2.12 The Origin of the Magnetic Field

This video was made for a junior electromagnetics course in electrical engineering at Bucknell University, USA. The video is designed to be used as the out-of-the-classroom component and combined with active learning exercises in class. This video covers the origin of the magnetic field in a somewhat qualitative way so that later derivations of magnetic field equations make more sense.

[@douglasstrother6584]

I first came across the connection between the magnetic field and relativity in "Electromagnetic Fields and Waves" by Lorrain and Corson. The Lorentz Force is a consequence of Coulomb's Law under a Lorentz Transformation.

[@warrenchu6319]

Totally correct. But the book explained it in a too mathematically rigorous way, without any intuitive insight. Decades later, I finally understood. 😮

[@jamesnasmith984]

Unusually fine speaker and crisp flowing content.....even a retired MD got something from this.

[@DelonSamourai]

Very straight and useful explanation with good diagrams and figures. I find these videos much helpful to understand some difficult subjects than other videos about the same subjects. Thanks a lot.

[@JustNow42]

Mag fields are very easy to understand. If you have a moving electric field of Energy E and speed v , then the mag field energy is 1/2 m v2 , which is the kinetic energy of the electric field. m = E/ C2 . Note C2 means C*C and v2 means v*v ( at not too high speed)

[@simonapalosan3208]

nice explanations, one thing: B is called magnetic induction = magnetic field vector and not magnetic flux

[@magncity1817]

And, since you state magnetic fields can also be understood as electric fields…this may be extended to permanent magnetic field as well…as internally to the magnet there are similar relativistic discussions…the center of the magnet represent a relativistic change in direction of motion similar to a dielectric…great video

[@fatroberto3012]

This is terrible. Somehow you have managed do mix up just about every aspect of this. The electrons in the wire move very slowly (drift velocity) but there are an astronomical number of them, so even though the Lorentz contraction is tiny it has an appreciable effect.

[@alwaysdisputin9930]

If we assume kridnix's flying past in a spaceship & the wire is on Earth, then he's perfectly entitled to regard himself as stationary & the electrons as moving at high speed (due to the principle of equivalence). So I think he's right in some frames of reference. But with respect to our normal everyday frame of reference you're correct.

[@catherinegrimes2308]

@@alwaysdisputin9930 Fat Roberto means that the relative velocity between the nuclei and the electrons is low and he is correct on this.

[@HitAndMissLab]

yeah, great video. I was looking for relativistic explanation of currents and magnetization. Aldough this is a very good explanation, there is an error. According to many other videos, by professors of physics, speed of charges inside electric wire is not more than few centimeters per minute. Relativistic effects are created by shearling enormous numbers of charges, ruther than their speeds.

[@kentdavidge6573]

+HitAndMissLab Actually field contraction occurs at any speed. The greater speed, the greater intensity of the field. But the effects are easily measurable even at low speeds because the electromagnetic interaction is very strong, so that a small "magnetic" field produces a measurable force.

[@kentdavidge6573]

+HitAndMissLab Since there are many charges in the wire, it's more convenient to look at charge density in such situations.

[@glynnevans4531]

The video is excellent. However the comment is well made and deserves a correction in subsequent versions. The clear implication is that near light speeds are reached because the mass of the electron is so small. The video is accurate that there are indeed highly observable relativistic effects but these are not due to electrons reaching "close to light speed".

[@CreationTribe]

​@@glynnevans4531 yup - in fact, electrons travel at ~0.0000000001% c ... I'm pretty sure that's slower than an opiate induced mosey.

[@DeadCatX2]

The drift velocity is a result of collisions. The collisions cause a change in direction. So we have the gigantic relativistic speed in the (technically opposite) direction of the current being modulated by the frequent collisions, which results in a much slower average speed for a given electron.

[@warrenchu6319]

Question: If magnetic fields are the result of unequal relativistic contraction of charge densities as seen by a moving charge and only is a manifestation of the electric field in a moving frame, how do we explain the magnetic field produced by an isolated electron? There is an analogy of the spin of the electron having a magnetic field akin to the magnetic field produced by a wire loop. Classically (although we know quantum mechanically the electron is not a point charge), the electron's spin is like the current in the wire loop, but the magnetic field of the wire loop can be explained by bending the straight wire into a loop. All explanations of what causes the electron's own magnetic moment (ratio of magnetic field to mass) is that it is a fundamental property of the electron, which implies magnetic fields are a fundamental force. Are magnetic fields a fundamental force, like electric fields? (I know force is the result of a field acting on a body but I'm using the terms loosely. :-p )

[@zazlari5601]

I don't understand why the effect doesn't occur too when the outside charge is static. It see electrons moving too so must see them contracted so at a higher density ? The effect is present too, maybe smaller but present?

