How Special Relativity Makes Magnets Work

  • Опубликовано: 23 сен 2013
  • MinutePhysics on permanent magnets:видео.html
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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

Комментарии • 4 297

  • Shubhay Dikkar
    Shubhay Dikkar 2 часа назад

    I think I didn't matters wheater cat is moving or not it still experiences
    The positive force

  • Stas Lu
    Stas Lu 2 часа назад

    видео под сомнением

  • Ruiyang Xu
    Ruiyang Xu 2 дня назад

    It has been several years, but I still want to clarify that there is a misunderstanding in this video, and I will quote someone else's explanation:
    TL;DR: There is no causality between electricity and magnetism!
    It is a mistake to think that special relativity (SR) causes magnetism. The article you're reading is arguing that
    (A) "If electricity exists, and SR is true, then magnetism has to exist".
    This statement is true, I admit it. But it's equally true that
    (B) "If magnetism exists, and SR is true, then electricity has to exist".
    In reality, electricity and magnetism are equally fundamental parts of physics. Special relativity unites electricity and magnetism into electromagnetism, in exactly the same way that it unites space and time into spacetime. Time does not cause space, space does not cause time, and SR causes neither space nor time. SR merely reveals the relatedness of space and time. Similarly, electricity does not cause magnetism, magnetism does not cause electricity, and SR causes neither electricity nor magnetism. SR merely reveals the relatedness of electricity and magnetism.

  • mithilesh bharati
    mithilesh bharati 4 дня назад

    I think magnetic field is a probability of a space and the electric field is a probability of finding electrons

  • Ven Sar
    Ven Sar 5 дней назад

    Great and in the same time simple explanation. Thank you very much. And the cat is great :)

  • cgreat9
    cgreat9 7 дней назад

    cat is not moving in the "eveready battery"".

  • Markus Arntzen
    Markus Arntzen 8 дней назад +1

    I thought electronics and therefore current moved at very near the speed of light (c) since they have such small amount of mass and are so tiny. I saw this in another video. Someone edjucate me plz :D

  • Kornelije Kovac
    Kornelije Kovac 9 дней назад

    2:09 "Remember, objects take up more space when they're not moving , than when they are."
    Why doesn't that happen with protons at 1:30?

  • Zen vir
    Zen vir 10 дней назад

    ⚠️ Serious research question!!!!!
    Please reply.
    1:25 no current flowing means charge densities same
    1:28 he says when current flows the charge densities are still same.
    The negative charges are in motion with respect to cat (+ve), shouldn't negative charge density increase due to motion of negative particles?
    Which would mean that, when current flows, the cat in "rest position" should experience force of attraction.

  • Harles Balanta
    Harles Balanta 10 дней назад

    Question, is the example of the cat, when the electrons are moving related to the cat and the protons, due to relativity the density of electrons will increase, isn't it?

  • It's Got Electrolytes
    It's Got Electrolytes 14 дней назад

    I Googled cat videos and it took me down this rabbit hole

  • Clarence Anglin
    Clarence Anglin 15 дней назад

    Where are the equations that derive the force of an electromagnet based on relativity?

  • Owen Loh
    Owen Loh 17 дней назад

    Cat: did you just assume my gender?

  • Zeljko P
    Zeljko P 17 дней назад

    How comes: we stand, electrons move, they are not getting dense but the whole wire remains neutral? Then suddenly you start moving (how about in the opposire direction from the electrons?) and only positive electriciti is getting dense!!?

  • B R
    B R 18 дней назад

    This is one of my favorite visual description videos

  • Miteran Official
    Miteran Official 18 дней назад

    wrong imagination

  • harshita sharma
    harshita sharma 19 дней назад

    What about direction of the force?

  • Oruj Ahmed
    Oruj Ahmed 20 дней назад

    What is the meanig of negative or possitive charge? Why electron have negative charge and proton have possitive charge? What does those propertise actually mean?

  • Boggarapu Lokesh
    Boggarapu Lokesh 20 дней назад

    When the electrons are moving in fast forward direction,won't the electrons experience a huge bunch of positive charge opposite to their direction of motion? So will this huge positive charge at their back effect their motion in forward direction??

