How Special Relativity Makes Magnets Work

  • Опубликовано:  3 года назад
  • MinutePhysics on permanent magnets:
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    Magnetism seems like a pretty magical phenomenon. Rocks that attract or repel each other at a distance - that's really cool - and electric current in a wire interacts in the same way. What's even more amazing is how it works. We normally think of special relativity as having little bearing on our lives because everything happens at such low speeds that relativistic effects are negligible. But when you consider the large number of charges in a wire and the strength of the electric interaction, you can see that electromagnets function thanks to the special relativistic effect of length contraction. In a frame of reference moving with the charges, there is an electric field that creates a force on the charges. But in the lab frame, there is no electric field so it must be a magnetic field creating the force. Hence we see that a magnetic field is what an electric field becomes when an electrically charged object starts moving.

    I was inspired to make this video by Prof. Eric Mazur

    Huge thank you to Ralph at the School of Physics, University of Sydney for helping us out with all this magnetic gear. Thanks also to geology for loaning the rocks.

    This video was filmed in the studio at the University of New South Wales - thanks to all the staff there for their time and support.

    Music: Firefly in a Fairytale, Nathaniel Schroeder, and Love Lost (Instrumental) by Temper Trap licensed from
  • КатегорииОбразование
  • Длительность: 4:19
  • Тэги для этого Видео:  veritasium  science  physics  magnet  magnets  magnetic  how do electromagnets work?  how do magnets work?  special relativity  length contraction  attract  repel  minutephysics  charged cat  

Комментарии: 4 290

  • fergal farrelly
    fergal farrelly 1 день назад

    I'm an electrician and I was taught that electricity flows at the speed of light. I challenge you on the last part of this video. this would give electrons mass and momentum. I was always taught electrons were almost massless.

  • Freddie Vandyck
    Freddie Vandyck 2 дня назад

    While I love your videos this is the first one to make my jaw physically drop for a full minute before I realised my mouth was open

  • nischay singh
    nischay singh 3 дня назад

    but why will magnetic field have no effect in the charge's frame of reference

  • Taylor Hedge
    Taylor Hedge 5 дней назад

    But, as far as the cat and the wire, how can a physical tangible force be affected by perspective? Weather the positively charged cat and the negative electrons in the wire are moving and the positive electrons are not , or the positive electrons are moving and the negative electrons and the cat are not, the force applied is still the same regardless of perspective, correct?

  • LowReg Gamer
    LowReg Gamer 5 дней назад

    What is magnetic field? I've learned that magnetic field can travel on matter and space. So what is that thing? alpha, beta or gama rays. A different lamda of particles of that 3 thingy? In class and these video you explain how they work and in relativity. What is that magnetic wave made of?

  • Radioactive Music 4 U
    Radioactive Music 4 U 7 дней назад

    So somehow gravity and the strong nuclear force are basically like this and technically the same thing? Isn't that what humans will get to in future science?

  • Suwarna Gummuluru
    Suwarna Gummuluru 8 дней назад

    But if the electrons are moving in your frame of reference, then when the cat is not moving, the density of electrons is greater , so the cat should be attracted to the wire . I'm still confused.

  • Dale Anderson
    Dale Anderson 9 дней назад

    Thanks Veritasim. Mind blown.

  • Kumar Sanu
    Kumar Sanu 10 дней назад

    thanks a lot very much because u made it so simple to understand...... thank u.....☺

  • forestsoceansmusic
    forestsoceansmusic 10 дней назад

    Fascinating. Just on that tiny fraction of the speed of light that electrons move through a wire, could you do a video on how long it takes from the time we flick the light switch to the time the light lights up?
    There was a huge debate in an Australian Electronics magazine in the early 1990's about this, with people of all qualifications (from PhDs down to trade electricians weighing in on ALL sides--)--> which ranged from below the speed of sound to close to the speed of light! After several issues (it was a monthly) the editors had to close the debate because of no conclusive result and too many letters on that topic.

  • Wendy Dravis
    Wendy Dravis 11 дней назад +1

    If the iron in the middle of the world 🌎 is over the curry temperature then why does it make a magnetic field?

  • cody hedges
    cody hedges 12 дней назад

    The speed of light in copper?

  • pinku kakati
    pinku kakati 14 дней назад

    he's a genius.