[@marimarmarimar25]

Thanks a lot. If the conductor is shielded with a conducting foil eg. Aluminium, your theory does not work because electric charges do not interract thru the shield. But the constant magnetic field passes thru the non magnetic shield as every pupil knows. How comes that?

[@nikan4now]

great video...one question so this force is felt by the charge in the non-inertial reference frame isn't it? however why is it that the same force is measure from a stationary reference frame too (a person not moving with the charge)

[@silversurfer493]

If your frame of reference is the moving election (you sit „on“ the electron), then the positively charged atoms (ions) are „Lorenz“ contracted. They „move“ in the opposite direction of the electrons. If you are stationary with the atoms (ions), then the electrons are „contracted“ (their distance). The electrons move relative to the atoms and therefore Lorenz contraction applies to them. Both these views explain the imbalance in charge distribution that eventually leads to attraction (or repulsion, depending on the direction of movement). This is relativity.

[@haikkokorian3323]

You got it all wrong from the beginning. First, 1 Coulomb is not = to 1 Amp per second, 1Amp is = 1 coulomb per second. The speed of electrons is not defined by electric field E and the electron mass, as the wire can be zero resistance hence no electric potential. The speed of electrons is the number of electrons per second due to current divided by the number of atoms per cross section depending on the thickness of the wire.

[@nbsr1]

This was very interesting. Thank you! Two questions: - if magnetic field originates from a (non-linear) relativistic correction, then why it is proportional to the particle velocities? - what about the "displacement current" (change of the electric field in time) - it produces magnetic field without any charged particles moving.

[@luismaoer7145]

John Green Without charges the electric field is 0 everywhere. When you say the electric is non-zero it means there have to be charges somewhere and when you say the electric field is changing it means the locations of the charges must change, they must move

[@marcinna8553]

If you have a stationary positive charge next to a wire where a current is flowing, why would that charge not sense a magnetic field? In this case, the positive charges inside the wire are in the same inertial frame as the stationary positive charge outside the wire, but the moving electrons will be compressed in space and produce a higher negative charge density. One would therefore expect that there would be an attraction of the positive charge towards the wire.

[@VinOnline]

Because the stationary positive charge is *stationary* and therefore doesn't have its own magnetic field. In order for a positive or negative charge outside to experience this magnetic field created by the current in the wire, the charge has to be moving because moving charges create magnetic fields. And so then, from an observers perspective, the charge's magnetic field interacts with the current's magnetic field, resulting in attraction or repulsion. Now if you replace the charge with a permanent magnet, then its velocity wouldn't matter, and the magnetic field it already has would interact with the current's magnetic field.

[@marcinna8553]

@@VinOnline I see, this would be like trying to influence the static charge on a balloon surface with a magnet. Nothing happens. But what do "stationary" and "moving" mean? There is no absolute reference frame in the universe. We could just as well say that both charges are moving relative to one another. If you had exactly the same setup (i.e. a stationary charge next to a wire carrying a current) but you were zipping by in a spaceship, now from your point of view both charges are moving. Do they now influence one another?

[@CreationTribe]

Great video - thank you :)

[@jacobvandijk6525]

"Moving charges see relativistic effects". Because these "huge forces" give them ... driftvelocity???

[@criggeruk]

quick question, I thought electrons moved at about 0.1mm/ second in an electric current, nowhere near the speed of light, what am I missing?

[@joshicune]

I can't agree with this explanation. If density of electrons is increased or decreased that mean wire will not be electrically neutral - meaning some charges are added or removed and we know it is not true. Electrical current can flow in metal circle without battery, due to change in magnetic field around and remain neutral in term of charge. Also, single moving electron also produce magnetic field around so this is not about distance between charges. And last but not least, if I'm observer sitting in front of this lab and this magnetic force you want to present as electrostatic force, than change of direction of free moving charge out of wire would not cause change of direction of force. I would like that this is that simple, but it is not.

[@davidbrewer9030]

As the electron accelerates closer to the speed of c, as its length decreases in the direction of motion, does its mass increase?

[@frankmccann29]

Not too many people can explain complex issues where a dummy like me can understand. 👍 Thanks. The Einstein factor and the shortening effect of charges is wonderful. But what happens when they smash into each other from opposite directions?

