Talk:Electromagnetism

Misc.

Next, there's how to transform the electric field under relativity, then there's defining and calculating B (an extremely long section since it turns out to be a combination of relativity and E), then how B and E transform under relativity, and finally a derivation of the wave equation for electromagnetic waves. So much to do, so little time. The vector potential is probably also worth mentioning...

Some points:

The magnetic field due to a wire is stationary only if the wire is at rest in an inertial frame and carries a stationary current. The electric field between the plates of a capacitor is stationary only if the plates are at rest in an inertial frame and have a stationary charge distribution.

The form did not work (on my computer).

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I think the derivation of the wave equation should go under electromagnetic wave or under wave equation, maybe even under electromagnetic wave equation. Otherwise, this article is going to get too long. I may take a crack at deriving the WE soon, since I want to refer to it for nonlinear optics. As for the equations, I can't find a font with ∇ in it. Perhaps curl and div would be better, though this won't match the rest of the Wikipedia -- DrBob

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Ok, right about the article's length. You may want to try Mozilla as a browser, and from what I've heard the special characters used in Wikipedia are an HTML standard. At any rate, the wave equation isn't that tough to derive (now that I have my notes to look at), just take the curl of the two Maxwell's equations that have the curl in them, and use the mathematical identity:

∇×∇×A = -∇²A + ∇(∇·A)

for any vector field A (there may be some caveats, but I'm fairly sure that they don't apply to actual magnetic and electric fields). Then use the fact that you want speed of light in a vacuum to say that div(E) = 0 and curl(j) = 0. It gets considerably more sticky in matter since those last two are no longer true and they're tied to particles that have mass and their own electric fields. --BlackGriffen

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I just wanted to ask about the use of the permittivity of free space. I would think it is better to use the permittivity as a general variable (&epsilon) so the formulas would be valid if the charges were suspended in glass or any other medium besides free space. I wasn't sure whether there was a reason to use the free space value, so I didn't change it.

There is unnecessary comment in one part of this article which says « Carlo is gay» which i think should be removed. a little serious please

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Yup. I agree

Units of electromagnetism

[http://de.wikipedia.org/wiki/Gr%F6%DFen_und_Einheiten_der_Elektrotechnik_und_des_Magnetismus to be translated out of German]

Esoteric?

I have issues with the second paragraph if the article:

• While the electric and magnetic forces may sound fairly esoteric — Esoteric? anyone who has combed their hair, used a magnet, or gotten a shock from a metallic door handle has first-hand experience of electric and magnetic forces. What is esoteric about it?
• the attractive forces between atoms in a solid that give rise to the rigidity of solids are predominantly electromagnetic — the rigidity of solids is a result of the balance of attractive electromagnetic forces and a non-electromagnetic repulsive force coming from Pauli's exclusion principle. There are repulsive electromagnetic forces, but it is impossible to obtain mechanical equilibrium using only electromagnetic means. It is still true that all the phenomena described are electromagnetic.

Also, someone complained that Maxwell's equations is hard to understand. Maybe we can design a suggested itinerary through the electromagnetism hierarchy of articles, starting from the easiest, most physically intuitive articles and advancing to more specialized topics such as Maxwell's equations.

Miguel 19:08, 5 Jun 2004 (UTC)

yes, they are unnecessarily hard to understand in Wikipedia. I will try to work on them. I like Wikipedia's depth but sometimes its accessibility is poor. Andries 19:22, 5 Jun 2004 (UTC)

You'll notice that the sentence only describes the concepts as "sounding esoteric", while in reality not being as esoteric as they sound to laymen. While it is true that hair combing and metallic door handle shock are simple expressions of electromagnetic forces, most "normal" people merely think of it as "something involving electricity that makes me go ouch", and the full concept of electromagnetism is still thought of as esoteric. -- 193.11.221.16 11:41, 16 Oct 2004 (UTC)

This article isn't esoteric, it's just extremely vague in its wording.

If you understand what to fix, go right ahead and change said wording. Otherwise, you can just copy any word/phrase in the article which may be unknown to you, paste it in the search box, click Go and read the resultant links. Or google it, if that does not suffice. Be sure to write down what is unknown to you so you can keep the question for yourself. It takes years to understand this subject, if you are working alone. Ask others for help if you do not have the time to resolve your question . Ancheta Wis 10:31, 2 January 2007 (UTC)

Electrodynamics

I have redirected electrodynamics to this article. The definition given of electrodynamics, that of "electromagnetism with quickly changing fields", is simply i

Electromagnetic field only?

Is this not also related to electromagnetic radiation as well as electromagnetic feilds If it is only about elecromagnetic feilds then should it be merged with this article? http://en.wikipedia.org/wiki/Electromagnetic_field Alan2here 18:39, 22 October 2006 (UTC)

someones messing around, deleting the article and replacing with jokes (the jokes aren't funny either!), i don't know how to revert so i copied and pasted an older revision.

Simple Introduction

Some other science articles are starting to produce introductory versions of themselves to make them more accessible to the average encyclopedia reader. You can see what has been done so far at special relativity, general relativity and evolution, all of which now have special introduction articles. These are intermediate between the very simple articles on Simple Wikipedia and the regular encyclopedia articles. They serve a valuable function in producing something that is useful for getting someone up to speed so that they can then tackle the real article. Those who want even simpler explanations can drop down to Simple Wikipedia. I propose that this article as well consider an introductory version. What do you think?--Filll 22:46, 12 December 2006 (UTC)

Opening Introduction

The opening sentence of this article was obscure and inconsistant with all other encyclopedic sources. I have added a simpler one. An encyclopedia needs to educate the lay reader first and not begin as a vehicle to demonstrate someones "sophisticated" scientific knowledge.Lumos3 09:35, 10 January 2007 (UTC)

First paragraph is just wrong

The first paragraph reads:

Electromagnetism is the physics of the electromagnetic field; a field encompassing all of space which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles.

