Talk:Eddy current

Untitled
I had made major edits because I was concerned that, while the text allowed for the creation of eddies in a constant, uniform, stationary magnetic field (through which a conductor is moving) the diagram (which is otherwise truly excellent) implies that eddies only occur where the conductor experiences a magnetic field that is either growing or weakening. As I have no skill at drawing these sorts of diagrams, I have changed the text instead. Ideally, someone will modify the diagram to show eddies within the central field region, as well as at the edges. We should all be aware, too, that the real shape of the eddies is probably very different to those in the graphics: individual electrons may be attempting to move in circles, rather than forming macroscopic currents that we can conveniently depict as curved arrows. The problem with big, curved arrows in a uniform field is justifying where the arbitrary axis of symmetry should be! StuFifeScotland 13:33, 27 March 2007 (UTC) On second thoughts, I've done some calculations which show that, for the kinds of resistivities metals have at room temperature, the tightly-circulating eddies are heavily damped. The currents become quite long and straight, so the diagram is pretty accurate. StuFifeScotland 15:27, 7 April 2007 (UTC)


 * Yes, I don't think you're right. If you spin a cylindrical magnet that has a pole endwise on to a plate for example, you'll get no eddy current. It's the rate of change of the field intensity/direction, not simply the fact that there's a relative motion between the field and the conductor that matters. If you like, for any loop the moving, constant, field removes some field from the loop (negative field change over time) and adds an equal amount of new field (positive field change over time). So the two field rates would perfectly cancel, and so a constant field intensity causes no current.- (User) WolfKeeper (Talk) 20:44, 29 January 2008 (UTC)

I have a problem with the hypothesis that lamination's or grooves etched into the conductive material reduce Eddy Currents by blocking the flow of electrons or by causing the buildup of static electricity. The drift rate of electrons is far to slow to account for a drop in Eddy Current induction that takes place in microseconds. With drift rates in the order of a millimeter an hour or less, it seems impossible they could reduce the Eddy Currents in say a rail gun projectile traveling at thousands of feet per second, as they clearly do.

While the rise time of induction in the conductive material, at first seems a likely candidate for explaining the reduction in induction, this also is impossible as in most cases it is too fast to cause much difference.

The only explanation that stands up to close scrutiny is that by breaking up the overall magnetic gradient into discrete regions, the magnetic gradient in each is vastly reduced and this (in accordance with established theory) is the cause of the reduction in induction. It is possible that standing waves at the bottom of each groove (in the case of a etched conductor) add to the reduction in induction.

Eddy Currents resulting from the relative movement, over time, of a permanent magnet and a conductive material are very complex as skin depth, phase shift at different depths, relative gradient over time, coupling of the gradient into larger flows due to a increase in equilibrium of the magnetic tension, harmonics and numerous other factors all play a part.

In contradiction to traditional induction theory it has been established (1) that in the case of a permanent magnet moving relative to a conductive material, Eddy Currents increase quickly to a peak at around 300rpm and then decline to about 10% by 8,000rpm. This is probably due to phase cancellation and the increasing effect of rise time on the magnetic gradient differential but remains an unresolved phenomenon.

(1). L. H. Cadwell, Am. J. Phys. 64, 917923 (1966) and Langman, Mikheev and Tabachnicht et al and Dr. Peter Campbell in his book "Permanent Magnet Materials and their Applications". Aimulti (talk) 05:58, 15 April 2008 (UTC)


 * Eddy currents are greatest in low resistivity materials. Drift velocities in such conductors are of the order of 0.1 mm/s, not mm per hour as stated above (see the drift velocity and electron mobility articles). By definition, charge builds up on laminations at the same rate the eddy current flows: 1 A is 1 C/s. This is not static electricity, as in an excess of positive or negative charge on a body: it is the separation of charges that happens at any open-circuit that is 'live'.


 * An analogous situation happens when a conductor (eg an aircraft's wing) moves through a perpendicular magnetic field (eg the Earth's). The Lorentz force acts on the charges within the conductor, driving positives and negatives in opposite directions. Electrons collect at one end of the conductor and positive lattice ions are exposed at the other. A potential difference is therefore established between the conductor's ends that opposes any further flow of charge due to the induced emf. Equilibrium is reached when the separation of charges produces a force due to the electric field that is equal and opposite to the force induced by the B-field. The current ceases to flow.


