How Special Relativity Makes Magnets Work
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Published 2013-09-23
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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 mazur.harvard.edu/emdetails.php
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 CueSongs.com
All Comments (21)
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On my fourth viewing of this video, I realized I was looking at... a cat-ion.
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"You are looking slim." "Only in your frame of reference." - how is this not a famous quote? XD
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Just saw this 7-year-old video. "A magnetic field is just an electric field viewed from another reference frame." Extraordinary!
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Dude wtf are you serious , I have been asking this question like millions of times too professors and teachers but noone wanted to give me an answer. This channel is sacred for me from now on
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What Schroedinger was trying to say with his cat analogy: Quantum superposition makes no sense. What Derek learned from Shroedinger's analogy: All analogies are better with cats
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"A Magnetic field is just an Electric Field viewed from a different frame of reference." Thank you for that mind-blowing definition. I always viewed Electromagnetism from a classical point of view and just started studying Special Relativity, but thanks to this video I see another connection between both theories. Thanks a lot man, you literally made my day!
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I’ve been teaching high school physics for 32 years, I and I’m still learning such cool things thanks to amazing videos like this one.
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For those who ask themselves, why the electrons dont come closer together in the lab frame: I think its because only the electrons become contracted not the space in between. Hence, the density of charge doesn't change. When the cat moves, everything it sees is contracted, since everything (also the space between the postitiv charges) moves and the density of positive charges increases. Just my approach though
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Wife (While jogging): Honey, am I looking slim? husband: Not even in my frame of reference?
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Our Prof linked this video as part of his lecture. So I have to ask: Is this part of the exam?
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Why the wire has no charge in the rest frame: The electrons do initially experience length contraction as they start moving, but they repel each other and spread out to restore neutral charge. The same can't happen with the protons because they can't move freely in the material. Why a single moving electron produces a magnetic field: The electric field caused by the electron also moves along with the electron, causing it to contract and increase its density.
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I was always told electromagnetism is a relativistic correction on a moving electric field but I’ve never actually had that explained. Thanks heaps for that.
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In the case when cat is at rest and it sees electrons moving, the electrons are more densely accumulated and the positive charges are spread out. So according to this logic, even rest charge must interact with magnetic field.
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A positively charged cat is a cation.
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Why is the wire neutral when current flows through it? Shouldn’t the electrons experience length-contraction from our point of view? Shouldn’t a stationary cat therefore be attracted to the wire?
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I absolutely love you guys, this is a such a great explanation it literally filled me with joy since I have been trying to understand what magnetism is for months now, but have been unable to find a satisfying answer. The work you guys do is so so so important and I am extremely grateful for this video and all the work you guys do.
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At primary school I never understood why magnets work... Now I see why it never gets explained to kids;)
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Trying to be flirty with nerds: "You're looking slim" "only in your frame of reference"
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1:20 "so if there were... a positively charged cat nearby," cat dying sfx
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I can not thank you enough for this brilliant video! For years now this question has been bugging me but most explanations weren't sticking with me. This was presented so clear that it will finally stick in my head. I always look forward to your videos!
