4 ms·

Confused. Are you saying that, if we took a light bulb (off) and a metal rod (0 charge) beside on another. Then were somehow able to turn the light bulb on and

Confused. Are you saying that, if we took a light bulb (off) and a metal rod (0 charge) beside on another. Then were somehow able to turn the light bulb on and apply a charge to the that rod at the same time. While also having a detector that can sense a photon and a change in electric field some equal distance away from the bulb and rod. Then the photon (from the bulb) would reach our detector before the detection of the change in electric field (from the rod)? Let's suppose the medium is just plain air, and not particularly humid.

Spivak 2y ago
Yes, you can see one of the best people do a demonstration of it https://m.youtube.com/watch?v=2Vrhk5OjBP8 https://m.youtube.com/watch?v=2Vrhk5OjBP8 It's close enough to c that you should just use c but it can be observed that it's less.
- sightbroke 2y ago
  https://en.wikipedia.org/wiki/Electromagnetic_radiation https://en.wikipedia.org/wiki/Electromagnetic_radiation "In a vacuum, electromagnetic waves travel at the speed of light, commonly denoted c." I would expect that in air, that the photon from the light source and the perturbance of the electric field from the charge to reach the detector at the same time.
  - mr_toad 2y ago
    Yes, essentially. The perturbation of the electrical field causes EM radiation (radio), which moves at the speed of light.
    - tsimionescu 2y ago
      It moves at the same speed as light (it is light), but not at the "speed of light" (c). Light/EM waves move slower than c in air (and much slower than c in a copper wire).
  - B1FF_PSUVM 2y ago
    Correct, you're detecting electromagnetic fields in the same medium.
panxyh 2y ago
Yes. What's confusing there?
- sightbroke 2y ago
  u/cogman10 comment reads to me as if they are saying that EM radiation and light travel at different speeds.
  - Tuna-Fish 2y ago
    They travel at the same speed, in the same medium. c is the speed of light in a vacuum, all EM radiation travels slower in any other medium. The complicating factor here is that when you have electricity flowing in a wire, the fields are generally mostly outside the conductor, not in it. That is, the signal propagation delay depends more in what you are using as an insulator around wire than the material of the wire. This has had practical consequences in the past; if you replace the insulating jacket of one wire in a twisted pair with a slightly different material, on long runs it will ruin your signal.
  - mr_toad 2y ago
    EM radiation, whether radio or visible light is photons, and photons only have one speed. However, electrical conduction is the movement of electrons, not photons.
    - Tuna-Fish 2y ago
      > EM radiation, whether radio or visible light is photons Is the only part that's not wrong in your post. Photons only have one speed in empty space. They slow down when traveling through any medium other than vacuum. > However, electrical conduction is the movement of electrons, not photons. Electrons move because they influence each other through their fields, which are transmitted by photons. Electrical conduction happens at the speed of the fields, not at the speed of the electrons. When you push one more electron into one side of a conductor, an electron flows out the other side when the fields reach the other side, not when the electron does. (As an analogy, consider a rubber hose full of steel balls. When you slowly push an additional ball in from one side, another ball starts to fall out of the other side as you push the first ball in, perceptually instantenously⁰, regardless of the speed you are pushing the new ball in. (0): After a delay of (length of tube)/(speed of sound in steel)
j5155 2y ago
Yes. As a more common example, fiber optic cabling is known to transmit information significantly faster then copper.
- sightbroke 2y ago
  I am sorry if my experimental setup was not clear. Copper is clearly a different medium than fiber optic. That is why I stipulated a single medium for the experiment.
- chongli 2y ago
  The speed of fibre optic cabling has more to do with signal integrity and bandwidth than any difference in propagation delay. In fact, in some cases a signal can travel faster on a copper wire (0.8c) than it can through an optical fibre (0.6c).
tsimionescu 2y ago
No, the two would arrive at the same time. What the poster above was saying is that the speed of EM propagating through copper is much lower than c, which is the speed of EM in a vacuum. Overall the picture is like this: - massless particles like photons travel at speed c in empty space, in a straight line - massive particles like electrons travel at a speed slightly less than c even in empty space, in a straight line - inside a medium, any particle traveling in a straight line will quickly bounce off/be absorbed and re-emitted in a new direction because of some field given off by another particle; so, the average speed at which particles actually traverse through the material is lower than c. How low depends on properties of the medium, mainly how dense it is; for copper and most metals commonly used in electronics, this varies between 60-80% of c. - When applying a potential difference to a metal wire, the electrons which normally are moving at speeds very close to c in the empty space between atoms in random directions (amounting to an overall speed of 0 along the wire) will start collectively moving in the direction of the potential difference (towards the positive node), at a very low average speed called "drift speed"; this is caused by their normally completely random bounces now being biased in the direction of the electric field; - However, the current in the wire (the EM radiation) moves extremely fast along the wire, at the same speed that light moves (on average) through the wire. You can think of this as being caused by photons (since EM waves are photons) moving much more easily than electrons through the wire, simply because they don't have a charge of their own and so don't get caught so easily by other atoms as electrons do. So you have 5 speeds relevant to this: the speed of light/photons/EM waves in vacuum (c), the speed of an electron in vacuum (very close to c), the average speed of all electrons in a metal wire without any electric potential (0), the average speed of all electrons in a metal wire with an electric potential (drift velocity, very small), and the speed of EM waves in a wire (60-80% of c in typical conductors). Edit: one more note that complicates this picture, but the EM waves in a charged wire don't really move inside the wire, or not entirely - they move mostly around the wire - which means that their actual speed depends not (just) on the material from which the wire is made, but also the insulation outside the wire. That is, the EM field will propagate a different fraction of c for a copper wire than for an aluminum wire; but also for a copper wire wrapped in plastic versus one exposed directly in the air.
- fransje26 2y ago

