Energy stored in *what* in a magnetic air gap? - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: July 14, 2022, 03:17:49 am
content: Say we have an iron cored inductor with a steady DC current flowing through it and the core has a small air gap. Furthermore, the inductor is not saturated. The inductor holds a certain amount of energy. AFAIK, for the most part that energy is in the magnetic flux in the airgap, not the actual iron core.Question - how does this energy exist in nothing? It's not like an iron core that has it's magnetic domains rotated, or a capacitor that has it's dielectric stressed somehow? Energy seems to be stored in nothing at all. What has changed in that air gap?

author: ejeffrey
author_rank: Super Contributor
timestamp: July 14, 2022, 03:56:53 am
content: In the magnetic field. It takes energy to create a magnetic field.The same goes for a vacuum gap capacitor.

author: BravoV
author_rank: Super Contributor
timestamp: July 14, 2022, 04:09:18 am
content: Quote from: Circlotron on July 14, 2022, 03:17:49 amAre you sure there is really "nothing" there ?As like eg: electron or quantum level gremlins there ?

author: robert.rozee
author_rank: Frequent Contributor
timestamp: July 14, 2022, 04:36:53 am
content: surely there will be a force pulling together the two faces of the gap? this force will cause energy to be stored in the elastic (mechanical) properties of the core. i do not know how much energy this would account for, but presumably it would be at least some of it.cheers,rob :-)

author: ejeffrey
author_rank: Super Contributor
timestamp: July 14, 2022, 05:17:09 am
content: Mechanical energy storage is negligible.Some (magnetic) energy is stored in the core. The density is much less but gap usually has much less volume so it can be a significant fraction.

author: Circlotron
author_rank: Super Contributor
timestamp: July 14, 2022, 05:34:20 am
content: Quote from: BravoV on July 14, 2022, 04:09:18 amNo, I'm not sure at all.I should have said "whatappearsto be nothing"Maybe the magnetism in the air gap distorts spacetime like gravity does?Maybe dark matter is the storage medium. I really don't know.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 14, 2022, 06:22:47 am
content: You can certainly talk about quantum field theory, spacetime, and virtual particles. That is certainly interesting, but I don't think any of them are going to make it more understandable or intuitive. Honestly that would mostly be just using fancy sounding words to try and convince you to stop asking the question.At a simple but perhaps not terribly satisfying level, the gap isn't empty: there is a magnetic field in it. The lowest energy state of a region of free space has zero electric and magnetic fields. If you want to create a field there you have to supply energy to do it. If it didn't cost energy to create a magnetic field we would have million tesla fields all over the place causing havoc.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 14, 2022, 12:20:39 pm
content: Try pushing two identical magnetic poles together. That's the nothing.

author: tszaboo
author_rank: Super Contributor
timestamp: July 14, 2022, 12:57:47 pm
content: Quote from: Circlotron on July 14, 2022, 05:34:20 amThere are 4 fundamental forces.Gravity, electromagnetism, the strong and the weak nuclear force.So yes, "It's like gravity"And it doesn't distort spacetime, it does it's thing with the photons.

author: Picuino
author_rank: Super Contributor
timestamp: July 14, 2022, 01:11:37 pm
content: By definition, the energy stored in an volume of vacuum (if air changes little) due to a constant magnetic field will be:E = B^2 * V / (2 * u0)Where:E = Energy of the magnetic field in .B = Intensity of the magnetic field in .V = Volume of the magnetic field in .u0 = 1.2566 x 10-6 Magnetic permeability of vacuumKeep in mind that when a magnetic, electric, gravitational, etc. field appears, the bodies that are within that field become potential energy, so that the field is not "nothing", it is something that shapes the space where it is located and allows potential energy to exist.For example, the fact of moving an electron to a certain position will cause another electron that is within its "electric field" to have a potential energy that manifests itself with a separation movement of the second electron towards another position far away from the first electron. This can be understood as an energy that has created the magnetic field of the first electron.With magnetic fields it is not so obvious because there are no monopoles, but the effect of the "field" can be understood as in the electric case.

