Stupid electromagnetics question

-------------------- 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 :).

Reply to
Don Kelly
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The magnetic field is just one perspective of the superposed fields of the drifting charged particles. The B field is just a perspective of the current that we associate that B field with, rather than a cause of the current. Only E fields can accelerate charges, which implies that currents are produced solely by E fields.

Conservation of energy prohibits a current from generating itself.

Though it is common to speak of changing B fields as generating currents, this is not technically correct either. A B field can only change a particle's direction, not its speed, and this is why your static B field cannot produce a current. The so-called changing B field that was mentioned by others is in fact literally and exactly an E field.

Reply to
RP

Allow me to point out a few commonly and widely accepted fallacies with regard to this matter. One is that a changing E field produces a B field. I recently checked here on the latest experimental results attempting to prove the existence of Maxwell's displacement current. So far as anyone here knows no definitive direct proof of the existence of a displacement current has EVER been forthcoming!

Point two is that Amperes law is somewhat less than fundamental! It only works part of the time. Calculations by "flux linkages" will not give a correct result in all cases.

Point three is that famous "one E field" dogma is completely bogus! An electrostatic E field is a TOTALLY different animal from the "supposed" "E" field accelerating electrons in a Betatron. Proof is that if you place an electron on the pole piece of an electromagnet and then start increasing the current, the DIRECTION the electron will be accelerated will depend on it's tiny initial velocity. Electro- static E fields always provide a force direction!

Point four is that Maxwell's equations, are incomplete as they do not include the relation, qV x B. Thus Maxwell's equations in spite of commonplace wild-ass assertions on PBS, do NOT "explain" "All electro- magnetics" anymore than Newton (or Einstein for that matter) "explained" gravity.

Benj

Reply to
Benj

E fields don't come in different types. They're just a mathematical way of saying that a unit point charge Q will experience such and such force if located at such and such point in space relative to some other charge(s). Both the E and B fields are artifices, but there are real em fields. E and B are just different mathematical perspectives of the em field.

But there is a problem with common conceptions. To correct those: A changing B field doesn't cause an E field, it is the changing current that causes the E field, and that change in current can in turn be traced to changes in charge densities caused by yet other E fields, or in other words the induced E field is produced by other E fields, and all are precisely what you just said that one of them wasn't, Coulomb forces in action. Even the force associated with the B field is just superposed Coulomb forces. See Purcell if you want a mainstream derivation of these conclusions, or see Weber for a previous version of pretty much the same ideas.

Reply to
RP

In an electric wire, the E field is contained within the wire. The E field operates between the electrons moving at the surface and the atomic nuclei inside.

A distinction must be made between the mathematical theoretical Gaussian general description of an E field and an actual E field, and the same for the B field.

In a transformer, you feed an alternating current into the primary winding wire, meaning that the flow of electrons will reverse cyclically in the primary wire. But it is the associated fluctuating B field about the primary wire that induces motion of electrons into the secondary wire winding.

To produce an real E field external to a wire, you need to charge surfaces, one with atoms maintained into an electron depleted state, that will then be electrically positive and the other with excess electrons that will then be electrically negative.

An E field will then be present between these plates.

In a Betatron, you accelerate charged particles with such an E field, but you need a very precisely calibrated B field (separately produced by electromagnets) to contain them on the very precise circular path.

In linear accelerators, you need to calibrate both E and B fields such that v=3DE/B for the path to be straight

Ref "Principles of Charged Particle Acceleration" by Stanley Humpries.

Andr=E9 Michaud

Reply to
srp

The proof has been around all along. Just check the fundamental LC relation.

Andr=E9 Michaud

Reply to
srp

Is the cat's name Uncertain? :-)

Reply to
ehsjr

Don't do this at Ohm.

Bill

-- Fermez le Bush--about two years to go.

Reply to
Salmon Egg

If that is the case, explain how betatrons work. Betatrons, for all you you youngsters, were electron accelerators using a magnetic field to drive them.

At the guts of this is that the electromagnetic field is a consolidated concept of electric and magnetic fields. This combination is a 4 by 4 tensor that is invariant under the Lorentz transformations of special relativity. The components may change values in different uniformly moving coordinate systems, but represents the sam electromagnetic field in all these coordinate systems.

Bill

-- Fermez le Bush--about two years to go.

Reply to
Salmon Egg

Ampere's law calculates the magnetic field produced by a flow of electric charges (in a wire). The magnetic equivalent of this statement would be: Erempa's law calculates the electric field produced by a flow of magnetic charges.

Because there are no magnetic charges (monopoles) in nature, no such calculation can be carried out.

In the original posting, the fallacy is to argue that if A causes B then B will cause A.

Bill

-- Fermez le Bush--about two years to go.

Reply to
Salmon Egg

The Great Attractor wrote in news: snipped-for-privacy@4ax.com:

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

Reply to
qwerty

We're not sure. ;-)

Reply to
Michael A. Terrell

If they existed at all they must always have been around.

Maybe we just don't recognize them yet for what they are (if they do exist).

Andr=E9 Michaud

Reply to
srp

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

They all spin in one plane.

Reply to
The Great Attractor

The distinction is that the actual field is not an E field, it is an em field. Wether it is describable in terms of E and/or B is a matter of relative motion, meaning that E and B are frame dependent fields and by virtue of that fact they cannot be representative of reality. It is only those things that are invariant that even have hope of being described as real causes.

The B field is a composite E field. Again, see Purcell.

The E field is a mathematical representation of the force that a group of charged paticles exerts on point charges. It is no more than that, and no less. The B field consists of E fields with opposing vectors producing a net lateral force on point charges in motion with respect to the sources of those crossed E fields. All conductors consist of both negative and positive charges, thus at least two E fields are associated with any conductor. In current carrying conductors those two charge components are moving relative to each other, giving rise to the velocity dependence of the effect known as the B field. This effect is in turn produced via relativistic changes to the apparent charge densities of those two components in the conductor with respect to the external moving charge.