Spivak 2y ago

Yes, you can see one of the best people do a demonstration of it https://m.youtube.com/watch?v=2Vrhk5OjBP8 https://m.youtube.com/watch?v=2Vrhk5OjBP8 It's close enough to c that you should just use c but it can be observed that it's less.

sightbroke 2y ago
https://en.wikipedia.org/wiki/Electromagnetic_radiation https://en.wikipedia.org/wiki/Electromagnetic_radiation "In a vacuum, electromagnetic waves travel at the speed of light, commonly denoted c." I would expect that in air, that the photon from the light source and the perturbance of the electric field from the charge to reach the detector at the same time.
- mr_toad 2y ago
  Yes, essentially. The perturbation of the electrical field causes EM radiation (radio), which moves at the speed of light.
  - tsimionescu 2y ago
    It moves at the same speed as light (it is light), but not at the "speed of light" (c). Light/EM waves move slower than c in air (and much slower than c in a copper wire).
- B1FF_PSUVM 2y ago
  Correct, you're detecting electromagnetic fields in the same medium.

panxyh 2y ago

Yes. What's confusing there?

sightbroke 2y ago
u/cogman10 comment reads to me as if they are saying that EM radiation and light travel at different speeds.
- Tuna-Fish 2y ago
  They travel at the same speed, in the same medium. c is the speed of light in a vacuum, all EM radiation travels slower in any other medium. The complicating factor here is that when you have electricity flowing in a wire, the fields are generally mostly outside the conductor, not in it. That is, the signal propagation delay depends more in what you are using as an insulator around wire than the material of the wire. This has had practical consequences in the past; if you replace the insulating jacket of one wire in a twisted pair with a slightly different material, on long runs it will ruin your signal.
- mr_toad 2y ago
  EM radiation, whether radio or visible light is photons, and photons only have one speed. However, electrical conduction is the movement of electrons, not photons.
  - Tuna-Fish 2y ago
    > EM radiation, whether radio or visible light is photons Is the only part that's not wrong in your post. Photons only have one speed in empty space. They slow down when traveling through any medium other than vacuum. > However, electrical conduction is the movement of electrons, not photons. Electrons move because they influence each other through their fields, which are transmitted by photons. Electrical conduction happens at the speed of the fields, not at the speed of the electrons. When you push one more electron into one side of a conductor, an electron flows out the other side when the fields reach the other side, not when the electron does. (As an analogy, consider a rubber hose full of steel balls. When you slowly push an additional ball in from one side, another ball starts to fall out of the other side as you push the first ball in, perceptually instantenously⁰, regardless of the speed you are pushing the new ball in. (0): After a delay of (length of tube)/(speed of sound in steel)

j5155 2y ago

Yes. As a more common example, fiber optic cabling is known to transmit information significantly faster then copper.

sightbroke 2y ago
I am sorry if my experimental setup was not clear. Copper is clearly a different medium than fiber optic. That is why I stipulated a single medium for the experiment.
chongli 2y ago
The speed of fibre optic cabling has more to do with signal integrity and bandwidth than any difference in propagation delay. In fact, in some cases a signal can travel faster on a copper wire (0.8c) than it can through an optical fibre (0.6c).

tsimionescu 2y ago

No, the two would arrive at the same time. What the poster above was saying is that the speed of EM propagating through copper is much lower than c, which is the speed of EM in a vacuum. Overall the picture is like this: - massless particles like photons travel at speed c in empty space, in a straight line - massive particles like electrons travel at a speed slightly less than c even in empty space, in a straight line - inside a medium, any particle traveling in a straight line will quickly bounce off/be absorbed and re-emitted in a new direction because of some field given off by another particle; so, the average speed at which particles actually traverse through the material is lower than c. How low depends on properties of the medium, mainly how dense it is; for copper and most metals commonly used in electronics, this varies between 60-80% of c. - When applying a potential difference to a metal wire, the electrons which normally are moving at speeds very close to c in the empty space between atoms in random directions (amounting to an overall speed of 0 along the wire) will start collectively moving in the direction of the potential difference (towards the positive node), at a very low average speed called "drift speed"; this is caused by their normally completely random bounces now being biased in the direction of the electric field; - However, the current in the wire (the EM radiation) moves extremely fast along the wire, at the same speed that light moves (on average) through the wire. You can think of this as being caused by photons (since EM waves are photons) moving much more easily than electrons through the wire, simply because they don't have a charge of their own and so don't get caught so easily by other atoms as electrons do. So you have 5 speeds relevant to this: the speed of light/photons/EM waves in vacuum (c), the speed of an electron in vacuum (very close to c), the average speed of all electrons in a metal wire without any electric potential (0), the average speed of all electrons in a metal wire with an electric potential (drift velocity, very small), and the speed of EM waves in a wire (60-80% of c in typical conductors). Edit: one more note that complicates this picture, but the EM waves in a charged wire don't really move inside the wire, or not entirely - they move mostly around the wire - which means that their actual speed depends not (just) on the material from which the wire is made, but also the insulation outside the wire. That is, the EM field will propagate a different fraction of c for a copper wire than for an aluminum wire; but also for a copper wire wrapped in plastic versus one exposed directly in the air.

fransje26 2y ago