Talk:Electronics/Magnetic Field

Constructive critism (I hope)
''A electron moving through space creates a magnetic field that spins around the charge according to the right hand rule. The magnetic field is created by the spin of the moving electron. If the wire is bent in the shape of a ring, when its current is flowing it magnetic field will resemble water flowing through a hose. In order for the ring to have a magnetic field, its magnetic field must first displace the magnetic field that is already there. This is why inductors initially resist any changes in current when a voltage is applied. Over time the magnetic field changes to reflect the magnetic field of the ring and current starts flowing''

This paragraph needs a lot of work. The field is created by the moving charge of the electron not it's spin (Well there is a tiny field due to the spin, but i don't think you need to wory about it). if you bend a wire into a ring you gat a torus shaped field, I'm not sure what you mean by "resemble water flowing through a hose". Magnetic fields don't need to displace each other, they just add together.Inductors resist changes in current because the changing current creates a changing magntic field wich in turn creates a back EMF. Apart from these problems the paragraph looks fine ;-) Theresa knott

no it is the quantum spin of the particles that make the field because of gravity waves. User:67.71.37.148

Completely false. Any interaction between quantum spin and gravity is negligible above the Planck scale -- with gravity waves, more so. Magnetic fields are correctly predicted by Maxwell's equations, which are purely classical. If quantum spin were a necessary part of the explanation for magnetism, Maxwell would have not been able to explain it. Carandol 05:39, 28 Apr 2004 (UTC)

This chapter is not pitched at a consistent level. Some of the material -paramagnetism, diamagnetism, etc- should go to the expanded edition, probably in the introductory basic principles chapter

I agree. That's because I am still reworking the book to figure out what I should include where. Hence the warnings all over the place. When I am done the book will be pitched at a more consistent level and some things will go to the expanded edition and others will vanish. Wanderer

But maxwells equations are wrong becaus quantum theory is better


 * Maxwell's equations aren't wrong, in the sense that 1=2 is wrong.

Rather, they are solutions of QED in the classical limit, which means we can with care use them as a guideline to the nature of QED.


 * That magnetic fields in the classical limit are accurately described by Maxwell proves that quantum spin doesn't affect the classical behaviour of magnetic fields, which is what this book is concerned with, and hence that an hypothetical spin-0 charged particle will produce a magnetic field when in motion, as can be confirmed by QED. If it didn't Lorentz invariance would be broken, which would be a real mess. Carandol

Are you saying that quantum spin isn't responsible for gravity and magnetism??

are u saying that quantum spin isnt responsibal for gravity and magnatism??


 * I am. We don't have a working theory of quantum gravity yet, which makes any claim quantum spin is responsible highly tentative, at best. We can though safely say that any common cause of gravity and magnetism is only going to be relevant at the Planck length, and uttterly irrelevant to this book. Carandol

From what I can tell spin has to do with neutralizing the effects of the magnetic moments of electrons. For a given orbital there are two electrons in it with opposite spins and magnetic moments. If both electrons are there then the moments cancel and if one electron is there than it has a moment and hence a magnetic field.


 * Electrons do have a magnetic field proportional to their spin, but that is not the sole cause of magnetism. Anyone moving relative to a charge will see a magnetic field, even if the charge has neither spin nor magnetic moment.Carandol

''Spin has to do with exclusion. Electrons come in pairs due to the Pauli exclusion principle, and if only one electron exists then it is affected by magnetism and has a magnetic dipole moment. If both electrons exist the effects cancel.''

The previous paragraph, removed from this chapter, is incorrect.
 * Spin has to do with behaviour under rotation. Exclusion is an indirect conseqence.


 * Electron states come in pairs because of both the exclusion principle and electrons being spin 1/2


 * Paired electrons are still affected by magenetism. All effects do not cancel completely, and the Lorentz force wouldn't cancel anyway. That force is eVxB, proportional to electron charge, not electron spin

I'm not sure where people are getting this misconception from, but there's no doubt it's wrong

Self inductance
This is what happens when you turn on DC in an inductor.


 * 1) The voltage increases very rapidly ( in a matter of ms) as you turn on the switch.
 * 2) This causes a very rapidly changing current to flow.
 * 3) This current has an associated magnetic fiels which also changes very rapidly.
 * 4) The rapildly changing field induced an e.m.f. (Faraday’s Law) that tends to oppose the increase of current (Lenz’s  Law).However it cannot stop it increasing cdompleatly, just slow it down.
 * 5) Once the current reaches it's maximum (determined by the voltage ad the ordinary resistance of the wire) it ceases to change, the magnetic field becomes constant and the back EMF goes to zero.

All remains calm until you try to turn the thing off


 * 1) The field collapses very suddenly
 * 2) The changing magnetic field induces a back EMF which tries to oppose the collapse of the field (Lenz’s  Law again)

For large coils the EMF can be huge, sparks can fly!

Isn't really an issue for DC though except when turning on and off. But for AC the current is varying all the time, so things never settle into a stready state. The higher the frequency, thwe faster things are changing and so the greater the inductance. I hope all this is reasonably clear now. Theresa knott 13:49, 30 Apr 2004 (UTC)


 * Surely the inductance (L) doesn't depend on frequency? Norman Weiner


 * Not for AC purposes, but the impedence due to inductance does (&omega;L), which -- talking loosely -- comes to the same thing.


 * At sufficiently high frequencies, capacitators acquire non-negligible inductance and vice versa. Inductance starts changing with frequency, just below the point where it stops being a useful concept, and you have to resort to full EM. For the purposes of this chapter, and of circuit theory generally, we don't need to consider gigahertz frequencies, so we can say inductance and capacitance are constant.Carandol
 * Sorry about talking about inductance when i should have said impedence. I don't know why I'm so sloppy all the time, it drives me round the bend it really does. Fortunately there is always some noce person around to correct me. Theresa knott 08:32, 5 May 2004 (UTC)


 * The simple relation

$$   L\frac{di}{dt} + iR + \int \frac{i}{C} dt = V(t) $$ should handle everything just fine, where L and C depend on only the geometry. Norman Weiner

So what do people know about toroids? Wanderer