author: Picuino
author_rank: Super Contributor
timestamp: July 14, 2022, 01:18:02 pm
content: On the other hand, you can see that when you move a magnet through a coil, it generates an induced current and electrical energy in the coil. The changing magnetic field translates into an induced current.That magnetic field can also be the field created by the coil itself (hence it is called self-induction). That way you can see the "energy" not in the field itself, but in the change of the magnetic field from one value to a different value.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 14, 2022, 01:59:53 pm
content: Quote from: tszaboo on July 14, 2022, 12:57:47 pmMaybe not on a PCB, but out there...https://en.wikipedia.org/wiki/Magnetar"remarkable things happen within a magnetic field of magnetar strength. "X-ray photons readily split in two or merge. The vacuum itself is polarized, becoming strongly birefringent, like a calcite crystal. Atoms are deformed into long cylinders thinner than the quantum-relativistic de Broglie wavelength of an electron." In a field of about 105 teslas atomic orbitals deform into rod shapes. At 1010 teslas, a hydrogen atom becomes 200 times narrower than its normal diameter."The only thing that scares me more than Jupiter is a magnetar, the stupid things. Probably because it sounds like Sinistar, the video game that freaked me out as a kid.

author: tszaboo
author_rank: Super Contributor
timestamp: July 14, 2022, 03:31:51 pm
content: Quote from: Alex Eisenhut on July 14, 2022, 01:59:53 pmFor sure on the PCB. What do you think radio waves are? Or EMC?You also make photons if you throw a magnet in the air. Not visible ones, but photons nontheless.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2022, 04:25:31 pm
content: Quote from: ejeffrey on July 14, 2022, 05:17:09 amIndeed, it's easy to see why:Quote from: Picuino on July 14, 2022, 01:11:37 pmDividing by V gives an energy density, which is also a pressure: the Maxwell stress. Typical materials have elastic modulus 120GPa or higher, while Maxwell stress is rather modest at flux densities reasonable for materials (2T ~ 0.8MPa). So, strain in the 1ppm range -- equivalent to a degree temp change (thermal expansion). Magnetostriction is a much stronger effect (~0.1% strains for typical materials; some much, much higher!), hence the buzzing (at twice line frequency: magnetostriction is a square-law effect!) of mains transformers for example. Or CRTs, for those who remember.This also tells why, for example, coilguns will always and necessarily suck: 0.8MPa is all of 8 atm; a potato gun typically does as much. To compete with chemical propellants, over 30T would be needed. (Which on a related note, might give you some appreciation for just how much work is really needed to pull off lab magnets of such level: they consume massive power (to dump all that current through copper slabs; superconductors run out around 15-20T!), and need massive mechanical strength to keep from blowing apart!)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2022, 04:33:15 pm
content: Quote from: Circlotron on July 14, 2022, 03:17:49 amNo medium needed; or if you like, what we now understand to be "space" or "vacuum", used to be called the "luminiferous aether".It's simply a property of space, that at every point, there can be several values simultaneously: electric field; magnetic field; various quantum fields; and there are laws which govern how those fields vary over space and time. (You can't have a pointlike magnetic field somewhere, it has to loop around something in a smooth, continuous manner; etc.)Indeed, materials tend to reduce the energy storage of free space; since energy density goes as B^2/(2 mu), we see that increasing mu = mu_r mu_0 reduces energy storage locally. This is still beneficial for inductors, as we're likewise limited on how much magnetization (A/m) we can apply with copper coils; there is an ideal mu_r which depends on conductivity of the winding, and our desired L/R time constant. We're normally satisfied with L/R ~ ms for power switching inductors, for example, or 100s to 1000s ms for mains frequency, etc. Along with a few other scaling and geometric factors, this sets the core dimensions, air gap, etc. needed for a particular value and quality of inductor.Put another way: if we didn't mind the relative lack of shielding, or the relatively high leakage inductance, we could do perfectly well with superconducting coils and no core at all -- indeed, given a few ten thousand turns in a tiny lump, we could have a mains frequency transformer capable of some kW in the palm of your hand, if not for its cryogenic temperature causing frostbite on contact.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 14, 2022, 05:13:15 pm
content: Quote from: T3sl4co1l on July 14, 2022, 04:33:15 pmYou summed it up, when dacus goes on and on about his "energy storage device", well, every cubic angstrom of the universe is an "energy storage device"....