  • Drake Beheaven
    Drake Beheaven 22 дня назад

    fun but not informative

    VARUN.N RAO 22 дня назад +1

    Veritasium please answer these questions
    1. As there is no magnetic field how will you explain EM wave propagation
    2. Can we explain the spin angular momentum and its corresponding magnetic field using relativity. Apparently I heard somewhere that De-broglie did it, but wasn't quite sure about it

  • Kalpesh Popat
    Kalpesh Popat 23 дня назад

    whatever is not understood goes under the bucket of special relativity, how convenient !

  • orange70383
    orange70383 24 дня назад

    We have no idea what magnets are or how they work, the same goes for electricity. They talk like electrons and protons are real but it's all just a guess and it's all wrong. We may be able to do tricks to prove what we're saying but if you get right down to it we don't know anything.

  • Nikhil Vidhani
    Nikhil Vidhani 24 дня назад


  • Adenosine triphosphate
    Adenosine triphosphate 25 дней назад

    Hello, could you or anyone here explain me how would you apply this principle to two wires with A-same direction of current and B-oposite direction? I did some thought experiments and I kind of cant figure it out.

  • alwan mani
    alwan mani 26 дней назад

    the video provide a good way of answering why moving charge produce magnetic field
    but I need to understand the mathematics behind it in a simple way

    NEHA SAKKA 28 дней назад

    damm awesome man.
    i mean seriously out of words dude

  • Nick Tokar
    Nick Tokar Месяц назад

    I love watching videos I don't understand

  • Vlog and Thoughts New Spin
    Vlog and Thoughts New Spin Месяц назад

    Very good our. Kids need good teachers ours are terrible

  • Luis Maoer
    Luis Maoer Месяц назад

    What about the cat-ion moving away or towards the wire, shouldn't it get deflected to the right or left?
    How is that explainable?

  • Deniz Coşkun
    Deniz Coşkun Месяц назад

    Why should “cations” be moving? Any not-moving object is also attracted by magnets.

  • hema sundar
    hema sundar Месяц назад

    What is conventional current

    VARUN.N RAO Месяц назад +1

    Somebody please answer this, why is the proton not pulled towards the wire when its stationary, as the negative charge density is increased because the elecetron is moving??видео.html

    • 2017/2018 1SE EDUARD EMANUEL ROMAN
      2017/2018 1SE EDUARD EMANUEL ROMAN 13 дней назад

      As you can see, in one of the commentaries of the video you posted, it says that the cat isn't attracted to the wire by the contracted moving electrons, because, as the electrons move, in their own frame of reference, the protons move and contract. Finally, their force (of the electrons and protons) cancel each other, resulting in a neutral wire.

    VARUN.N RAO Месяц назад +1

    Now that we have removed magnetic field and replaced it with a relativistic self interacting electric field, how will you describe electro"magnetic" wave propogation?

    • 2017/2018 1SE EDUARD EMANUEL ROMAN
      2017/2018 1SE EDUARD EMANUEL ROMAN 13 дней назад +1

      I'm not at all into physics or math. But, I guess, that the electro"magnetic" wave propagation is still valid for the stationary observer, as it interprets the phenomenon as a magnetic field. Whereas, for the moving cat, the electro"magnetic" wave propagation, could be replaced simply by static electricity laws.

  • Sang H
    Sang H Месяц назад

    Thank you. Really it possible for me to differentiate electrostatics and magnetism.

  • Aravindh gokul
    Aravindh gokul Месяц назад

    awsome thank you very much

  • Md Mizan
    Md Mizan Месяц назад

    awesome metaphor

  • RCB
    RCB Месяц назад

    OMG... this... this is how magnets work.

  • bharathreddy gudibandi
    bharathreddy gudibandi Месяц назад

    saying that..... Electric and magnetic fields are same is fantastic

  • bharathreddy gudibandi
    bharathreddy gudibandi Месяц назад


  • Lambda
    Lambda Месяц назад

    Where’s the symmetry? Relativity effects, time, length and mass, but the individual charge is left unaffected.

  • hacker_8162
    hacker_8162 Месяц назад

    At school we were taught that current flows opposite to the flow of electrons. However, this doesn't make sense and no-one has given me a proper explanation. Can you please?

  • Sazzad Hussain
    Sazzad Hussain Месяц назад

    Feels nice to see good channels working together

  • phoenix1925
    phoenix1925 Месяц назад

    so if we give energy to the electrons to travel way way faster this magnet will be extremely strong.

  • Sahil S.j
    Sahil S.j Месяц назад

    Can you please explain why and how electric charge exists

  • StanJan1958
    StanJan1958 Месяц назад

    This. Is the text book definition of mental masterbation ..... the culmination of many grants. Nice graphics.