[@EugeneGaufman]

А, что произойдёт, если мы предположим, что электрический ток «никуда не течёт», а есть результирующее состояние перераспределения энергии волновой функции «периферии атома» и «его ядра». В этом случае конфигурация внутренней структуры феномена (кристаллической решетки и броуновского движения, плохо структурированной пульпы- этапов динамики стремления энтропии к максимуму, формирующей формы эволюции от динамической системы «порядка» к динамической системе «детерминированного хаоса», за которым просматривается состояние «квантовой сингулярности», состояние подпороговых «квантовых флюктуаций», недоступных физической определяемости рецепцией человеческого организма (скоростям процессам более коротким, чем способность формировать потенциал действия при деполяризации клеточной мембраны- от постоянной Планка до кванта энергии Потенциала действия). В этой логике, границы любого феномена природы определены способностью совокупной перцепции человеческого организма осознавать физические характеристики сред существования в гармонии границ физических констант. Если это так, то атрибутам человеческого сознания полностью принадлежит способность генерации физических параметров определения себя самого и обстоятельств своего бытия Существующими. Физические параметры, генерируемые механизмами перцепции информационного воздействия на человеческий организм проявляются «Точками», «первичными двумерными слайдами реальности (32 мсек активности tpj) и трансформацией 3D образов через мультипликацию, генерирующую движение во времени и пространстве за счёт вовлечения структур энторинальной коры головного мозга (активность лимбической системы 90 мсек, которая после вовлечения префронтальной коры и связанной с ними функцией гиппокамповой извилины - 350 мсек предоставляет человеку в сетях информационных технологий социальной коммуникации глобализующегося человечества Вербальные символы эмоциональных образов поведения, составляющих суть физических моделей двигательной активности человеческого организма, сформировавшегося фокусом внимания здравого человеческого смысла, актуального состояни мотивации поведения, отражающего баланс гомеостаза внутренней среды физио- биологической природы, психо- социо- политического реагирования в динамике горизонтальной тенденции эволюции биосферы планеты Земля и вертикальной тенденции эволюции информационного пространства человеческого индивидуального и коллективного разума. Возвращаясь к физике электромагнетизма можно предположить его характеристики неотъемлемой фундаментальной частью информационного пространства антропоцентрического мироздания, перераспределение энергии в связях функциональных элементов внутренней структуры зависит лишь от квантовой фазы фиксации внимания наблюдателя, уровень которой подчинён взаимоотношениям блокчейна, преобразовывающего конфигурацию связей одновременно в граница фокуса внимания. Электоромагитизм, отсюда выглядит сингулярностью, отражающую различную степень организации внутренней структуры, определяемой перцепцией человеческого сознания в состоянии Бодрствования, чем исключает фактор скорости и направленности движения как такового. Все эти физические категории возникают в результате кодирования энергии воздействия на человеческих организм и последующим преобразованием этой энергии в образы памяти и физические символы паттернов эмоциональных образов поведения - Мемы и Артефакты ноосферы планеты Земля, представляющей собой геологический ресурс коллективной памяти глобализующегося человечества. Где же появился электромагнетизм? Он возник в интерпретации результатов эксперимента в научном познании себя и мироздания. Условия эксперимента вносят дисбаланс в гармонию констант антропоцентрического бытия, а энергия восстановления этого Баланса формирует иллюзию объективной и трансцедентной реальности (принцип MRI). Жду осмысленных и аргументированных возражений.

[@Virigis]

. Dear, kridnix! 1. Not a magnetic field but twisted electromagnetic fluxes, 2. If there are such flows, then we can talk about pressure / discharge and be able to transmit and accumulate electrical energy. ... This understanding can be obtained based on the vortex theory. Good luck! 1. Не магнитное поле а скрученные электромагнитные потоки, 2. Если есть такие потоки, то можно говорить о давлении/разряжении и иметь возможность передачи да накоплении электрической энергии. . Такое понимание можно получить на основе вихревой теории.

[@leeorshimhoni8949]

You get a force even if one of the charges is moving, and not both.

[@nikan4now]

So the magnetic field is the same in nature as the electric field?

[@silversurfer493]

No, but the force that charged particles feel is actually electrostatic force. There is no such thing as a magnetic force ...

[@nikan4now]

@@silversurfer493 This is not clear to me. A force is just a force though, but from different origins like gravitational, electric etc...why is there no magnetic force and only electric? I've never seen this in any classical physics book.

[@silversurfer493]

@@nikan4now Hello Nikan, i understand your point. This whole magnetic force thing was also unclear for me for a long time. In many classical physics books, only concepts like field and their mutual interactions (electric field vs magnetic field) are discussed. This is OK and it also leads to a good (although often complicated) understanding and possible calculation of interactions - yet, if you look a little bit closer, none of these classical approaches really explain what the origin of attraction or repulsion is. For example the „two wire“ experiment ist often used to explain magnetic interactions, yet using the circular magnetic fields that surround the wires. But this is no explanation, just visualization or „explanation“ in another way... The relativistic view on things however goes to the root cause: electrostatic interaction. As magnetic „force“ is occurring when charged particles are moving (and only moving charge!!) it is obvious that magnetic force must be related closely to electrostatics. And all this movement can be described in terms of relativity, even though velocities are often rather small. And then, when considering Lorenz contraction, the common aspect of magnetism and electrostatic „force“ becomes obvious. Eventually, you do not need the concept of magnetic force because you can trace it back to electric force. So „magnetic force“ in itself is a concept but not „the real thing“ so to say...