 * Note: there has to be a small transient current for charges to separate, but there could only be a sustained current if there was a return path for the electrons to flow around. Charge is 'pumped' sideways for any conductor moving through a perpendicular magnetic field. If that charge runs into a boundary, then an opposing E-field appears; but appreciable sustained eddy currents can only exist if part of the conductor is either outside the B-field, or in a region of reduced B-field, where the 'pumping force' is smaller and a net emf can be maintained between these two regions of the conductor. Laminations cannot prevent eddy currents completely because, in a practical dynamic (eg rotational) system: 1) the B-field and/or relative velocity is not uniform across the conductor, so some return path current is always present and 2) transient currents are constantly ebbing and flowing so as to re-establish E-fields across the boundaries as the conductor re-orientates itself.


 * For an excellent, barely-mathematical treatment of all these issues, see M. Nelkon & P. Parker, "Advanced Level Physics", Fourth Edition (1977), Heinemann Educational Books Limted StuFifeScotland (talk) 18:50, 14 July 2008 (UTC)

Picture
Hello to all concerned. I have the most basic of knowledge concerning magnetic fields, so I am having a little trouble interpreting the picture. More experienced persons may find my lack of comprehension amusing, but I can assure them that I have a better chance at understanding the picture than the average wikipedia user. It is for this reason that I suggest that someone may find it worthwhile to at least include what I,V and B represent. Thanks. 138.64.2.77 (talk) 16:10, 21 August 2008 (UTC)

Foucault?
The wiki page for Léon Foucault says that he discovered eddy currents, but this page says it was François Arago. Some clarification is needed here. At the very least, the article should explain why eddy currents are also known as Foucault currents. Erp Erpington (talk) 20:12, 16 October 2009 (UTC)


 * You are right. I checked the Foucault article and it says "discovered eddy currents". The Arago article says "discovered by French physicist François Arago in 1824". They're both French so it's not nationalistism. Needs clarification 
 * Big Anomaly here, Foucault was born 1819, 'discovered' by "Arago in 1824". If the dates are correct then it's 'unlikely' Foucault made a find like this at age 5!. It wasn't that uncommon for discoveries/inventions to be made near simultaneously in those days. Sometimes 'A' notices an effect, but don't publish it. Years later B discovers the same effect, publishes it, 'A' then searches their notes and find they discovered it first.
 * There is some vigorous discussion on who invented 'radio', or perhaps more accurately "who made the first long distance trans-mission" using radio waves! Probably ALL those guys made important contributions. If one dude brought it all together, he was perhaps standing on the shoulders of giants, "If I have seen a little further it is by standing on the shoulders of Giants", Isaac Newton(et al):--220.101.28.25 (talk) 10:13, 3 November 2009 (UTC)


 * The source of the confusion appears to be this edit. The Foucault article says he discovered eddy currents, the Arago article makes no such mention.  (Wikipedia is not a reliable source for itself, but it's better than nothing for the short term.)  I'm going to put the name and date back the way it was, and tag it with a "citation needed".  CosineKitty (talk) 18:54, 21 March 2010 (UTC)

Strength of Eddy Currents
I do not believe the supplied equations are correct. The units for f were unclear, and when I performed dimensional analysis I found that f2 would have to have the units of kg m-1 s-2. This is clearly erroneous. Eshansen (talk) 23:59, 24 November 2009 (UTC)
 * I concur. It wasn't the units of f that I was concerned about, it was the definition of D. What could the density of the material have to do with eddy current loss?  I think that D in this case is Dpen, the penetration or skin depth.  Skin depth has a major influence on eddy current losses, was missing from the formula, and has the correct dimension so the units on left and right side match.  Henkdeleeuw (talk) 05:54, 23 February 2010 (UTC)
 * Units still don't add up in the current article. When I check T^2*m^2*Hz^2/(Ohm*m*m) I get kg/(s^3*m^2), but power is kg*m^2/s^3. So there's a missing length^4. That's way off. The only thing I can think of is that it's using flux density (T) instead of flux (Wb). That's a difference of m^4, and it makes sense. My resources on induction heating all refer to flux rather than flux density when calculating power. V = -N(dFlux/dt) and P=V^2/R so (Flux*frequency)^2/R would have the right units. --W0lfie (talk) 14:51, 9 August 2010 (UTC)