  • Justice is CLEAR
    Justice is CLEAR 2 месяца назад

    This is so laughable.
    Do you want real knowledge?
    Here's some,видео.html

  • Naveen Rajan
    Naveen Rajan 2 месяца назад +1

    I always love your videos....though i am a school student, all of your videos make me understand physics more practically..thank you..

    2017/2018 1SE EDUARD EMANUEL ROMAN 2 месяца назад

    When you say that in the copper wire (as the positive ion(the cat) moves along with the electrons) because of special relativity, the cat sees less space between the positive ions and more space between the electrons, then shouldn't the positive ions also shrink and the electrons augment?? When you planted the car example, the car was shrinking when moving and augmenting when stopping. I say this, because, if this happens, then the effects of repulsion will disappear. The situation would be just as the initial one, just two times greater for the electrons and two times less for the positive ions. It would be like: 2/4 = 1/2, which finally is 0.5 (nothing happens).
    And also, I don't understand why, according to this theory, a force is exerted on the magnet(compass): because you say that a magnetic force occurs ONLY for a stationary observer, on a moving object/ion. If the magnetic force occurs just from the viewing point of a stationary observer and that the object has to move, why does the magnet respond?
    Thank you!

  • Heisenberg-SchrodingerEmc2
    Heisenberg-SchrodingerEmc2 2 месяца назад +1

    Einstein is the greatest genius ever

  • Igor
    Igor 2 месяца назад

    Magnetism is a lie!

  • Zarif Muhtasim
    Zarif Muhtasim 2 месяца назад +1

    to the stationary cat, the electrons are moving but the protons aren't moving. So, wouldn't there be higher density of negative charge? and wouldn't the cat be attracted to the wire?

  • Gnana Prakash
    Gnana Prakash 2 месяца назад

    This only explains the interactions of two current carrying conductors and no the interactions between a permanent magnet and a conductor and neither does the video linked to the minute physics. They just say they don't know why it's like that.

  • Anirban Dutta
    Anirban Dutta 2 месяца назад

    Charge is a conserved quantity in Special Relativity, But charge density is not (due to length contraction). So in your explanation, the correct scientific term would be "charge density is increased/decreased in cat's frame" instead of saying "more positive or negative charge in cat's reference frame."

  • Steven Dwello
    Steven Dwello 2 месяца назад

    That cat looks like a butthole

  • slavarus
    slavarus 2 месяца назад

    "magnetic field is an electric field seen from a different frame of reference" - BEST explanation ever!!

  • Algot Kristoffersson
    Algot Kristoffersson 2 месяца назад

    What you do not understan is objekts look like they take up less space than they do then moving they do not actualy take up less space

  • Wishbin Robin
    Wishbin Robin 2 месяца назад

    But, even as the electrone undergo 'slimming' there would still only be 1 charge carrying electrone for one particular atom.......'as in every atom'
    So how can the conductor attain a -ve charge overall?

  • Dora Demir
    Dora Demir 2 месяца назад +1

    You look slim
    Only in your frame of reference
    LMAOOOOOOOOOOOOO best roast ever

  • lisen kaci
    lisen kaci 2 месяца назад

    Does that mean that a metal that has higher drift velocities will become a stronger electromagnet when experiencing the same current as a metal with a smaller drift velocity?

  • yt678901
    yt678901 2 месяца назад

    When you think you know it all, this video comes up.

  • SuperVV2010
    SuperVV2010 2 месяца назад

    A number of commenters have questioned why the cation (+cat), sitting still next to the wire with current, is not attracted by 'contracted' electrons because they are moving past it. This is a good observation and the only answer I can suggest is that electrons are 'sucked' toward the positive attractor (the EMF generator) at the right side of the wire, therefore there is less electron density in the conduction electron set than if it were stationary. (A kind of Bernoulli principle for electrons.) This electron dearth would then be exactly cancelled by the relativistic 'scrunching' of them in the stationary +cat and wire frame of reference. Of course this requires an EMF to alway be a suction and this explanation goes to hell if it is a compressive force, aargh. And why should an EMF generated reduction in e- density be exactly compensated for by Lorentz contraction? Is there a physicist in the house! Otherwise it explains + and - test charges going back or forth relative to the current wire nicely.

  • Killian Defaoite
    Killian Defaoite 2 месяца назад

    I was really looking for an explanation on this. Thank you.

