# Stupid electromagnetics question

Discussion in 'Electrical Engineering' started by qwerty, May 10, 2007.

1. ### qwertyGuest

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.)

2. ### qwertyGuest

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?

3. ### Timo A. NieminenGuest

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).

4. ### qwertyGuest

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.

5. ### The Great AttractorGuest

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

6. ### The Great AttractorGuest

Permanent magnet.

7. ### The Great AttractorGuest

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. :-]

8. ### The Great AttractorGuest

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

9. ### The Great AttractorGuest

WOW... one kook posting to another.

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

10. ### The Great AttractorGuest

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

11. ### ehsjrGuest

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

12. ### Don KellyGuest

--------------------
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 .

13. ### ehsjrGuest

Is the cat's name Uncertain?

14. ### qwertyGuest

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

15. ### The Great AttractorGuest

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

They all spin in one plane.

16. ### Tzortzakakis DimitriosGuest

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....

17. ### Tzortzakakis DimitriosGuest

? "Autymn D. C." <> ?????? ??? ??????
ìÞíõìáI'm having some
trouble getting my head around this:
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?