The supplied equation for the eddy current per unit mass is not correctly transcribed from the source material. The original source gives on p. 31 of Characterization and Measurement of Magnetic Materials, Fiorillo, 2004: $$W_{cl}=\frac{\pi^2}{6}\frac{\sigma d^2 J_p^2 f}{\delta}$$. The unit in the book for J are Tesla, I assume the other units are in SI. Vario (talk) 11:16, 1 June 2015 (UTC)

A recent edit https://en.wikipedia.org/w/index.php?title=Eddy_current&type=revision&diff=758120974&oldid=755111446 replaced $$P = \frac{\pi^2 B_\text{p}^{\,2} d^2 f^2 }{6k \rho D}$$ with $$P = \frac{\pi^2 B_\text{p}^{\,2} d^2 f }{6k \rho D}$$. Dimensional analysis of the original form had units of $$\frac{W}{kg}$$ as described in the description of the equation. The new form has units of $$\frac{J}{kg}$$. The only referenced is a book. Does anyone have a public reference for this formula? Relliksadab (talk) 16:44, 13 January 2017 (UTC)

I have an issue with the formula provided in the page for the Eddy current power dissipation as mentioned above as it differs from the book as shown on page 31 of https://books.google.com.hk/books?hl=en&lr=&id=n3awvIkC7lYC&oi=fnd&pg=PP2&dq=measurement+and+characterization+of+magnetic+materials&ots=WTJftmNkNo&sig=AuvYdzYG3rz89gyFh0yY6fsQtHI&redir_esc=y#v=onepage&q&f=false. Could someone please explain why there is the addition of the constant k and shift of density to the denominator and the alteration of the power of frequency (f)? MorosIntrepidus (talk) 13:50, 25 July 2019 (UTC)

content that could be copied here
Faraday's law of induction has some nice text and illustrations related to eddy currents and mitigation. Maybe something could be copied here. --Steve (talk) 12:38, 29 July 2012 (UTC)

"Eddy currents in conductors of non-zero resistivity generate heat..."
This comment concerns the statement in this article "Eddy currents in conductors of non-zero resistivity generate heat..." and whether a citation is needed for that claim.

First, I would like to address whether the claim is true (which is not the same as the question of whether in belongs in a Wikipedia article; see WP:V).

There are two aspects to this. First, according to Ohm's Law, zero ohms equals zero watts equals zero heat generated. This is well-cited in Superconductivity. The converse and equally true statement is that any current in a conductor of non-zero resistivity generates heat. Alas. both Power and Electrical power are rather light on citations, so we can't just Wikilink to them.

Second, there is the question of whether eddy currents occur in a superconductor. Our article starts out by saying "Eddy currents (also called Foucault currents) are electric currents induced within conductors by a changing magnetic field in the conductor." An obvious corollary is this: no magnetic field = no induction = no eddy current.

It is a well-known property of superconductors that they are perfect magnetic shields (up to a point; a strong enough field will cause them to cease to superconduct). Sort of like Mu-metal on steroids. Perfect magnetic shielding = no magnetic field = no induction = no eddy current. Well known, but as I found out when I was designing such a shield for electron microscopes, the "perfect" isn't exactly perfect in real life. It does come close, and the nature of the imperfect magnetic shielding really doesn't affect eddy currents - see below for details.

The mechanism behind the lack of magnetic field in a superconductor is the Meissner effect, which expels all magnetic fields. Well, almost. This is actually only true once you get past the London penetration depth (a couple of hundred nanometers). Can we get an eddy current within the London penetration depth? I have no idea whether this is possible.

A lot of EEs get this wrong, because applying Faraday's law of induction with resistance equal to zero in the calculations gives a similar answer -- no change in magnetic flux -- but Faraday's law doesn't quite explain what happens when you have a magnetic field before the conductor-to-superconductor transition.

Another issue is that a perfect Meissner effect is difficult to achieve in practice. As the material undergoes the superconducting transition, small amounts of flux can be trapped in the metal, "pinned" there by tiny non-superconducting impurities.