author: ebastler
author_rank: Super Contributor
timestamp: July 14, 2022, 06:23:56 pm
content: I am not quite sure what the question is. In an electronics forum, why would anyone be surprised that electric and magnetic fields can store energy? If you let alternating electric and magnetic fields interact, they can eventransportenergy over a distance, and of course that works in a vacuum too. (Actually it works better in a vacuum than in a lossy medium.)By way of example, over the years the sun has managed to send us quite a bit of energy, over quite an expanse of vacuum...

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 14, 2022, 06:38:06 pm
content: I don't think it's the "that", it's the "how". We just take things for granted and repeat phrases but we really know how it stores energy? It's not obvious at our scale, we can see things like compressed air in a tank, or acid in a battery, or dry wood burning. How does something invisible work? It's the old action at a distance dilemma.I'll just leave this herehttps://youtu.be/MO0r930Sn_8

author: rfeecs
author_rank: Frequent Contributor
timestamp: July 14, 2022, 09:18:56 pm
content: You're only a step or so away from saying energy flows outside of wires.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 14, 2022, 10:34:58 pm
content: Hey, I'll have a go. This is intended to give the correct intuitive picture, and a good start if you want to research the stuff further, but it is not acompleteor precise picture by any means.Quote from: Circlotron on July 14, 2022, 03:17:49 amMagnetic field does not require any kind of medium to exist. The field itself exists, and is measurable (by how it affects matter and even photons passing through it). It is this field that "stores" the energy. (In reality, the field itself is energy, just of a particular type or form, just like matter is.)Do note that it indeed is the field itself that exists. Electromagnetic fields aremediatedby photons, which means that when the field interacts with something else, that interaction is described in terms of photons. It is exactly this why all electromagnetic radiation and photons are one and the same thing, just viewed from a different angle; the dichotomy is calledparticle-wave duality.Magnetic field itself is part of electromagnetic force, one of the four fundamental forces in nature, and aside from gravity (which we cannot really manipulate except by moving mass or (huge amounts of) energy around), is the one that basically governs human-scale phenomena like chemistry, materials properties, and electronics. (The other two only work at really short ranges. Short interaction is what makes quarks into protons and neutrons, and keeps them together, and weak interaction is responsible for nuclear fission and fusion. We cannot even measure these directly, only model them via their effects.)Magnetic fields are vector fields, that can be described using two related mathematical vector fields:Hdescribes the magnetic field strength (in e.g. units of amperes per meter), andBdescribes the magnetic flux density (in e.g. units of tesla or Newton meters per ampere). The reason for this split is that in vacuum,H=B/µ0, where µ0is a constant called vacuum permeability. In non-vacuum, the relationship varies by the magnetizationMof the matter present,H= (B+M)µ0. (Magnetic polarizationI(in teslas) is sometimes used instead, asI=Mµ0. There are a lot of terms for the related things, so it is more confusing than it really needs to be, but that's just us humans doing this physics thing.)Maxwell's equations describe the behaviour of magnetic fields, electric fields, charge density, and current density.Thus, the answer is, the magnetic field itself is/contains the energy, even in vacuum.Any suitable matter that retains magnetization can be used to "store" that energy long-term; this gives usmagnets.Because moving charges generate a magnetic field, currents induce useful magnetic fields and vice versa; this gives uselectromagnets,solenoids,inductors, andtransformers.(And we damn well know how magnets work, dammit. The problem is that exactly because it is avector field, it behaves differently to the scalar fields humans are used to and can understand intuitively, and you need to understand those and other related concepts before you can truly intuitively understand magnetism. Which means that it is not possible to explain magnetic fields in layman terms precisely, but that's only because it requires understanding concepts that do not occur to laymen, first. Furthermore, although we know how they work, we don't know yet all the useful ways they can be used – we only know a few, really! ––, so there's tons of research and engineering to do to find those out. It's like the wheel. Julius Caesar had wheels, but he didn't have the bicycle; he could have, they already had carts and roads and such aplenty, and you can build a bicycle out of wood, too. Just think what that would have done to the movements of the legions.)