  • Parth Suyal
    Parth Suyal 15 дней назад

    wow superb thanks!!!!!!!!!!!!!!!!!!!!!!!!!!

  • jacob hughes
    jacob hughes 15 дней назад

    why then in his real life example with the wire and the compass, isn't the compass moving? cus with the cat example the cat is moving which is causing the effect, but the compass isn't moving but we still see the effect.

  • TechyBen
    TechyBen 15 дней назад

    So the question is... what is an "electric charge", in relation to special relativity with Quantum mechanics (the big question of how to combine the two).

  • Aryan Derania
    Aryan Derania 17 дней назад

    Why on God's earth will the cat move along with the direction of electron flow?

  • Ross Petersen
    Ross Petersen 17 дней назад

    Once you start mentioning the contraction of positive charges and separated negative charges, is this related to Coulomb's law or something?

  • Achint Rastogi
    Achint Rastogi 19 дней назад +1

    its strange that ur video has less views than ur no. of subscribers

  • Willem Beltman
    Willem Beltman 19 дней назад

    this doesnt seem right, and it is not the first video i get this feeling. are you sure the wave pattren of the electrons are not just creating a magnatic flux? and those are 2 different things, like water waves and wind.

  • Kuldeep Saini
    Kuldeep Saini 20 дней назад

    why external stationary charge do not experience force when current is passing through wire? as e- are moving w.r.t. to it so acc. to special relativity -ve charge density should inc. than +ve and hence net attraction should precede?

  • Vince Noir
    Vince Noir 20 дней назад

    0:53 University of Sydney :D

  • Buzz Werd
    Buzz Werd 22 дня назад

    So where does the torque come in?

  • gottalikeit2010
    gottalikeit2010 23 дня назад

    i am reporting this as cat abuse

  • Nikša
    Nikša 23 дня назад

    This is actually incorrect in electron-vitron universe model, only true model.

  • Larry B
    Larry B 24 дня назад

    Wait! The positive charged particles are moving with respect to the cat, BUT the contraction due to relativity is for each particle, and THAT does not mean they are "ever so slightly contracted", they are NO closer together and the charge density is the same. And, additionally, the forces involved are units of charge, and there are no changes in the total charge of cat or wire. This sounds like a VERY bogus argument, at least as presented.

  • lolPantsProductions
    lolPantsProductions 24 дня назад

    Can someone explain what is happening at 2:40 with the compass and the current? Thank you.

  • physics physics
    physics physics 24 дня назад

    correction: the electrons are closer, the proton will be attracted

  • Alex Russett
    Alex Russett 25 дней назад

    Do you have a say in who does your ads? Because the ad on this video was propaganda. I don't need a lecture from a retired president of a third world country.

  • Gonzalo Chumillas
    Gonzalo Chumillas 26 дней назад

    Resulta curioso como algo tan subjetivo como es el cambio de sistema de referencia pueda tener consecuencias objetivas y medibles. Creo que la mecánica cuántica y la relatividad están íntimamente relacionadas, aunque no hayamos podido conciliarlas. Muy bueno el vídeo. Congratulations.

  • spelunkerd
    spelunkerd 26 дней назад

    Finally, a popular cat video that does not drag viewers into the mindless abyss.

  • Hobby Metallurgist
    Hobby Metallurgist 26 дней назад

    Why don't the moving negative charges contract in the original frame of reference?

  • FirstRisingSouI
    FirstRisingSouI 27 дней назад

    Mind = blown.
    And I'm a physics graduate student teaching assistant.

  • NurchOK
    NurchOK 27 дней назад

    Thank you for showing the electrons moving and not the protons and also for applying the right hand rule properly :)

  • Ethan Taylor
    Ethan Taylor 27 дней назад

    but electron spin tho

  • Boss Hogg
    Boss Hogg 28 дней назад +1

    Okay, so a (relative) concentration of positive charge will repel a moving cation, i get that part. However, I don't see how that explains an electromagnet. It just looks like a positive electric field. After all, an electromagnet attracts metal, yet a concentration of positive charge will not.
    One more question, if the cation were moving in the opposite direction of the current would it experience a negative charge since the electrons relative speed would now be greater than that of the wire.