When I was working on the electron microscope shield, we experimented with using eddy currents to reduce this trapped flux. It turns out that tin has a huge drop in electrical resistivity just before it becomes superconducting. So if you rapidly rotate a tin cylinder while cooling it through the transition region, any outside magnetic field induces eddy currents in the low-resistance-but-not-yet-superconducting tin. These eddy currents tend to drive out magnetic flux before the the tin becomes superconducting and thus able to trap the flux. Alas, the effect doesn't work for the axial component of the external field, and it doesn't survive a warmup/cooldown cycle in the field.

the question is, how much of the above needs to be in the Eddy Current article and how much needs to be backed up with citations. Is "Eddy currents in conductors of non-zero resistivity generate heat..." likely to be challenged? If so, is a Wikilink to Superconductivity good enough, or do we need citations for all of this in this article? --Guy Macon (talk) 16:00, 4 October 2012 (UTC)

Edit warring by user:Embrittled
User:Embrittled has started edit warring in order to push his preferred version over the objections of at least two other editors. I have placed a warning on Embrittled's talk page and encourage him to dicuss the reasons for his changes here. BOLD, revert, discuss cycle (often shortened to WP:BRD) has some excellent advice about this. --Guy Macon (talk) 01:39, 31 October 2012 (UTC)


 * I'd like to point out that Guy Macon is, in effect, involved in the edit warring himself, and is making personal attacks on this page, and is pushing his own personal views on Wikipedia as well; as well as misrepresenting the situation here. There are actually reliable sources that support my edits, as well as material that is produced by universities and many other sober websites as well.Embrittled (talk) 03:07, 31 October 2012 (UTC)


 * A single revert is not edit warring. Please read WP:3RR. And please give us citations to those reliable sources; if the sources support your version, then of course i support it as well. --Guy Macon (talk) 07:12, 31 October 2012 (UTC)

Please note that User:Embrittled is currently using the following pages as his battleground:

Induction motor

Eddy current

Wikipedia talk:WikiProject Physics

--Guy Macon (talk) 09:43, 31 October 2012 (UTC)


 * So you think that if you revert somebody else multiple times that's not battlegrounding, nor is it if you are posting lots of your insulting tags across numerous talk pages, nor is tagging your subject lines with multiple insulting tags.Embrittled (talk) 21:34, 31 October 2012 (UTC)


 * Again, please read WP:3RR and WP:BATTLEGROUND. Reverting you on Oct 1 and then again on Oct 31 after you ignored four editors urging you to discuss your edits on the article talk page is not unreasonable. Your repeated violations of WP:CIVIL and WP:NPA are. The path you are on will lead to you being blocked from editing Wikipedia. If you stop it now and instead follow WP:CONSENSUS your desired changes will be evaluated fairly. --Guy Macon (talk) 03:11, 1 November 2012 (UTC)

Power generation
I've recently seen some bicycle light generators that use eddy currents from magnets placed next to alloy rims.

How would efficiency compare to a dynamo hub or a friction-driven bottle generator? --Triskele Jim 01:29, 19 January 2016 (UTC)

Assessment comment
Substituted at 14:07, 29 April 2016 (UTC)

eddy current motor
You need a section on eddy current motors. — Preceding unsigned comment added by Arydberg (talk • contribs) 23:49, 1 July 2016 (UTC)

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Ultimate source of drag force
In the Explanation section, I think we should clarify that the Lorentz force on the electrons carrying the eddy currents is what opposes the conductor's motion in the brake diagram, rather than the opposing B-field in the perpendicular axis. The moving charges closer to the magnet feel more force, and that force is in the same direction (-v) on both ends of the magnet. We need to show that net force vector antiparallel to v. Seems like explaining it by just saying that the eddy currents induce an opposing B-field is hand-waving and misleading.

For example: https://opentextbc.ca/physicstestbook2/chapter/eddy-currents-and-magnetic-damping/ — Preceding unsigned comment added by 173.127.151.28 (talk) 10:58, 13 April 2020 (UTC)


 * Thanks! I wrote that section of the article. I can't believe I missed that. I corrected the text.  --ChetvornoTALK 20:26, 23 October 2020 (UTC)