  • Kenny Huang
    Kenny Huang 2 месяца назад

    some people say a stationary will be attracted to the wire in the same way ,He is right, I can explain. There is something important the video didn't make clear. You are right , when the cat don't move, the cat does be attracted to the wire, but the effect is too small (the speed of electron is too slow). On the other hand , If there are a number of stationary cat( a wire without current) ,the effect will added and stronger. But! Not all cat in the wire are stationary, they are moving at the random direction. A cat is attracted, while B cat is pushed away. Consequencely , the effect not appear. So, To make effect stronger, Only 2 ways, the first : all the cat is stationary ; the second, all the cat is moving at same way. Of cource, the second is more easy with a power source.(In the video , a moving cat may mean 'a number of cat' moving at same direction). On contrast , Keeping lots of cats stasionary is impossible.

  • Jack Lewis
    Jack Lewis 2 месяца назад

    Damn that's an impressive beard after 5 hours, 59 minutes and 59.999* seconds.

  • DrDress
    DrDress 2 месяца назад

    1:27. When the current flow, why aren't the electrons contracted and repel the stationary cat?

  • Haridinee Vikhe
    Haridinee Vikhe 3 месяца назад

    Amazing job. The realization of magnetism is really worthy

  • Mk Km
    Mk Km 3 месяца назад

    Finally I understand the tape recording.
    Well, not completely. You need time to think about it.

  • Electromorphous
    Electromorphous 3 месяца назад

    So we should be able to use this for making the theory of everything that is supposed to connect relativity and quantum mechanics.

  • T. Baggins
    T. Baggins 3 месяца назад

    Are you high dude?! ....I don't care to argue right or wrong, me just thinks you'd like a pipe and a crepe.

  • vanscoyoc
    vanscoyoc 3 месяца назад +1

    This video has made me catatonic!

  • Onkar Patil
    Onkar Patil 3 месяца назад

    U are awesom

  • Husni Jabir
    Husni Jabir 3 месяца назад

    Excellent. Love this.

  • Pankaj Jain
    Pankaj Jain 3 месяца назад

    As in ur frame of reference cat is stoped n so is +ve charge in wire but electron are moving so in your frame of reference linear density of electron must increase n that of +ve must decrease. This will make wire begave like negative charge n thus attract +ve cat. Plz help to solve me this puzzel ...

  • apple embedded
    apple embedded 3 месяца назад

    but what about the speed of a moving electron, i read in an article before that says the speed is so small than a moving snail.
    then how it will work.
    can you clarify my doubt?

  • OHTYCH 100
    OHTYCH 100 3 месяца назад

    Is there an object that when you have a compass, and a battery the needle gets weird but is there something you can put between them and nothing will happen to the needle

    MANIKANTT M 3 месяца назад +1

    But the electrons keep on colliding which means they are accelerating or decelerating which makes it a system with non constant velocity and unfit for the usage of special relativity right , but why are we using it then.

  • Yoyo Tsui
    Yoyo Tsui 3 месяца назад

    Wow! thanks for this

  • Tanay Mehta
    Tanay Mehta 3 месяца назад

    What about a single electron. When it moves it creates a magnetic field. But how can you apply special relativity apply here?

  • mumblernumber
    mumblernumber 3 месяца назад

    yo science wtf

  • Kane Sweet
    Kane Sweet 3 месяца назад

    Hell yeah dude good video

  • Noah Mikov
    Noah Mikov 3 месяца назад

    good song. Also, I recommend going to dr physicsA to understand all the math.

  • sammbo250
    sammbo250 3 месяца назад

    ARRGH, my head hurts now. I've never understood Relativity even as a physics nerd.

  • André A
    André A 3 месяца назад

    0.0000000001% of the speed of light is about 0.03 m/s for those interested.

  • Jonty K.
    Jonty K. 3 месяца назад

    If this all is based on the fact that nothing can travel faster than light than....... What about neutrinos?...... Does it mean that special relatively is all a hoax!

  • Dhaval Deshmukh
    Dhaval Deshmukh 3 месяца назад

    god bless you

  • Gonzalo Diaz
    Gonzalo Diaz 3 месяца назад

    so this is the quantum explanation of Faraday law?

  • Mike kirock
    Mike kirock 3 месяца назад

    This is all fine, but how do you explain that the force is a right angles, i.e come out of screen?