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 14, 2022, 10:57:56 pm
content: Quote from: rfeecs on July 14, 2022, 09:18:56 pmIt's like the silly debate where one side says current cannot pass through a capacitor, and the other side says energy does pass through a capacitor.Both are right, because a capacitor stores energy as an electrical charge. It does not pass any current through itself (if we ignore leakage), but you can easily use the energy in the stored charge in a suitable circuit, on either side of the capacitor. Thus, while no current passes, you can easily pass energy across the capacitor in form of a stored charge. Simples. (Except that theusesof this are myriad and wonderful, and definitely not simple!)Where does the energy flow? Well, fuck, everywhere. Where does the majority of the energy flow? In what we use now, for direct current, mostly in the conductor; for alternating current, mostly near the surface of the conductor. But it depends. If the conductor is hot enough, it will emit electrons, and these can carry the current and thus energy even across a vacuum gap. If the shape and placement of the conductor is such that the electromagnetic field generated by the current flowing in the conductor gets coupled to somewhere else, then most of the energy can flow in that electromagnetic field (but will also obviously flow in the conductor too).The entire question aboutwherethe energy flows is silly, because it depends exactly on the geometry of the system at hand.You know high-voltage high-current transmission lines? The magnetic field they generate is pretty weak, but the electric field they generate can be damned strong; so strong, in fact, that it can affect cell function in animals with prolonged exposure. (Fortunately, it's easy to shield against – conductive grounded enclosure suffices –, and because of Earth ground, is very limited in volume to the close region between the transmission line and the ground closest  to it.)If you want to steal energy from that line, you can trivially couple to it capacitively or magnetically: put another wire in parallel, or a few turns around the it.If we look at the actual electromagnetic fields involved, then we realize that it is exactly the geometry that we should be considering, because it is the shape of this field – which is affected by whatever it interacts with – that is the question.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: July 15, 2022, 10:05:18 am
content: if energy exchange could not exist in nothing (vacuum)then radio tubes and CRT's would not exist

author: Terry Bites
author_rank: Super Contributor
timestamp: July 23, 2022, 04:12:53 pm
content: The electromagnetic field is best understood as being ubiquitous. M. Faraday made this wild claim many years ago. Magnetic and electric devcies just locally distort "the field", there are not multiple insances of fields associated with individual devices. It just looks that way. All the energy is stored inthefield not a field.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 23, 2022, 05:13:31 pm
content: Quote from: oz2cpu on July 15, 2022, 10:05:18 amWe'd have a hard time seeing the Sun too.

author: coppercone2
author_rank: Super Contributor
timestamp: July 23, 2022, 05:22:20 pm
content: don't forget the mostly discredited theory of the 'ether'https://en.wikipedia.org/wiki/Aether_theories

author: TimFox
author_rank: Super Contributor
timestamp: July 23, 2022, 07:09:09 pm
content: Quote from: Circlotron on July 14, 2022, 03:17:49 amNote that magnetic flux density (B-field) exists in vacuo, for example in the interior of an air-core solenoid coil. If we add a permeable material (e.g., ferromagnetic iron or paramagnetic magnesium) to the system, the magnetic moment of the material aligns to that field to produce a large polarization, which adds to the field. A diamagnetic material (e.g., bismuth or graphite) on the other hand will polarize in the opposite direction and reduce the field.