Which can be translated as "a very precisely calibrated set of E fields (separately produced by electromagnets) to contain them on the very precise circular path".

All of the E fields.

Don't give a damn about such references :)

Reply to
RP

Sorry Andre, but you cannot use circuit concepts to prove field theory! Now if you wanted to say that the existence of EM waves proves the existence of "displacement current" by inference, Ok. That's what everyone has done since the 19th century! But the question is if displacement current is a real thing and not some mathematical error, then one should be able to come up with some clever experiment to demonstrate it directly. Apparently nobody has.

Benj

Reply to
Benj

Correct. It's often called a "capacitor" if configured to conform to circuit theory approximations.

Yes.

No not correct. In a betatron (which as Salmon egg pointed out is a relativistic device capable of accelerating electrons very close to the speed of light) electrons are NOT accelerated by an electrostatic E field. Allow me to quote the freshman physics text, Resnick and Halliday, 1960 ed. p 756.

" The induced electric field that are set up by the induction process are not associated with charges but with a changing flux. Although both kinds of electric fields exert forces on charges there is a difference between them."

And at p 757.

" Electric fields associated with stationary charges are

*conservative*, but those associated with changing magnetic fields are *nonconservative*. Since electric potential can only be defined for a conservative force, it is clear that it has no meaning for electric fields produced by induction as in a betatron."

Dare I say it? So much for the famous "one E field" dogma!

Of course there are lots of side engineering issues in betatron design including focusing the beam, keeping the beam circulation in its "doughnut" vacuum tube at a constant diameter, and injection and extraction of the beam at the right time. But these are details that do not impinge on the statements above.

Benj

Reply to
Benj

My blunder, thanks. I was thinking generally.

I noticed Salmon egg comment after I answered.

I also have this quote regarding the 100 Mev GE Betatron from Halliday and Resnick (I have the 1967 edition), page 884

"The magnetic field in the betatron has several functions: (a) it guides the electrons in a circular path; (b) it accelerates the electrons in this path; (c) it keeps the radius of the orbit in which the electrons are moving a constant; (d) it introduces the electrons into the orbit initially and removes them from the orbit after they have reached full energy; and finally (e) it provides a restoring force that resists any tendency for the electrons to leave their orbit, either vertically or radially. It is remarkable that it is possible to do all these things by proper shaping and control of the magnetic field."

It is a pity that the GE betatron is no longer in operation. I would very much have liked that more experiment would have been carried out regarding the unexplained energy loss reported by John P. Blewett. in his paper "Radiation Losses in the Induction Electron Accelerator", Phys. Rev.

69, 87 (1946).

What I have page 885 (possibly the same quote revised ? )

"The electrons are accelerated by electric fields set up by the changing flux [The current in the GE Betatron coils was made to reverse 60 times per seconds] "

Couldn't locate your quotes in my 1967 edition. Could you refer me to chapters and section numbers ?

Right. All issues apparently dealt with by the magnetic field as H&R quote.

Andr=E9 Michaud

Reply to
srp

Not my intention really. But simply to mention that displacement current is by structure part and parcel of the LC oscillation.

Well, since it is this intuition by Maxwell in 1865 that caused him to properly define em wave treatment afterwards, it seems to me that there must be some ground for its validity. For em waves to exist at all, in the absence of conduction current in vacuum, only displacement current could possibly be at play as a source of magnetic field and that the associated changing electric field in a region of space will induce a magnetic field in neighboring regions, even when no conduction current and no matter are present.

Sears, Zemansky & Young do discuss this in their "University Physics", 6th Edition, Addison Wesley, (1984), page 625.

To demonstrate it at our macroscopic level, I see only the LC circuit as an option.

At a more fundamental level, I think there would be need to experiment with individual photons (is it even possible ?)

De Broglie had an hypothesis in this regard that seems logical enough to me and that, according to him, satisfied at the same time Bose-Einstein's statistic and Planck's Law; and perfectly explained the photoelectric effect while obeying Maxwell's equations and conforming to the properties of Dirac's theory of complementary corpuscles symmetry.

His hypothesis was that discret photons would be made up not of one corpuscle, but of two corpuscles, or half-photons, that would be complementary like the electron is complementary to the positron.

According to him, "Such a complementary couple of particles (that he perceived as unsigned charges) is liable to annihilate at the contact of matter by relinquishing all of its energy, which perfectly accounts for the characteristics of the photoelectric effect. "

Furthermore, he said "The photon being made up of two elementary particles of spin h/4pi, it must obey the Bose-Einstein statistic as the precision of Planck's law for the black body requires."

Finally, he concluded that "this model of the photon allows the definition of an electromagnetic field linked to the probability of annihilation of the photon, a field that obeys Maxwell's equations and has all of the characteristics of electromagnetic light waves."

His two half-photons had to cycle towards and away from each other (displacement current), inducing a magnetic field that would increase as the two half-photons close in on each other, and would decrease as they receide from each other,

This would by the same token take care of the magnetic monopole issue, since in this configuration, both magnetic phases cannot be present at the same time. Meaning that while the charges symmetrically separate in space, the magnetic phases symmetrically separate in time.

But his hypothesis did not really take at the time.

Andr=E9 Michaud

Reply to
srp

I'm pretty dumb about these things, but doesn't a collapsing magnetic field (such as around an inductor) attempt to push the current in the direction it was going when the field was created? IOW, doesn't it become EMF in the same direction as the current was flowing already?

Reply to
Anthony Fremont

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