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simple E/M question

simple E/M question

simple E/M question

(OP)

So, I understand that in free space, E fields and B fields appear together, and they scale by a ratio of 377.  

But I think this is only true for moving E fields or B fields.

So, just to clarify, a permanent magnet sitting on a desk has a B field -- correct?

But, since it is not moving in time or space, it has no E field -- correct??

But, if I throw the magnet across the room, does it have an E field while it's in motion???

I understand that the moving magnet will produce a current in a wire, but I'm talking totally free space here.   I don't see how the E field can be occur in free space for some reason, but I can imagine it occurring in a wire.  I guess I'm confused!!!

I know these questions are fundamental, thanks for your time in answering them for me!


RE: simple E/M question

>So, just to clarify, a permanent magnet sitting on a desk has a B field -- correct?
Yes

>But, since it is not moving in time or space, it has no E field -- correct?
Yes

>But, if I throw the magnet across the room, does it have an E field while it's in motion?
Yes. Remember Faraday’s Law of Induction. A voltage is induced due to a rate of change of magnetic flux.

>I understand that the moving magnet will produce a current in a wire, but I'm talking totally free space here. I don't see how the E field can be occur in free space for some reason, but I can imagine it occurring in a wire. I guess I'm confused!

The E field is created regardless of whether or not there is a conductor and a current path. Don’t be upset with your self for not getting this immediately. These are really quite hard concepts to grasp and anyone who thinks it is easy, "just Maxwell’s equations", is probably just a parrot quoting from a text book.

When you start thinking harder about this you will see that a moving charge (like an electron or even a small charged metal sphere) is an electrical current. As such it produces a magnetic field. But if you happened to be sitting on the charged sphere you would only see an electric field an not a current. These ideas of electric fields and magnetic fields are therefore a simplification for our limited intellects. Physicists then talk about 4-vectors for these quantities.  

RE: simple E/M question

(OP)
Thanks logbook for the fast response.   Good info.

Now I'm confused as to the difference between the B field and the H field.   It seems like they are basically the same thing, with a constant mu multiplier being the only difference between the two.

I guess I don't see why *both* B and H are needed.  

RE: simple E/M question

Take a long straight piece of wire and put a direct current (DC) through it. Rather difficult to do since the current has to go somewhere! So make a square loop of wire and connect it to some battery terminals. Now you have a current flow through a straight wire, provided the square is so big that the other sides are a good distance from your experimental area. Make the plane of the loop vertical for convenience, in other words the straight wire we are investigating is also vertical.

Provided the current is strong enough, if you get a small magnetic compass you will find some sort of  "force field" around the wire which makes the compass needle point tangentially to a circle centred on the axis of the wire. (This is much easier to see with a picture) It seems as if this "force field" is making concentric circles of some sort of "force" around the wire. We call this a magnetic field because it affects iron filing and compass needles.

If we put 1A through the wire and the size of a particular concentric circle is 1 metre then the field along that path has a strength of 1A/m. This is the H field. If the concentric circle has a perimeter length of L and the current is of amplitude A the H field strength is simply A/L. It is that easy. Notice that the material through which the magnetic field does not come into the equation. If the wire goes completely through iron, water or air then the H field strength is constant.

The B field is the flux or flow of the magnetic field. Although we don’t think of magnetic flux as an actual flow of anything, the term "flux" means flow and that is how the subject originated. You can’t see it, touch it, taste it, smell it or hear it, so you have to infer its existence from various tests, like the effect on a compass needle. As electronics people we have to be able to visualise the intangible, something which some people find impossible. Anyone can look at a metal mechanism and see the gears and wheels. The intricacy of space-time is beyond everyone.

If we put a metal ring around the wire we say that the H field is constant but there is now more flow of magnetic "stuff" because the iron allows it to "flow" more easily. A material which allows more magnetic flux for a given H field is said to have a higher permeability. Iron for example has a permeability say 50,000x larger than air.

B= µH

In induction problems it is the amount of flux changing that gives the induced voltage. Thus our self-consistent scheme of calculation requires two separate quantities: the driving force (H) and the resulting flow (phi for total flux or B for flux per unit area.)

RE: simple E/M question

Hi!

B=uH is just a simplified Maxwell eqn.
In real, it should be written as B=J+uH (isn't it??? ).
J is ommited in some discussions because it's not present or its influence is so small that it can be ommited to make calculation easier. If we additionally assume that B, H and J are all vectors so it starting to seem more reasonable and maybe it will be easier to understand why H and B are not the same things.
It was a long time ago since I played with such a theory, so please correct me if I'm wrong.

Buding.

RE: simple E/M question

Well the current density, J, in free space is pretty low (zero) so it is reasonable to neglect it.

RE: simple E/M question

You also have B=J dimensionally, so that equation is obviously wrong.

RE: simple E/M question

Well, You're right. There is nothing like B=uH+J. I was wrong. As I said it was long time ago.
Current density is present in maxwell eqn.:
rotH=dD/dt + J
which says that variable electric field will cause magnetic field.

B=uH is material eqn. and it can be written as:
B=uH + M where M i magnetisation. M is very often ommited because it is not present or it is close to zero.

To answer hansen64 question if B and H are the same, eqn. B=uH is the answer. It says that the same magnetic field will cause different magnetic induction in different materials. And at the same time different B vectors will cause different electric fields. And it is obvious that there is a need for both vectors. In Maxwell eqn. it is seen that variable B (not H) cause electric field E. B is proportional to H but it is also dependent on enviroment where it occure (enviroments permability).

The same might be used to answer the whole thread. Flying magnet will change magnetic field and at the same time it will produce electric field according to microwave theory. But produced em field will be very weak and will last for a very short time at one place.
Putting a wire around moving magnet will help to measure or focus electric field. So, what I want to say to hansen64 is that electric field in such a situation will always occure not only when You put a wire around moving magnet. Current seen in the wire will be a result of gathering of already existing electric field.

Well I hope You understand what I meant. I have it in my head but it is hard for me to express it with easy words. I have to take a coffee. I wonder what You think about above comment.

RE: simple E/M question

For permanent magnets:

B = ?0 (H + M) or B = ?0H + ?0M

B = residual induction
H = applied magnetic field
M = magnetization [response of material to field].

We also say that the polarization J = ?0M.  For the right units B is equivalent to J - for others, you have to throw in a 4? term.

Gareth P. Hatch, Ph.D.
Director of Technology
Dexter Magnetic Technologies
http://www.dextermag.com

RE: simple E/M question

Hmmm - although the correct symbols showed up in the preview, they didn't get reproduced properly.  Let me try that again....

For permanent magnets:

B = u0 (H + M) or B = u0H + u0M

B = residual induction
H = applied magnetic field
M = magnetization [response of material to field].

We also say that the polarization J = u0M.  For the right units B is equivalent to J - for others, you have to throw in a 4-pi term.

Gareth P. Hatch, Ph.D.
Director of Technology
Dexter Magnetic Technologies
http://www.dextermag.com

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