Talk:Electric field/Archive 2

Archive 1

Archive 2

Where is it customary to pluralize a unit? (rhetorical)

Constant314, with reference to this edit comment, it is customary to pluralise units when these are used in a quantity (five seconds), sure. But in the context of giving the unit of a quantity? See Time, where you will read The SI base unit of time is the second, not The SI base units of time are seconds. Also see the infobox in Mass: its SI unit is given as kilogram, not kilograms. Since it is clear that your edit summary does not apply and you reverted a number of other edits without justifying this, I feel that my revert of your revert (without my first engaging in discussion) is warranted. 172.82.46.195 (talk) 21:41, 21 July 2022 (UTC)

Keep existing style per WP:STYLERET until there is a clear consensus to change. Constant314 (talk) 23:43, 21 July 2022 (UTC)

I'm a little taken aback. Please take the trouble to clarify what style you wish to preserve rather than just tersely objecting. The inference is that you may mean all of the edits amount to changing a style. For example, do you feel that the article should capitalize "Electric Field" mid-sentence? Do you wish to stick to a "style" that claims that the electric field transforms as a vector, when the applicable transform is that of a degree-2 tensor? Your terseness suggests that you think what I have said does not have merit. 172.82.46.195 (talk) 00:10, 22 July 2022 (UTC)

It may not be helpful to you, but it is policy. Also see WP:PLAINENGLISH which is not a policy but considered good advice. It is the way we talk. A 9-volt battery provides a potential of 9 volts or 9 V. When it is a measurement and the numerical part is anything but exactly 1, then we use plural units when we talk or write it out. The unit (or units, I don't have an opinion on that usage) of field intensity is V/m but if we write it out it is volts per meter. That is plain English.

My revert specifically referred to plurals. Your capitalization changes looked fine. Constant314 (talk) 00:26, 22 July 2022 (UTC)

With regard to whether the transformation is vector or bivector, you will need a reliable source WP:RS. Constant314 (talk) 01:17, 22 July 2022 (UTC)

Thanks for clarifying. It seems that we have different opinions on a matter of English grammar. I see this as a question for physics editors generally, not only those who are watching this article.

I see no examples in my edits of a change of the unit in a quantity from plural to singular, as in your "9 volts" example. I agree with the use of the plural here.

To keep the "transforms as a vector" in the article needs a RS, since I have challenged this. I did not insert anything about a bivector. 172.82.46.195 (talk) 01:44, 22 July 2022 (UTC)

This is an issue of grammar imo rather than style or common use. It is a matter of number agreement. One can correctly write units are volts per metre or unit is volt per metre (although I think unit is the volt per metre reads better) but not unit is volts per metre. Having said that, I won't deny the latter usage can be found, but this ngram is revealing. SpinningSpark 13:54, 22 July 2022 (UTC)

Here is another ngram showing prevelence of the plural form for electric field strength. Constant314 (talk) 14:10, 22 July 2022 (UTC)

That is not a relevant ngram. The context may not be, and in most cases isn't, the naming of the unit. For instance the field strength can be in the thousands of volts per metre at the perimeter fence is a valid construction, but bears no relation to the issue being discussed here which is a singular, not a plural, case. SpinningSpark 15:44, 22 July 2022 (UTC)

I concur with SpinningSpark that this is a matter of number agreement; units are volts per metre or unit is volt per metre would work, but not singular "unit" and "is" with plural "volts". XOR'easter (talk) 18:16, 22 July 2022 (UTC)

I might be losing the drift here. The article text reads "The units of the electric field in the SI system are newtons per coulomb (N/C), or volts per meter (V/m)". Should that be left as is or should it be converted to "The unit of the electric field in the SI system is the newton per coulomb (N/C), or the volt per meter (V/m)", or something else? Constant314 (talk) 19:30, 22 July 2022 (UTC)

The edit pointed to at the top of this thread also included changes in the infobox. In the infobox, I think "volt per meter (V/m)" works better, because "unit" is singular.

Talking of the infobox, I'm not sure what benefit the "Behaviour under coord transformation" line brings. First, saying "coord transformation" makes me wince. Second, the link points to Coordinate_system#Transformations, which only talks about different ways of writing vectors. I suppose the intent was to say how the electric field changes under rotations or translations, but the link is to transformations between polar and Cartesian coordinates. Then you have the issue that, as mentioned above, Lorentz boosts transform the E- and B-fields together. As it stands now, that box is an oversimplified mass of disconnected facts. XOR'easter (talk) 20:00, 22 July 2022 (UTC)

I don't know what the intention of that "Behaviour under coord transformation" line means either. I have no problem with leaving it blank. Constant314 (talk) 21:19, 22 July 2022 (UTC)

The NIST Guide to the SI, section 9.7 asserts that derived units like volt per meter should usually be singular.--Srleffler (talk) 05:56, 23 July 2022 (UTC)

I withdraw my objection. Constant314 (talk) 06:06, 23 July 2022 (UTC)

Electric fields from time-varying magnetic field

@Constant314 and Chetvorno: Re this revert, the relationship $\nabla \times \mathbf {E} ={\frac {\partial \mathbf {B} }{\partial t}}$ is quite generally applicable, not just to Faraday emf induction in a conductor. For instance, a magnet swinging on a pendulum gives rise to an electric field that will be felt by any nearby charges. This is how electric generators work, by moving magnets. An emf can be produced without necessarily producing any current. Light is an electromagnetic wave in which the varying magnetic field is giving rise to a varying electric field which in turn drives the magnetic field. No charges or currents involved there at all. SpinningSpark 15:52, 24 July 2022 (UTC)