  • Simon sez
    Simon sez 3 месяца назад

    So, if two wires are connected in parallel to the same power source and have the same total Resistance and have the same amount of current passing through (in phase in case of AC), does it mean there will be no magnetic effect between the two wires?

  • Tanay Kohale
    Tanay Kohale 3 месяца назад

    Thanks for solving my old dought but second question is what is relativity (both special or general) or to be specific why speed of light is constant???
    I tried Search it many times but no one tell me any equation or any phenomenon, or principle
    They just move around topic n finally give statement speed of light is constant.

    • Tanay Kohale
      Tanay Kohale 3 месяца назад

      In other comments I found people r saying its fundamental
      If it is then how humans come to know it's constant in all frame

  • Martin Schober
    Martin Schober 3 месяца назад

    great work! thanx for this insight - and i mean it! WOW!!!

  • Leo Yohansen
    Leo Yohansen 3 месяца назад +1

    Amazing how length contraction can be acknowledged for electromagnetism in special relativity but totally ignored for gravitation in general relativity exposing the fact that black holes cannot exist due to the impossibility of the total length contraction of the mass that would have to occur at the so-called event horizon.

  • fritzz2
    fritzz2 3 месяца назад +1

    einstein = wrong e=mc2 = Poincaré Relativity = Lorentz einstein = fraud

    • An artist theory on the physics of 'Time' as a physical process. Quantum Atom Theory
      An artist theory on the physics of 'Time' as a physical process. Quantum Atom Theory 3 месяца назад

      Or we can reformulate it: (E=ˠM˳C²)∞. With energy ∆E equals mass ∆M linked to the Lorentz contraction ˠ of space and time. The Lorentz contraction ˠ represents the time dilation of Einstein’s Theory of Relativity. We have energy ∆E slowing the rate that time ∆t flows as a universal process of energy exchange or continuous creation. Mass will increase relative to this process with gravity being a secondary force to the electromagnetic force. The c² represents the speed of light c radiating out in a sphere 4π of EMR from its radius forming a square c² of probability. We have to square the probability of the wave-function Ψ because the area of the sphere is equal to the square of the radius of the sphere multiplied by 4π. This simple geometrical process forms the probability and uncertainty of everyday life and at the smallest scale of the process is represented mathematically by Heisenberg’s Uncertainty Principle ∆×∆pᵪ≥h/4π. In such a theory the future is unfolding photon by photon with the movement of charge and flow of EM fields. This gives us a geometrical reason for positive and negative charge with a concaved inner surface for negative charge and a convexed outer surface for positive charge. The brackets in the equation (E=ˠM˳C²)∞ represent a dynamic boundary condition of an individual reference frame with an Arrow of Time or time line for each frame of reference. The infinity ∞ symbol represents an infinite number of dynamic interactive reference frames that are continuously coming in and out of existence.

  • Deepak K
    Deepak K 3 месяца назад +1

    So, this means that if either a positive charge or a negative charge is brought near the conductive wire, it will repel? But that is not the principle of electromagnet. And also how do you assume that the outside positive charge is moving with same velocity as that of electrons inside the wire? In an electromagnet, the electrons move in the conducting wire creating magnetic field around them, not electric field. This video is quite misleading.

  • Saleh Algadeer
    Saleh Algadeer 3 месяца назад +1

    I think that the video doesn't exactly explain the magnetism, but rather the electromagnetic force (applied on an electric charge).

  • yayJohn P
    yayJohn P 3 месяца назад +1

    0:38, Derek, are you sure you shaved?

  • syed hussain
    syed hussain 3 месяца назад +1

    Brilliant way of clearing concept....

  • Surneel Sehgal
    Surneel Sehgal 3 месяца назад +1

    Why there is not any electron contraction ? In the stationary frame of reference of cat.

  • Coco Deng
    Coco Deng 3 месяца назад +1

    One of the question in my magnetic field unit test (G12) asked, which direction would a ball move if a wire that has a current moving out of the page and a ball moving into the page. According to right hand rule, the ball is going into the page (v= the thumb) and the magnetic field created by the wire is acting on the ball is from left to right (field=fingers). Therefore, the force of magnetic field should be down. However, according to relativity, obj. compress in the direction of motion, so, in the frame of reference of the ball, the density of e- should be increased. Thus, force of magnetic field is up? I know that I must’ve messed something up but, I really don’t know how...

  • Yes I entered My Name Correctly
    Yes I entered My Name Correctly 3 месяца назад +1

    I think anyone who watches this video will be dumber for having watched it.