  • Will Stanks
    Will Stanks 28 дней назад

    Water fire air and dirt

  • Olsen Olsen
    Olsen Olsen 28 дней назад

    Good morning ! Magnets at speakers are very heavly damaging our magnetic field. Only. I am not will talk twice !!! Only tesla speakers are not damaging environment. Very bad souls of humans WAS TRYING TO TAKE CONTROL under speaker production. Speakers got magnets who is need to be magneted. That magneted force is scoming back to earth with time apperance. But now it is coming from earth to speaker side. Please kill every speakers !!! OR YOU WILL KILL US WITH TOO MUCH OF MAGNETIC FIELD "REAL ABSORPTION"

    Some of you have an 3 or more frequencies of telephone communication, antenas with Wifi or something(3 or more always), antenas everywhere for cable TV, (probably new types of TV LED !(holographic but not really visible by user)(you can be programmed by one smart arab with antena and drone)(your brain)(because it is now that technology)(we must kill electricity)(ad/dc///////////-)(change)(your brain is programmed by arabians

  • mdiem
    mdiem 28 дней назад

    Three years later, and I'm back to review this.

  • Bartosz Olszewski
    Bartosz Olszewski 28 дней назад

    his logo stands for V(ap)e 4:20 lol
    im so dumb

  • Andrew Combs
    Andrew Combs 29 дней назад

    My head hurts in the most wonderful way possible...

  • StarlightVisual
    StarlightVisual 29 дней назад


  • George K
    George K 1 месяц назад

    no force on a kitty. <-- this should be the leading comment

  • Marco Discendenti
    Marco Discendenti 1 месяц назад

    Are really wires electrically neutral in general?

  • bhargaw gaikwad
    bhargaw gaikwad 1 месяц назад

    if special relativity is the reason then charge must attract in your frame of reference as the moving electrons suffer from length contraction.My friend I read many books but no one give explanation of electromagnetism through special theory of relativity.
    please check the hypothesis.

    • DiabloDBS
      DiabloDBS 26 дней назад

      So what kind of books have that been? Any titles you remember on EM or SR?

  • Rajat soni
    Rajat soni 1 месяц назад

    love it

  • Sander Wildeman
    Sander Wildeman 1 месяц назад

    The video is also quite confusing to me. Maybe the dilution of the electrons (from the point of view of the cat) can also be viewed as an effect of time dilation.

    If we imagine the full circuit, i.e. including the battery which pushes out the electrons (for simplicity assume that the electrons are injected from the left and absorbed on the right), then in the frame of the cat the "battery" is moving and hence its "clock" runs slow. In other words, as far as the cat is concerned the battery pushes out electrons at a lower rate, and hence the density is less.

    The distance between ions is contracted (along with the battery and the rest of the circuit, and including the space between the electrons) as explained in the video.

    Also note that in the example the magnetic field according to the cat is not zero, as there is still a current of ions. So it is not true that the magnetic field simply gets "converted" into an electric field and so it is not true that a magnetic field is "explained" by special relativity. It just means that "the" electric and magnetic field are relative concepts (depending on your motion), just like space and time.

    Just my ten cents to add to the confusion :)

  • Josh Richter
    Josh Richter 1 месяц назад

    why is it that electromagnets can act upon stationary objects as well? (not just moving ones)

  • Hyperflip
    Hyperflip 1 месяц назад

    Woah, after learning about both elecromagnetism and special/general relativity for my final exam, this is such a mindblowing connection!

  • Saw Teck Rul
    Saw Teck Rul 1 месяц назад

    thanks Henry and Derek. :D

  • David Zare
    David Zare 1 месяц назад

    sorry the explanation is sooooo wrong from beginning to the end. not to mention that regardless of protons/electrons charge neutrality every moving charge is a source of magnetic field.

  • Paul Anderson
    Paul Anderson 1 месяц назад

    I still can't wrap my head around this. - Why do things change? I can't get past the fact that it seems that we're only talking about perception. Not reality. Just perception.

  • Paul Anderson
    Paul Anderson 1 месяц назад

    watched this twice...head still hurts...

  • Professional Insultant
    Professional Insultant 1 месяц назад

    are you trolling us?