I concede that "currents" doesn't usually include moving permanent magnets. Perhaps we can find a better way to say it. the equation $\nabla \times \mathbf {E} ={\frac {\partial \mathbf {B} }{\partial t}}$ implies a relationship between E and B, but not cause and effect. The fact that the magnetic vector potential, or A field, contributes both to E and B is the cause of the relationship. Perhaps we can change " Electric fields originate from electric charges and time-varying magnetic fields" to " Electric fields originate from electric charges and time-varying magnetic vector potential. Constant314 (talk) 18:36, 24 July 2022 (UTC)

Agree with Spinningspark the original sentence should be restored. I understand Constant314's point, the Aharonov–Bohm effect shows the vector potential is the ultimate source, but his alternate sentence above is way too complicated for the introduction. The introduction should be written for general readers (MOS:INTRO, WP:EXPLAINLEAD). Time varying magnetic fields are the proximate source of circular electric fields through Faraday's law, and that is important enough that it should be mentioned in the introduction. --Chetvorno^TALK 19:42, 24 July 2022 (UTC)

I can accept restoring the original sentence while we talk about it. Constant314 (talk) 21:45, 24 July 2022 (UTC)

That being said, I really don't like saying that "electric fields originate from electric charges and time-varying magnetic fields." I regard it as a lie to children that gets repeated over and over and over. Constant314 (talk) 00:21, 25 July 2022 (UTC)

Field value on the shell of a conducting sphere

It has been suggested that the field value on the shell is half the value just outside the shell. It follows from Purcell's analysis in his 1985 textbook. I have the 1985 version reprinted in 2011 and it does not seem to have survived. Maybe Purcell had second thoughts. Anyway, it we assume that the surface charge has a small but non-zero thickness, then by the mean-value theorem, the value has to be halfway between the value well inside the surface and the value well outside the surface. So, yes, in the real world where there are no true surface charges with zero thickness, then we are assured that somewhere the value must be half. But is it on the shell? No, I don't think so. When looking at real conductors, the surface charge is distributed between the surface radius and a radius a few atomic radii inside the surface. So, yes, I will accept that somewhere the value of the field is half, but not on the surface, but rather a few angstroms beneath the surface. This result is too inconsequential to be cluttering up the article. Constant314 (talk) 22:13, 19 December 2022 (UTC)

Leaving this for referring how one can derive the result rigorously: https://www.ias.ac.in/article/fulltext/reso/023/11/1215-1223.

We have used this formula successfully to find its energy as well as problems such as force exerted by a part of the sphere to another. It matches results by other methods and makes the steps simple. In fact I don't think you can solve the later problem without the use of this formula. For sake of completeness, I think we can add a note that mentions it's use as well as the link to give further information. If an alternative to having to use this exist, then there may not be any need to mention this. EditingPencil (talk) 10:09, 20 December 2022 (UTC)

I object for the following reasons

• This is primary research. We need a reliable secondary source that says Lima got it right.
• Lima himself admits that Assad disagrees. Assad has as much credibility as Lima.
• Lima admits that other respected secondary sources disagree.

That puts Wikipedia in the position of deciding who got it right. Wikipedia doesn’t do that. The result is not ready for Wikipedia.

I have a couple of lessor reasons.

• This result is highly specialized. The result for the field value outside the sphere applies whether the surface charge has zero thickness or non-zero thickness. Lima’s result only applies to the case of zero thickness, which does not happen in reality. That means that it only applies in an unphysical hypothetical case. For all physical cases, the result is incorrect. The half value point occurs a few angstroms inside the surface of the sphere.
• After reviewing the math, I believe that Lima made a mistake.

Constant314 (talk) 14:20, 20 December 2022 (UTC)

I don't see any obvious mistake in the paper nor do I think unphysical things need not be included in Wikipedia but it's fair if this is not included until it is well established. I also think it's good that this discussion was opened here anyways.

Thank you! EditingPencil (talk) 12:11, 21 December 2022 (UTC)

Yes, well-meaning discussions are usually helpful. The article talk page is not the place to discuss math errors in primary research, but I think in step 15 the assertion $dA=2\pi ds$ assumes that $r>>ds$. However, this is not the case for r→0. You can continue the discussion on my talk age if you wish. Constant314 (talk) 12:31, 21 December 2022 (UTC)

The energy stored in EM fields

This page gives a formula for the energy per unit volume stored in an electromagnetic field. A formula for the total energy is also given. The source is Griffiths, Intro to Electrodynamics, 3rd edition. Grffiths himself says that these formulas were derived, in earlier chapters, by computing the "work necessary to assemble a static charge distribution" against the Coulomb force, and the "work required to get currents going" against the back emf.

I added this physical explanation of the formulas, but my edit was taken down because "there is no static charge or back emf" in electromagentic waves. Okay, but at this point in the "Electric Field" page, are we talking about waves? It appears that the energy formula is for the energy stored in any EM field, even a static one.

I don't understand why my edit was removed. Am I missing some fundamental point? Please enlighten me.

Thank you, Tzvi Scarr Scarrtzvi (talk) 11:38, 4 May 2023 (UTC)

Thanks for starting a discussion. I believe you are using synthesis. See WP:SYN. That is combining facts to reach a conclusion. If the reference does say it explicitly, it cannot be in Wikipedia. If you have accurately paraphrased the source, you should be able to quote a single, continuous passage that says the same thing. As for what you said, it is incorrect. The energy to assemble a static charge distribution is used to compute the total electric field energy in a static situation. The formula in the article is for the energy density at a point for all cases including static, dynamic, near-field, far-field, and traveling wave. You can use it to compute the total energy in the static case, but you cannot go the other way. Griffiths does give a derivation of the formula and it does not involve the items you mentioned. Constant314 (talk) 13:25, 4 May 2023 (UTC)