particle flow, either steady or changing, creates a magnetic
field around the flow of charged particles.

The other form of current is more sneaky. It goes by the
name "displacement current". To illustrate, suppose that we
have two parallel plates, with a Voltage applied between
them. The applied Voltage causes an electric field between
the two plates. For example, if you put an electron in that
electric field, then the electron sees a force that tends to
move the electron toward the positive plate. If the applied
Voltage is steady, then the resulting steady electric field
does not create a magnetic field. Now suppose that the
Voltage, hence the electric field, between the two plates,
is changing. That *changing* electric field, very loosely
called "displacement current", creates a magnetic field
around the changing electric field. The magnetic field is
proportional to the rate of change of the electric field, as
Kevin Kilzer said above.

Summary: (1) A changing magnetic field creates an
electric field which is proportional to the rate of change
of the magnetic field. (2) An electric current, steady or
changing, consisting of a flow of charged particles like
electrons in a wire, creates a magnetic field which is
proportional to the electric current, whether the current is
changing or not. (3) A changing electric field creates a
magnetic field which is proportional to the rate of change
of the electric field.

The proportionality constants can get messy, and can
involve tensors, which probably you don't want to get into.
At radio frequencies, things also get more involved. At
velocities close to the speed of light, relativistic effects
enter the picture. But the above description is reasonably
good for ordinary situations.

The assymetry that you mentioned, arises because flowing
electrically charged particles are one form of electric
current; whereas magnetically charged particles (magnetic
monopoles) do not exist, at least not in our ordinary world,
so magnetic current does not exist.

We owe a lot of this to the great physicist James
Maxwell, of the middle-1800s. Among other things, he laid
the foundation of much of modern electronics.

Dick Alvarez
alvarez at alumni dot caltech dot edu

 

[Tzortzakakis Dimitrios]

I can give you my point of view:scientists (and applied science) always
looks *the use* of certain inventions, and not in-depth-view of what
magnetic field is really.So, you have a rotating magnet inside a
stator:voila, an alternator. You have a stator that creates a rotating
magnetic field and inside it a "squirrel cage" rotor:voila, an asynchronous
rotor.Without both the inventions, todays world would not exist. Imagine a
refrigerator motor with brushes.Or a fan motor with them.Or generating
electricity only with DC.Actually, the equations of Maxwell, describe an
electromagnetic field, used to transmit TV and radio, and have nothing to do
with pure magnetism (even in the more complex form of the rotating magnetic
field).The magnetism...is something.