  • charles joshi
    charles joshi 1 месяц назад

    awesome video sir, thank u so much

  • Siddarth Arcot
    Siddarth Arcot 1 месяц назад +1

    The "Only in your frame of reference" made me laugh idk why

  • Sean Munro
    Sean Munro 1 месяц назад

    So, in the real world, what are the cats? E.g. Are the protons in the compass the cats? How and why do they move when he sends current through the wire? :\

  • Nikhil Joshi
    Nikhil Joshi 1 месяц назад

    Question and a possible experiment : If the magnet and or the cat is itself shaped like a wire and of the same length as wire, will it still experience the repulsive force ?

  • Math
    Math 1 месяц назад

    @ 3:23- Therefore, electrons drift through wires at about 5.9 or 5.86 miles per month

  • Noel Ruppenthal
    Noel Ruppenthal 1 месяц назад

    The compass needle is neither moving with the direction of current in the wire, nor is positively charged, so I'm not seeing how length contraction is responsible for the wire deflecting the compass needle

  • Akshay Kumar
    Akshay Kumar 1 месяц назад

    In our reference frame, charged cat is stationary and some current is in wire, current means electron is moving and positive charge is stationary. If electron is moving, distance between electron will contract. and it will produce some electric field that will attract the positive charged cat. Why this is not happening?

  • Istasis Mishra
    Istasis Mishra 1 месяц назад

    In Derek's frame of reference, since the electrons are moving, the electrons should seem like they have contracted and the wire should seem positively charged and this accounts for the repulsion of cat. Why isn't this the case?

  • AstralPotatoe
    AstralPotatoe 1 месяц назад

    my mind is blowned, magnetic fields are a lie, they are just electric fields

  • Saad Selwan
    Saad Selwan 1 месяц назад

    why is a stationary magnet attracted to a copper wire with a current flowing in it ?

  • Orion Michael Guy
    Orion Michael Guy 1 месяц назад

    Its the Motion of the Earth that gives the Magnet its energy

  • Aaron Forbes
    Aaron Forbes 1 месяц назад

    there were way too many different metaphors/similes in this to make coherent sense for me

  • Simon Barry
    Simon Barry 1 месяц назад

    well said

  • Chandan Bhuyan
    Chandan Bhuyan 1 месяц назад

    I am bit confused can anyone help.?
    1)when cat is stationary & electrons were moving then from cats reference due to length contraction electron density increases and proton spaced out making overall - ve charge which should attract the cat.. But didn't
    2) In this video a positive object which is
    Moving at speed of electron is repelled by electromagnet but what about stationary object or - ve charge object

  • Nosknut
    Nosknut 1 месяц назад

    Almost quote 1: "No force on the stationary cat next to a wire conducting electricity."

    Almost quote 2: "Force on a stationary cat next to a wire conducting electricity."


  • Science Asylum Fan
    Science Asylum Fan 1 месяц назад

    Why moving magnets move the electrons in a conductor? A moving magnet induces current in a conductor.Why?

    Changing magnetic field creates a current in a conductor.What is the reason at the microscopic scale. Explain without any technical jargon?

  • יאיר ערד
    יאיר ערד 1 месяц назад

    now I will pretend that I understood this video

  • Atif Wahab
    Atif Wahab 1 месяц назад

    Why don't electrons contract in the first case? Because their motion in reality is random?

  • Jon Do
    Jon Do 1 месяц назад

    well i think quantum Physicians would expect the universe to split. for the cat the wire is positive so in its universe it gets pushed away. And you should enter a universe there the cat does not get pushed away.

  • sanmveg saini
    sanmveg saini 1 месяц назад +1

    To all those who are asking that why that cat wasn't attracted when it was at rest (or more specifically, moving with the velocity of the positive ion in the wire, if any ), then just remember that in a neutral wire (a wire with no current in it) has electrons moving with random velocity, yeh! the velocity (speed+direction). and the number density is so high that you will always find two electrons moving with velocity in opposite direction in a particular volume of wire, and as you have heard that current is the flow of electrons(very basic definition ), so over all there is no current, hence neutral implies no force.

    although i do not know special theory of relativity in detail, but from the given explanation we figure out that, in the video he said that the length got contracted IN DIRECTION OF MOTION (not shouting just highlighting), so two electron will move in opposite direction and hence length will contract in opposite direction hence no net change in the length hence there is no change in the number density of the electrons hence no net force.

    hope you understand, have a grate day!

  • abc def
    abc def 1 месяц назад

    you need to come back with the correct answer, at 2:26 . moving electrons didnt get the length contraction, this is an inconsistancy, illiogical.

