wrote in :

What Maxwell's Equations say is a moving magnet produces a moving magnetic
field.

A moving magnetic field is not an electromagnetic field.


Never mind Maxwell, why is no-one asking "why is an electromagnet not used as
an antenna when driven by AC?" That should be a basic reality check because
I've neve heard of such a thing working, and if it did it would be widely
known.

gareth wrote:
"Brian Reay" wrote in message
...
Waving a magnet will not generate an EM wave, it won't even induce a
current unless there is a conductor to hand.

From Maxwell, del cross E = -dB/dt

Maxwell's equations come as a 'set' to generate an EM wave,

I regret that you have only part of the story, for Maxwell's Equations
describe _ALL_ electrical phenomena.

Nope; disproved mid-20th century.

Maxwell's equations are a classical approximation to quantum electrodynamics
much like Newtons Laws are a classical approximation to situations much
slower than the speed of light.

Maxwell's equations do not apply when the particle nature of electromagnetic
radiation is concidered or for very strong electric fields, for example.



--
Jim Pennino

wrote in message
...
What Maxwell's Equations say is a moving magnet produces a moving magnetic
field.

Maxwell's Equations say nothing about moving magnets.

However, Jimp's equations are a different matter.

gareth wrote:
wrote in message
...
What Maxwell's Equations say is a moving magnet produces a moving magnetic
field.

Maxwell's Equations say nothing about moving magnets.

A moving magnet produces a time varying magnetic field.

Are you now saying that Maxwell's equations do not apply to a time
varying magnetic field, gas bag?



--
Jim Pennino

In message ,
writes
Jeff wrote:


You are not 'being dense', you are perfectly correct.

Waving a magnet will not generate an EM wave, it won't even induce a
current unless there is a conductor to hand.

Likewise, waving a battery around, won't generate an EM wave either.

Maxwell's equations come as a 'set' to generate an EM wave, you can't
start with just one. That was one of the flaws in the Cross Field
Antenna theory-or the original one, it varied as it was challenged. It
had other flaws, eg the idea that the Poynting vector was some 'extra'
physical phenomenon which could be 'synthesised', rather than just a
mathematical vector representation of the power in the E and M fields.

As I pointed out in a previous post, the differential term is zero in
the absence of one of the fields so the equations have no, non-trivial,
solutions.

As I recall, this is one of the standard things you are taught when you
attend a lecture on Maxwell's Equations. Perhaps someone missed a
lecture (or more),has lost some crucial pages from his notes,or hasn't
got a clue.

Like all equations, if you apply them correctly, Maxwell's equations do
work. However, if you can't understand them, you will mislead yourself.

Thank you, Brian, I was beginning to doubt myself there. Not having
had the benefit of a University education like Gareth I tend to take
an empirical approach to this sort of thing.


Unfortunately this time Brian is not correct and a rotating magnet will
indeed produce an EM wave. The point that you have missed Brian is that
the differential term is zero for a static magnetic field, but when the
magnetic field is rotating analysis of the equations reveal that there
must be an E field produced in order to satisfy the full set of equations.

Have a look at the link in my previous post.

Jeff

Even if true, the presence of an E field and an H field alone is NOT
the same thing as an electromagnetic field.

If a permanent magnet was oscillated inside a loop of wire, it would
produce an oscillating current in the loop. An oscillating current would
produce RF radiation (especially if it was tuned with a capacitor to the
frequency of the magnet's oscillation).

The question then is, "is the wire loop necessary to produce RF
radiation?"


--
Ian

On 09/11/2014 17:55, Jeff wrote:


You are not 'being dense', you are perfectly correct.

Waving a magnet will not generate an EM wave, it won't even induce a
current unless there is a conductor to hand.

Likewise, waving a battery around, won't generate an EM wave either.

Maxwell's equations come as a 'set' to generate an EM wave, you can't
start with just one. That was one of the flaws in the Cross Field
Antenna theory-or the original one, it varied as it was challenged. It
had other flaws, eg the idea that the Poynting vector was some 'extra'
physical phenomenon which could be 'synthesised', rather than just a
mathematical vector representation of the power in the E and M fields.

As I pointed out in a previous post, the differential term is zero in
the absence of one of the fields so the equations have no, non-trivial,
solutions.

As I recall, this is one of the standard things you are taught when you
attend a lecture on Maxwell's Equations. Perhaps someone missed a
lecture (or more),has lost some crucial pages from his notes,or hasn't
got a clue.

Like all equations, if you apply them correctly, Maxwell's equations do
work. However, if you can't understand them, you will mislead yourself.

Thank you, Brian, I was beginning to doubt myself there. Not having
had the benefit of a University education like Gareth I tend to take
an empirical approach to this sort of thing.


Unfortunately this time Brian is not correct and a rotating magnet will
indeed produce an EM wave. The point that you have missed Brian is that
the differential term is zero for a static magnetic field, but when the
magnetic field is rotating analysis of the equations reveal that there
must be an E field produced in order to satisfy the full set of equations.

