Stupid electromagnetics question

[qwerty]

I'm having some trouble getting my head around this:

A current flowing through a (static) conductor produces a (static)
magnetic field around it, as described by Ampere's Law.

But the reverse appears not to be true; A magnetic field around a
conductor doesn't induce current to it, unless the conductor is
moving or the magnetic field changes (or both.)

Isn't this a paradox?

 

[qwerty]

Paradox? No. Why should they behave the same? What is
the apparent contradiction? E behaves one way, B another.

But, Ampere's Law states that:

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/amplaw.html

So if we have B around a conductor, surely we have to have I as well.
That's what the Law states. I know I'm missing something, but what do
I miss?

 

[Timo A. Nieminen]

I'm having some trouble getting my head around this:

A current flowing through a (static) conductor produces a (static)
magnetic field around it, as described by Ampere's Law.

But the reverse appears not to be true; A magnetic field around a
conductor doesn't induce current to it, unless the conductor is
moving or the magnetic field changes (or both.)

Isn't this a paradox?

I wouldn't call it a paradox (or a stupid question, either). The
situations are not symmetric - in the first, there is a static electric
field as well, producing the current.

You could say that, in the first case, the electric field driving the
current produces the current and the magnetic field. You can get rid of
the current in Ampere's law by substituting J = sigma E, where J is the
current density and sigma is the conductivity.

Another thing that stops it from being symmetric is that the current is
moving electric charge, driven by an electric field. The symmetric case
would be a magnetic current, composed of moving magnetic monopoles, being
driven by the magnetic field and producing a static electric field. No
magnetic monopoles, so we don't observer this.

(Exercise for the reader: assume magnetic monopoles exist, and design a
perpertual motion machine thereby.)

You might be interested to know that sometimes magnetic currents are
assumed to exist, to simplify calculations in problems where
electromagnetic waves interact with objects (ie scattering).

 

[qwerty]

Now what you *can* say is that if there is a magnetic field in a
certain region of space, then there is likely a current
*somewhere* that is responsible for that field. But if you just
happen to put a wire in that field doesn't mean that the current
responsible for the field has to live in that wire.

I understand it now. If there's a magnetic field somewhere then that
magnetic field is *always* caused by a current. So there's no way to
create the magnetic field of a current-carrying conductor without
using one.

Thanks.

 

[The Great Attractor]

I'm having some trouble getting my head around this:

A current flowing through a (static) conductor produces a (static)
magnetic field around it, as described by Ampere's Law.

Yes, and AC current makes for continually moving flux. DC makes a
standing field.

But the reverse appears not to be true; A magnetic field around a
conductor doesn't induce current to it, unless the conductor is
moving or the magnetic field changes (or both.)

To INDUCE (key term here), the flux MUST be in motion. The field
doesn't have to "change" as you put it. It HAS TO MOVE, as in lines of
flux must "cut" through the conductor, and the current induced in it will
always be varying, not static.

Isn't this a paradox?

Nope.

 

[The Great Attractor]

you can't get a static magnetic
field unless there's a current somewhere.

Permanent magnet.

 

[The Great Attractor]

Having said that, permanent magnetic materials throw
a little monkey wrench into the theory. The source of
magnetism in magnetic material is electron spin, tiny
magnetic dipoles. There isn't actually a classical current,
but you can *model* them as little imaginary current loops
for the purpose of Maxwell's equations.

Add up a gazillion of 'em, and get a cumulative effect we call a field.

Get a big planetoid sized orb full of iron and it will likely be
magnetic to some degree. That's Gazillions to the Gazillionth power. :-]

 

[The Great Attractor]

As this magnetic field intensifies about your conductor, current
will flow in it, but will stop flowing when the magnetic field is
made to stop increasing.

No. whenever it stops *moving*. It doesn't have to change strength,
just position.

 

[The Great Attractor]

I've never heard of a photon being called a "perpetual motion machine"
before, but they do seem to travel a long way.

WOW... one kook posting to another.

Why don't you guys discuss it so we can all have a laugh?

 

[The Great Attractor]

Keep in mind the weirdness that I added as a footnote. Even a changing
*electric* field can produce a magnetic field. Though this may seem
like a little oddball thing that doesn't happen very often, it is in
fact half responsible for your being able to see anything. (Light.)

PD

ESD damage can occur to chips due to electric fields, so yes, they DO
have influence.

 

[ehsjr]

I'm having some trouble getting my head around this:

A current flowing through a (static) conductor produces a (static)
magnetic field around it, as described by Ampere's Law.

But the reverse appears not to be true; A magnetic field around a
conductor doesn't induce current to it, unless the conductor is
moving or the magnetic field changes (or both.)

Isn't this a paradox?

No. The logic is faulty. It is like
stipulating that "all sandboxes contain sand" and
because of the stipulation assuming that
"all sand is in a sandbox".

From Ampere's law you stipulate that a current through
a wire results in a magnetic field around the wire.
(That's the sandbox). You incorrectly assume, because
of that stipulation, that a magnetic field around a wire
must induce a current.

Ed

 

[Don Kelly]

No. The logic is faulty. It is like
stipulating that "all sandboxes contain sand" and
because of the stipulation assuming that
"all sand is in a sandbox".

From Ampere's law you stipulate that a current through
a wire results in a magnetic field around the wire.
(That's the sandbox). You incorrectly assume, because
of that stipulation, that a magnetic field around a wire
must induce a current.

Ed

--------------------
Ah but Heisenberg's cat thinks it is a litter box. To crap or not to crap,
that is the question.


Actually you put it very well but I couldn't resist .

 

[ehsjr]

--------------------
Ah but Heisenberg's cat thinks it is a litter box. To crap or not to crap,
that is the question.


Actually you put it very well but I couldn't resist .

Is the cat's name Uncertain?

 

[qwerty]

The Great Attractor

Permanent magnet.

The magnetic field of a permanent magnet is caused by currents inside
the body of the magnet.

 

[The Great Attractor]

The Great Attractor


The magnetic field of a permanent magnet is caused by currents inside
the body of the magnet.

It is caused by alignments of atoms in the lattice of the medium.

They all spin in one plane.

 

[Tzortzakakis Dimitrios]

I'm having some trouble getting my head around this:

A current flowing through a (static) conductor produces a (static)
magnetic field around it, as described by Ampere's Law.

But the reverse appears not to be true; A magnetic field around a
conductor doesn't induce current to it, unless the conductor is
moving or the magnetic field changes (or both.)

Isn't this a paradox?

Depending on your point of viewActually your question is a very smart
one.Maybe it's God's will (or a higher being's) that we can't get energy for
free (there's no such thing as a free lunch).Because then we could place a
piece of wire in Earth's magnetic field and get electricity,plenty and free,
wouldn't we?
But maybe He doesn't want to spoil us;-)So we have to rotate generators with
prime movers, which on their turn need some fuel....Fuel costs money, and
their supply is limited....Google for the perpetuum mobile, for an
encore....

 

[Tzortzakakis Dimitrios]

? "Autymn D. C." <[email protected]> ?????? ??? ??????

Ï "qwerty" <[email protected]> Ýãñáøå óôï

ìÞíõìáI'm having some
trouble getting my head around this:

Depending on your point of viewActually your question is a very smart
one.Maybe it's God's will (or a higher being's) that we can't get energy for
free (there's no such thing as a free lunch).Because then we could place a
piece of wire in Earth's magnetic field and get electricity,plenty and free,
wouldn't we?

Fool, learn what induction is. There is already a current in such a
field in other media, magnetic or Coriolis or otherwise. It's not
much thouh.
<end quote>
Might be, but then, who am I, to tell right from wrong?