There is no one field encompassing all of space; the second clause of the first sentence is just wrong. Fields only emanate from charged particles or the motion thereof (propagating waves originate from same), and are in fact only an abstraction of the effects of charged particles on each other. I think the fact that they are an abstraction should be emphasized. Somehow the description seems to say that there is one field: a substrate upon which charged particles act (also wrong). CjPuffin  23:19, 9 April 2007 (UTC)

Cj, it's true that each electron (and other charged particle) creates its own EM field, but it's also fair to say that the sum of all such fields constitutes a single field that fills all of space. (It follows from linearity of Maxwell's equations.) In fact, the more general view is that the charged particles themselves are also excitations in fields that permeate all space. The idea of field is distinct from suggesting there is some substrate containing it. Gnixon 23:23, 9 April 2007 (UTC)

Alright, I'll buy that you can consider there to be one field that is the sum of the effects of all particles, but it is sort of a stretch. When we talk about gravitational fields, do we talk about one huge field? Mostly we are concerned with fields from particles in the immediate vicinity which are strong enough to be measured. CjPuffin  23:44, 9 April 2007 (UTC)

It's definitely common to think of one field, and thinking at the same time of the many fields that sum to it is common, too. The same is true for gravity, although often the only important field comes from one object (indeed, you can consider it to come from one point mass if it's a spherical object). In E&M, where more complicated charge distributions are common, idea of one overall field is even more useful. Gnixon 23:52, 9 April 2007 (UTC)

I agree; I just don't see the point of considering the entire universe to be the charge distribution of interest and then saying that there is one field "encompassing all of space". I guess I am taking issue with the use of the definite article in that phrase. CjPuffin  00:56, 11 April 2007 (UTC)

Yeah, I could see an argument for dropping "encompassing all of space." Not sure it adds anything to have it there. Gnixon 01:12, 11 April 2007 (UTC)

electromagneticc field energy in processors

W*h*f, W - A*V h - plank constant f- frenquency for example if f = 3GHz, Umax=1.35V Imax=50A then electromagnetic energy is: 1.35/2^0.5 (V)*50/2^0.5 (A)*6.626*10^(-34)(J*s)*3*10^9(1/s)=6.7*10^(-23)(J) —The preceding unsigned comment was added by 213.190.46.52 (talk) 20:31, 2 May 2007 (UTC).

Gilbert's "De Magnete", probable error about

I have a copy of Gilbert's "De Magnete" (Dover edition). I am 99% sure that Gilbert DID NOT discover electrostatic repulsion, surprising as it sounds. Unfortunately I don't have time immediately to check this. In any case I feel the onus to do so should fall upon the author. —The preceding unsigned comment was added by Eljeh Yendig (talk • contribs) 01:21, 4 May 2007 (UTC).

Electromagnetic units

I have moved to this section material from the "abampere" article that relates to these units in general, including a list of the basic ones, along with links to separate articles containing their definitions.Fbarw 23:04, 17 September 2007 (UTC)

Template:Electromagnetism vs Template:Electromagnetism2

I have thought for a while that the electromagnetism template is too long. I feel it gives a better overview of the subject if all of the main topics can be seen together. I created a new template and gave an explanation on the old (i.e. current) template talk page, however I don't think many people are watching that page.

I have modified this article to demonstrate the new template and I would appreciate people's thoughts on it: constructive criticism, arguments for or against the change, suggestions for different layouts, etc.

To see an example of a similar template style, check out Template:Thermodynamic_equations. This example expands the sublist associated with the main topic article currently being viewed, then has a separate template for each main topic once you are viewing articles within that topic. My personal preference (at least for electromagnetism) would be to remain with just one template and expand the main topic sublist for all articles associated with that topic.--DJIndica 16:54, 6 November 2007 (UTC)

Forest of articles, a quick explanation of "why electromagnetism"?

There are so many articles in this series I couldn't figure out whether this explanation/derivation exists in one of them, or not. (I can contribute some drawings etc if anyone thinks its worth the while). The explanation is easy, and I've used it before with good results (I think I found it in one of my old college texts -- Resnick and Halliday probably). It shows why, given relativity, moving electrons create magnetic fields.

We start with a column of free electrons in a given length of copper wire, say a meter lenth of it. The electrons are free because they escape their atoms' outer orbitals (definition of a metal). The electrons just wander about thereby cancelling their minus-charge with respect to the copper ions' positive charge -- all is neutral. But if they are compelled to drift because of an applied electric field along the wire, their drift velocity causes the apparent 1-meter length of ions to contract, due to the Lorentz contraction. The result is an excess concentration of electrons in the now shortened "tube" of moving electrons. This excess charge is no longer neutral with respect to the stationary column of positive ions. Thus this electrostatic-derived force represents an external force, albeit tiny, that is (i) measureable and (ii) related to the electrons' velocity. This is the so-called "magnetic" force. The vector part of it (i.e. the apparent "direction" of the force" is much harder and probably not necessary to explain. Bill Wvbailey (talk) 16:10, 24 November 2007 (UTC)

See Relativistic electromagnetism. The attribution of this to 1966 Purcell is a bit odd, since I used a text (not by Purcell -- Halliday and Resnick, I believe) in the mid 1960's that had this explanation in it. Bill Wvbailey (talk) 14:04, 12 March 2008 (UTC)

New section added

I have added a new section, Applications, I hope it helps the article. --Twicemost (talk) 16:01, 26 April 2008 (UTC)