I'm reading Griffith's electrodynamics and in it he mentions in section 10.2.1 that news travel with the speed of light in electromagnetism. To my understanding the news he is talking about are the electromagnetic waves, which travel through space. Later in the example 10.4 he calculates the electric and magnetic field of a point charge moving at constant velocity. Now my question is: if a charge moving at constant velocity doesn't produce EM waves, how does it create an EM field in a point in space if the charge and the point can communicate only via EM waves?
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$\begingroup$ The computer you are using - and if you use DSL internet connection, that as well - are not using EM waves yet you have information coming to you. You are confusing EM waves with electric (and magnetic) fields. $\endgroup$– José AndradeCommented May 20, 2020 at 15:00
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$\begingroup$ I know I'm confusing something but I can't understand how an electromagnetic field can be created without electromagnetic waves, also in Griffith's in example 10.2 he talks about how a suddenly created current creates and electromagnetic field. For time less than s/c (where s the distance from the cable in which the current is produced) there is no electric field. This means that the electric and the magnetic travel through space. Still even in this case we don't talk about em waves. I hope I made clear what exactly is causing my confusion $\endgroup$– Μπαμπης ΠοζουκιδηςCommented May 20, 2020 at 15:10
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$\begingroup$ Unfortunately I dont have time to post a comprehensive answer, but I will try to alleviate confusion with a, maybe not completely accurate, metaphor. Image EM waves as sound. They exist and are regions of low and high pressure. Now the fields is just the pressure. It can be higher or lower. You have wind from big regions of high and low pressure for example and it is not sound. Yet if sound exists or not comes from how this pressure is excited. Maybe it is not the most correct metaphor. I am hoping that someone with a good grasp of ED can give you a comprehensive answer, though. $\endgroup$– José AndradeCommented May 20, 2020 at 15:20
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$\begingroup$ (just a quick comment. A suddenly created current will have to create an EM wave, a pulse with large bandwidth. But then you need to distinguish between what is an EM wave and which are the created electric and magnetic fields) $\endgroup$– José AndradeCommented May 20, 2020 at 15:23
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$\begingroup$ To my understanding EM waves travel to create the EM field. Just like my voice for example will create the sound wave that will create the pressure difference at someones ear. Is this reasoning wrong ? $\endgroup$– Μπαμπης ΠοζουκιδηςCommented May 20, 2020 at 15:29
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2 Answers
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In-depth research shows that clearly: Magnetic field and electric field are unified, magnetic field is just the one appearance of electric field, there is not independent magnetic field in the world; To say it simply, magnetic field interaction is the electric field interaction that be effected by motion. Motion can take weak effect to the electric field interaction, the electric interaction between two electrons increase by the increase of relative motion velocity of them (Lorentz force had said it), the electric interaction is the weakest when the two electrons keep relatively static (is electrostatic interaction, but not Coulomb interaction); Compare with electrostatic interaction, the ratio of the increase of electric interaction is 1:10^11 when the relative motion velocity is 1m/S; And can be 1:10^4 to the two thermal movement electrons. These changes can take some effect to the world obviously, and the effect is just magnetic interaction. To say it accuturely, magnetic field is not a pure field, it is the combine of electric field and motion.
I recommend that you read this first，about the nature of magnetic field： https://phyyz.kinja.com/the-theoretical-analysis-of-the-field-1828705659?rev=1589649761587 The article is written in Chinese
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If a charge is moving at constant velocity for inertial frame $A$, it is always possible to make a change of coordinates, applying Lorentz transformation, to the inertial frame of the particle, say $B$.
For frame $B$, the field is static all over the space. And frame $A$ is moving around this static field. So, there is no signal or information to be transmitted.
Of course we can think that the charge was not always there. At some moment a device separated positive and negative charges, and surely some acceleration was involved. EM waves were emitted then, sending the information through the space.
answered May 20, 2020 at 18:27
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$\begingroup$ So at some point the electric or magnetic field created by the charge at a certain point in space had to be created by an EM wave, but once the charge stops accelarating the emition of EM waves stops? $\endgroup$ Commented May 20, 2020 at 19:00
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$\begingroup$ yes, because any inertial frame is equivalent in SR. $\endgroup$ Commented May 20, 2020 at 20:35