Have a look at the link in my previous post.

Jeff

You are confusing a rotating magnetic field with one which is changing
due to an alternating current producing it.

On 09/11/2014 18:07, Lostgallifreyan wrote:
Jeff wrote in :

E cross H in mathematical terms.


'Cross product' of E and H? As in, related to 'dot product'? If so that might
be my wau in to understanding Maxwell, I've used dot product before,
translated myself to find intersections of lines, and saw cross product
closely related, documented pretty much beside it a couple of times.




A cross product of two vectors is a third vector orthogonal to the first
two. In the case of the EM wave, the E and H vectors are orthogonal so
the cross product is in the third dimension, the direction of propagation.

A dot product is a scalar quantity, ie it has magnitude but no
direction. As you say, it is used to find where lines, represented as
vectors, intersect.

"Brian Reay" wrote in message
...

You are confusing a rotating magnetic field with one which is changing
due to an alternating current producing it.

It is a frequent sneer of yours that some subject or other is taught in the
first
couple of years at secondary school, and yet you are setting yourself up
as the target for such a sneer by making comments such as that quoted above.

In AC motors you can have both; a rotating magnetic field produced by
alternating
currents.

Magnetic fields are universally the same and do not "care" whatever was the
agency
that creates them

On 09/11/2014 19:37, Ian Jackson wrote:
In message ,
writes
Jeff wrote:


You are not 'being dense', you are perfectly correct.

Waving a magnet will not generate an EM wave, it won't even induce a
current unless there is a conductor to hand.

Likewise, waving a battery around, won't generate an EM wave either.

Maxwell's equations come as a 'set' to generate an EM wave, you can't
start with just one. That was one of the flaws in the Cross Field
Antenna theory-or the original one, it varied as it was challenged. It
had other flaws, eg the idea that the Poynting vector was some 'extra'
physical phenomenon which could be 'synthesised', rather than just a
mathematical vector representation of the power in the E and M fields.

As I pointed out in a previous post, the differential term is zero in
the absence of one of the fields so the equations have no,
non-trivial,
solutions.

As I recall, this is one of the standard things you are taught when
you
attend a lecture on Maxwell's Equations. Perhaps someone missed a
lecture (or more),has lost some crucial pages from his notes,or
hasn't
got a clue.

Like all equations, if you apply them correctly, Maxwell's
equations do
work. However, if you can't understand them, you will mislead
yourself.

Thank you, Brian, I was beginning to doubt myself there. Not having
had the benefit of a University education like Gareth I tend to take
an empirical approach to this sort of thing.


Unfortunately this time Brian is not correct and a rotating magnet will
indeed produce an EM wave. The point that you have missed Brian is that
the differential term is zero for a static magnetic field, but when the
magnetic field is rotating analysis of the equations reveal that there
must be an E field produced in order to satisfy the full set of
equations.

Have a look at the link in my previous post.

Jeff

Even if true, the presence of an E field and an H field alone is NOT
the same thing as an electromagnetic field.

If a permanent magnet was oscillated inside a loop of wire, it would
produce an oscillating current in the loop. An oscillating current would
produce RF radiation (especially if it was tuned with a capacitor to the
frequency of the magnet's oscillation).


That is not the situation under discussion.

No one disputes that, if you CHANGED the situation to the one you
describe, then it would produce RF (ie EM waves).

BUT, there is no wire etc.



The question then is, "is the wire loop necessary to produce RF radiation?"



Or something similar, yes.

Essentially, the rotating field can cause a current to flow in a
conductor, that current will produce another magnetic field. The current
is charge movement, therefore you have an electric field. That
and the 'new' magnetic field can produce an EM wave. No different to any
other coil 'excited' with an AC current- perhaps the method of exciting
the coil is a little unconventional ;-)

On 2014-11-09 20:00:10 +0000, Brian Reay said:

On 09/11/2014 18:07, Lostgallifreyan wrote:
Jeff wrote in :

E cross H in mathematical terms.


'Cross product' of E and H? As in, related to 'dot product'? If so that might
be my wau in to understanding Maxwell, I've used dot product before,
translated myself to find intersections of lines, and saw cross product
closely related, documented pretty much beside it a couple of times.




A cross product of two vectors is a third vector orthogonal to the
first two. In the case of the EM wave, the E and H vectors are
orthogonal so the cross product is in the third dimension, the
direction of propagation.

A dot product is a scalar quantity, ie it has magnitude but no
direction. As you say, it is used to find where lines, represented as
vectors, intersect.

Does a rotating magnet in vacuo (no conductors around to carry current)
produce an electric field? Sounds a simple question, which has been
answered each way by several people. Anyone who is interested but not
absolutely sure should go and check this question out, I think.
Starting to try to do so illustrates the well-known fact that there is
a lot of nonsense on the web. I take it you are firmly in the 'no'
camp, Brian?



--

Percy Picacity