Speed of Propagation of a Magnetic Field. - xkcd

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Hey!  Bert, Remember the Bill Beaty "Cease of motion wave"???



      Re: Speed of Propagation of a Magnetic Field.
      <http://forums.xkcd.com/viewtopic.php?t=62899#p3337605>

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<http://forums.xkcd.com/viewtopic.php?p=3337605&sid=132db8a49dfdcf0cd5585ce025f461e8#p3337605>by 
*gmalivuk 
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» Thu Apr 25, 2013 5:25 am UTC

    voltardark <http://forums.xkcd.com/viewtopic.php?p=3337601#p3337601>
    wrote:Magnetism propagate at the speed of light but when when it
    cease to exist it's nearly instantaneous...

No, *all* changes in the magnetic field propagate at the speed of light, 
including the cessation of the magnetic field.
Unless stated otherwise, I do not care whether a statement, by itself, 
constitutes a persuasive political argument. I care whether it's true.
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Top <http://forums.xkcd.com/viewtopic.php?t=62899#top>

On 8/22/2018 4:41 PM, Norman Wootan [email protected] [EVGRAY] wrote:
> http://forums.xkcd.com/viewtopic.php?t=62899
>
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Hi Norman,

They are discussing the propagation speed of magnetism and calls it c (the speed of light in vacuum) versus instant propagation to any place (infinite speed of propagation). They should study transmission line theory or actually do the experiments. The propagation velocity is depending on the medium in which the signal propagates. If transmitting more than one wave (modulated wave) different things can happen.

The phase velocity can exceed the velocity of the signal: https://wiki2.org/en/Phase_velocity https://wiki2.org/en/Phase_velocity. This doesn't mean that the front of the waves will exceed the speed of the signal. Also see group velocity: https://wiki2.org/en/Group_velocity https://wiki2.org/en/Group_velocity

Also related is Cherenkov radiation: https://wiki2.org/en/Cherenkov_radiation https://wiki2.org/en/Cherenkov_radiation. This is made by particles traveling at a speed greater than the propagation velocity of the signal. This is like the sonic boom from a supersonic plane or the boom from a supersonic bullet of a gun.

Regards
Ole

 

---In [email protected], <nwootan@...> wrote :

 Hey!  Bert, Remember the Bill Beaty "Cease of motion wave"???
 
 Re: Speed of Propagation of a Magnetic Field. Post http://forums.xkcd.com/viewtopic.php?p=3337605&sid=132db8a49dfdcf0cd5585ce025f461e8#p3337605by gmalivuk http://forums.xkcd.com/memberlist.php?mode=viewprofile&u=5887&sid=132db8a49dfdcf0cd5585ce025f461e8 » Thu Apr 25, 2013 5:25 am UTC
 voltardark http://forums.xkcd.com/viewtopic.php?p=3337601#p3337601 wrote:Magnetism propagate at the speed of light but when when it cease to exist it's nearly instantaneous...
 No, *all* changes in the magnetic field propagate at the speed of light, including the cessation of the magnetic field.
 Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
 ---
 If this post has math that doesn't work for you, use TeX the World for Firefox http://thewe.net/tex/ or Chrome https://chrome.google.com/webstore/detail/mbfninnbhfepghkkcgdnmfmhhbjmhggn
 
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 Top http://forums.xkcd.com/viewtopic.php?t=62899#top
 
 On 8/22/2018 4:41 PM, Norman Wootan nwootan@... mailto:nwootan@... [EVGRAY] wrote:

   http://forums.xkcd.com/viewtopic.php?t=62899 http://forums.xkcd.com/viewtopic.php?t=62899
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