Why moving charges produce magnetic field?

Does it, really? Let's explore what Einstein has to say about this question

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[Mahesh_Shenoy]

What’s the next topic you want the video on? (I prefer specific deep nuanced questions of high school science topics)

[douglasstrother]

It's pretty wild that the generation of magnetic fields by currents is an everyday relativistic phenomena, and ferromagnetism is an everyday quantum phenomenon.

[davidofearth]

I remember first deriving in grad school the force between two current carrying wires with only coulomb attraction and Lorentz, and getting the same answer as the standard formula that uses magnetic fields. This absolutely blew me just like it did the OP. Awesome to see another person having the same experience.

[Feynman_Fries]

Hi Mahesh, I don't know much about you but your passion for physics seeps through and resonates with me. I quit my high profile MNC job for my sheer passion for physics and turned to school teaching. During one of my brainstorming sessions while preparing for a lesson I stumbled upon a video from khan academy. Now I generally do not appreciate the way physics is treated by Indians or Indian teaching fraternity, reducing it a rote body of formulas and derivations (one of the reasons that drove me to be a teacher), and so I only really head to Feynman's lectures, Resnick Halliday or lectures by Walter Lewin, but jeez I was shocked and intrigued by the passion that your voice had, and the way you explained it (I think it was Maxwell's correction of ampere's law). Since then I've regularly followed you, and your work and admire the what you're doing. India needs good physics teachers, that know that physics is not a subject or a tool to crack petty exams like JEE or PMT, but an attempt to understand the laws that govern the universe with the most critical assessment of evidence. Kudos to you! Keep up.

[dodokgp]

This explanation is quite widespread, but should be taken with a extreme caution. It only validates special relativity by showing that it is consistent with the "magnetic" forces that we observe in everyday life for moving charges, like in a current carrying conductor. However, many people wrongly consider this explanation as the cause of magnetism. It is not. On the atomic and particle level, magnetism originates due to magnetic moments and spins of fermions, which are fundamental properties that we know to exists in matter. Of course, this explanation does help in understanding how special relativity and its consequence is consistent with the observations of forces in moving charges.

[reinerwilhelms-tricarico]

I was taught this as a physics student about 50 years ago. I didn't believe it, and seriously thought the prof was trying to troll us. Then we did the math; it made sense, but I still didn't believe it. Eventually, when I got deeper in the math I believed it because I could do the math myself. I admit, it felt like it took a certain amount of brainwashing before you begin to not just trust your common sense. In that respect you made an important argument that made a difference, namely the intuition about the effect reaching macroscopic relevance because of the enormous number of particles in the wire. On the other hand, the story with time dilatation explaining the reduction of the Coulomb force, that still feels as weird as before. To this day, at 70 years old now, I quite frankly find most of these results still a bit weird just like the first time in the 2nd or 3rd semester of undergrad physics.

[maxv.d.]

For the question at the end: if we were to look at a complete eletric circut, there would also be a part of the wire where the current would move in the opposite direction. At this part the laws of length contraction would still aply, and the protons would move away from each other and the electrons would move closer to each other. So there wouldnt be a lack of protons at the end because the Charge of the whole wire would add up to Zero.

[MuhammadJasoor]

You know what, Einstein's explanation to the above questions reminds me of my school days when I used to blabber some random excuses to my teacher to justify why i didn't complete my homework 😂... except in Einstein's case he was not lying and had mathematical proofs

[civildiscourse]

Your enthusiasm is a pleasure to share.

Sometimes I teach basic electrical theory to beginner electricians. We don't need to delve into the physics to this degree, but I always make it crystal clear that we are using simplified models and that the underlying mechanisms are much more nuanced, magical and utterly fascinating. I like to feature examples like this occasionally in case there is a future engineering student in my class who just doesn't know it yet.

[kishoreprabakar]

You didn't stop from just making videos on Khan academy (Ch:4 Moving Charges & Magnetism), but you also made this video. This is the sort of education everyone wants, this is the sort feeling one should get after learning. Thanks for inspiring me to educate myself!
(PS: I have boards coming up in 3 weeks, can't help myself from looking at your content for Physics. >.<)

[mmmh]

This guy needs a nobel prize in teaching

[TheLoadedLongboard]

My answer to your final question, which was "what about conservation of charge?"

The wire's charge is conserved, it's spatial dimensions are not. The wire changes size because the particles change in size. The change in size creates a dipole that is oriented non-normal to the length of the wire. The negative charge of the wire exists, but it is the orientation of the dipole within the wire that creates the net positive force.

Great video, love your enthusiasm.

[douglasstrother]

In Ch. 6 of "Electromagnetic Fields and Waves" (2nd ed.) by Lorrain & Corson, they derive the Lorentz force from the Lorentz transformation of Coulomb's Law. They also do a similar derivation of the magnetic field near a straight wire carrying a steady current.

Ch. 12 of "Classical Electrodynamics" (2nd ed.) by Jackson contains further discussion and analysis.

[dsolis]

Very good video. I have a degree in physics but sadly I had to move to Data Science and this is helpful to remember this concepts

[PenguinSharkHybrid]

I came here to watch some fun thought experiment before going to sleep. Didn't expect to have my whole mind catapulted to the ends of space and turned upside down... Absolutely amazing. I never really understood the "right hand rule" of magnetic fields and their forces. This explains it so much better!

[samiraesmaili]

Very well explained. What's so interesting is that even when we don't know the exact mechanism of a particular phenomenon, we can still use the laws and equations with reasonable accuracy for a lot of everyday applications.

[finnvankoutrik]

quik question, from the electrons reference frame the protons are moving so they get closer to each other, yet from the protons reference frame the electrons are moving and they should get closer to each other. This doesn't happen, why?? why doesn't the wire become negatively charged and repulse the electron on the outside?

[BigJ_FPV]

Your videos answer the questions I tried to ask my science teacher, and got the “because they do” response.

We are tought that these rules are just facts, and that there’s no real answer to them, other than that’s just how the univers works.

Until seeing your videos, I have just learned to not question it, because I know that no one will ever give e a real response.

Its so satisfying to watch your videos and Finally answer the questions I have wondered my entire life.

Thank you!

[jayall]

As soon as I ditched the idea of particles and started to think of electricity in terms of waves and relativity, everything started to make more sense, including electromagnetism. I still don't know why I'm just learning about this now, 100 years after Einstein conceptualized it.

[Songfugel]

Ok, not only did explain relativity in electro-magnetism, but also time-dilation better than anything I have come across so far! Oh boy, if only I had seen this sort of video (well, YT didn't exist yet) in University when first studying these topics