  • Landon Fitzgerald
    Landon Fitzgerald 1 месяц назад

    DEREK CAN YOU MAKE ANOTHER VIDEO CLARIFYING THIS PLEASE?? We're confused here because the car analogy doesn't pair perfectly with the stationary electromagnet. What is the car representing? You had no moving objects other than a current that generated an attractive force in your electromagnet. Additionally, why don't the electrons contract? Can you explain this using BOTH electrical motors and generators? Any help is much appreciated, thanks!

  • Landon Fitzgerald
    Landon Fitzgerald 1 месяц назад

    Please help!! It's been three days and I still can't find any good material on this. Is the cat a magnet? Why are things attracted to the electromagnet when the current is turned on?

  • Science Asylum Fan
    Science Asylum Fan 1 месяц назад

    😍 love you so much Ve- The element of truth

  • Aabhas Dhaubanja
    Aabhas Dhaubanja 1 месяц назад

    so does this mean that magnets are affected by charges, because since the density of proton increases and the wire becomes positive the magnet in the compass is deflected..plz help

  • SoCloseToTheSky
    SoCloseToTheSky 1 месяц назад

    ehm, I thought magnets are not that freaking complicated.

  • Martin Ducharme
    Martin Ducharme 1 месяц назад

    I really like cooperative videos between youtubers.

  • Benjamin Lehmann
    Benjamin Lehmann 1 месяц назад

    Best, most mind blowing video I've seen on YubTubs for a year.

  • José B. Junior
    José B. Junior 1 месяц назад

    Einstein still blowing my mind.

  • Caue Rego
    Caue Rego 2 месяца назад

    what would happen to physics if instead of using cats to explain theories they'd use armadillos?

  • kristoffer gaard
    kristoffer gaard 2 месяца назад

    I hope somebody can answer this question:

    According to special relativity, objects moving relative to one-another are subject to length contraction. So when the cat is static and the electrons in the wire start moving, wouldn't the electrons be subject to length contraction? Thus creating a higher density of electrons thus attracting the kitty?

  • ski
    ski 2 месяца назад

    I really think a video about drift velocity would clarify this topic even more so than this excellent clip already does.

  • kdmc40
    kdmc40 2 месяца назад +1

    Ah! This explains why I'm attracted to my sister-in-law! I knew it wasn't my fault!

  • Sean Dafny
    Sean Dafny 2 месяца назад


  • Sean Dafny
    Sean Dafny 2 месяца назад

    Thank u so much Feynman

  • SillieWous
    SillieWous 2 месяца назад

    In the stationary cat case: why is the cat not attracted to the wire? Moving electrons would be more densely packed, thus overall negative charge and hence the cat should be attracted.

  • O Gor
    O Gor 2 месяца назад +1

    it's a shame. delete this video

  • ​ ​
    ​ ​ 2 месяца назад

    For anyone that came there trought the question "Why is magnetic force perpendicular to current/charge velocity/magnetic field?", this video perfectly explains.

  • chari Muvilla
    chari Muvilla 2 месяца назад

    So when the electrons are moving it is like there are more of them everywhere?! Lol I'm never gonna understand how magnets work. Stupid world

  • Nikhil Aggarwal
    Nikhil Aggarwal 2 месяца назад

    why not non-moving cat is attracted to wire?

  • Diluksh Fernando
    Diluksh Fernando 2 месяца назад

    0:45 Adolf Hitler

  • Rober oat
    Rober oat 2 месяца назад

    So how does this attract ferromagnetic stuff

  • Hello_:D
    Hello_:D 2 месяца назад

    So if I'm moving fast enough i could occupy less space that someone who is stationary, what if blackholes are just moving really fast therefore taking up much more space than they seem to be, so it can look like they are squeezing matter into a fixed position to make a really strong gravitational force, but they are really just spinning really fast.

  • Minecraftster148790
    Minecraftster148790 2 месяца назад

    Does this mean you can determine the speed of light using an electromagnet?

  • Ido Ben Lulu
    Ido Ben Lulu 2 месяца назад

    why the density of the electron proton be the same if the electons move ? Dont the electons suppose to cover less space and hence the wire would be negatively charge from Vertasium reference? why only when the protons "move" the density isnt the same?

  • Samantha Kun
    Samantha Kun 2 месяца назад

    kawaii cat <3