More personal fascination than specialist subject Galu.
Of interest here to me is the fact that particles have to be accelerated in order to radiate an EM wave. So we are back to energy and changing time reference of the accelerated particle wrt it’s surrounding.
Of interest here to me is the fact that particles have to be accelerated in order to radiate an EM wave. So we are back to energy and changing time reference of the accelerated particle wrt it’s surrounding.
More personal fascination than specialist subject Galu.
Of interest here to me is the fact that particles have to be accelerated in order to radiate an EM wave. So we are back to energy and changing time reference of the accelerated particle wrt it’s surrounding.
Actually there are similar problems receiving them. At more usual wavelengths a usual method is to make a shorter aerial appear "electrically" long for both transmitting and receiving them.
The interesting question is how to encode some form of message so that it can be transmitted at VLF's. Also what form of modulation is used. I'd guess a variation of this one
https://en.wikipedia.org/wiki/Phase-shift_keying
The rf frequency sets the time taken to send the info - there isn't any limitation on the amount of info. It could even be very slowed down audio,
ELF 1 million watts class A can penetrate seawater, but you can’t send much info ELF so you tell the sub to surface to be able to communicate VLF. ELF is the one with limited comm capability. Obviously microwave, satellite communications are much higher frequencies, C band, Ku band, etc. MHz, GHz. And much more prevalent. Run silent, run deep.
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I'd argue that Bonsai's description involved a change of movement - ie accelereation... but... (abpout 1m 20s in)
If you have sufficient power and antenna size on one end you don’t need a large antenna on the other side, as long as receiver sensitivity is sufficient. I would sometimes run the HF propagation prediction software for FAA team that went to crash sites in remote areas where HF was the only available comm. It’s a link budget thing. All FAA centers and HQ had very large log periodic HF antennas. It was also a strategic comm system theoretically.
I thought that even the rate at which phase could be changed was limited? So for example, if you had a 100 kHz carrier, ultimately even that had some constraints placed on it. I’ll have to go back and look at it - I’m a bit out of my depth here 🙂
(Edit: just realized there’s a bit of a nerds tech pun buried in there 🤣)
Thinking about this again and my earlier statement about an accelerating electron - thought experiment:
Say you have a conductor in free space not moving and along comes an electron moving at constant velocity. When it passes near the conductor, it will induce current flow in the conductor (and a resultant field around the conductor as electrons in the conductor move). No acceleration involved, just relative movement between the charged particle and the conductor.
Electrons move back and forth rapidly in an audio AC circuit, right? So, if their speed is constant how to they get to “full speed” from moving in the opposite direction in 1/20000 sec? Sounds like UFOs if you ask me.
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The influence of the EM field is what moves at C. The electrons themselves don't go very far or very fast. The copper atoms are very good at keeping them in one place most of the time. Stuck in their orbitals.
Bonsai used the word "moves" which does not imply an acceleration.
When we say a car moves at 60 mph we can not assume that it is accelerating.
What may be good enough for the layman is insufficient for the physicist who would distinguish between movement and changes in movement.
Your thought experiment is not in relation to electromagnetic radiation and that is where the acceleration of charges is mandatory.
In copper, the electrons in the outermost orbitals are so loosely bound that they move randomly around in the space between the copper atoms.
That's why we call them "free electrons".
They are still spend most of their time in the orbitals. Jumping from one atom to the next - and not much time in the space in between them. That’s why the drift velocity is pretty slow.
I don't know which post engendered your response, Geoff. Perhaps if you used the quote function when you are not referring to the post directly above your reply?
An ideal description (i.e. all other factors being ignored) of the motion of an electron in an AC circuit would be that of Simple Harmonic Motion (SHM), where the acceleration is always directed towards the centre of oscillation. The electron speed would therefore vary from zero at the extremes of the oscillation to maximum at the centre of the oscillation.
The maximum acceleration must be very high. Perhaps an expert mathematician could calculate it for a typical audio frequency?
you can, if it was previously stationary, which is how I took it. Now, if you are still splitting heirs.... ( 🙂 )
Unfortunately I was splitting hairs 🙂. Acceleration always implies the action of some force (= energy expenditure). I think the only place where that won’t apply is with a body being accelerated in a gravitational field due to space time curvature - but Galu may have more insight.