[@nikan4now]

@@silversurfer493 Thanks for your comprehensive response. Is there a textbook that bears this out i.e. no such thing as a magnetic force only electrostatic?

[@silversurfer493]

@@nikan4now Hello again. I remember having read this explanation in a textbook long ago. I think it was Purcells book about electromagnetism. I also remember that the complete mathematical treatment included 4 dim Tensors and the like. If i remember correctly, it was this book where the connection between coulomb interaction (== electrostatics) and magnetism was explained, basically by transforming the electrostatic equations into the "magnetic" equation with the use of relativistic contraction. And therefore (more or less my conclusion), because the nature of forces cannot depend on a subjective choice of reference system, there is only one force that is responsible for the magnetism.

[@jacobvandijk6525]

This is education: we know the electronflow is the real thing, but we still teach the wrong conventional flow because we taught it that way for the last 100 years or so. Very scientific approach by these conservative minds ;-)

[@nehasakka4833]

what if any one of them is stationary? if only one set of charges moves still there would be relative motion and hence length contraction should follow...kindly correct me where am wrong

[@malindal7458]

I agree with you, don't understand why it's wrong

[@silversurfer493]

This is a thing that also worries me ... there must be more to this than just this rather simplified picture. Maybe a detail is missing. However, most explanations & videos assume that the charged particle IS moving along a wire. But i agree: I do not see a reason why a stationary charged particle is not affected (as suggested). Eventually it means that a particle MUST move relative to both charged particles in the wire: the positively charged nuclei and the electrons. But then, if a moving charged electron is assumed to have the same velocity as the electrons in the wire, the whole concept breaks down again. Is there someone with a good explanation?

[@vladimirkindl4802]

@@silversurfer493 You're right, I was also interested in this, so I performed a simple experiment with an air-cored coil (wound by my hand :)) powered by a dc current. I have a charged a piece of paper with a plastic bar (by rubbing against hair) which was attracted to a coil. So your concept is right!

[@silversurfer493]

@@vladimirkindl4802 Dear Vladimir, thank you for your response. I wonder however why these results are not properly described in textbooks. I have only found examples where the charge is "MOVING" somehow. This is what made me curious !? Is the explanation just wrong? Is something missing here? If you can confirm that stationary charges are indeed attracted by a coil, generating a magnetic field, this would save the whole concept of "special relativity" as the correct and (possibly) ONLY reason vor magnetic phenomena. I finally can assume that i have understood it (on that level).

[@vladimirkindl4802]

@@silversurfer493 Dear Silver Surfer, I think the problem is in a way how the textbooks deal with force effects separately, i.e. they speaks of either a magnetic or an electric field. This simplification works well for education, but it does not follow the reality. In my experiment, I first situated the coil so that the (charged) piece of paper lied in a geometric center of the coil - no force interaction was noticed. Then I put the paper close to the wire (outside the coil) and the force finally showed up. In other words, the static charge was affected by an electric field coming out of surface of the conductor. The same field is usually used to detect live conductors, see Fluke 1AC-A1-II VoltAlert Non-Contact Voltage Tester. It is also quite easy to simulate (or visualize) such an electric field using finite element method. Personally, I believe that this "relativistic" concept is general and predicts the simultaneous acting of electric and magnetic fields. This approach can even explain how the magnetic field acts on ferromagnetic materials (due to orbital and spin motion of electrons). Very nice mathematical derivation is here: kzbin.info/www/bejne/l5CygKV7gbiSbZI&ab_channel=TheoreticalCondensedMatterPhysics

[@live4Cha]

i wonder if that is the case and b is only e under relativity we should by placing a charged capacitor instead of the q outside and by moving it displace the charges on capacitor. doesn't feel right to me based on faraday law of induction that makes change of fields source of induction. thanks anyway.

[@catherinegrimes2308]

Good video, but one of your equations is incorrect. Around 1:25 you have the equation: 1 Coulomb (C) = 1 amp per second It should be: 1 amp = 1 coulomb (C) per second

[@164procar4]

Clap your hand anytime he say “field”

[@aliaatarek7106]

wow

[@jimtwisted1984]

Your explanation is exactly backward.The electrons are moving in a wire and hence the negative charge is compressed.

[@adairjanney7109]

God is the origin

[@liminal27]

conventional current is dumb

[@TeachTechToe]

People think that magnetic field comes due to special relativity but actually special relativity came out due to magnectic field and lorentz transformation

[@silversurfer493]

Well, not really. Special relativity is a result of the absolute speed of electromagnetic waves (light). This speed is equal for ALL reference systems (inertial systems) irrespective of their relative movement, thus violating Galileis transformation rules. Special relativity theory resolves this problem.