3 ms·

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 th

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
So if I understand this correctly, the information is moving along the wire, at a speed between 60 to 100% of c. This brings me to a Veritasium video of a few years back that I didn't quite understand at the time. [1] The claim being made was that if you connected a light bulb to a switch, with 300'000 km of wire left and right, and if the switch and the light bulb were 1 m away, the light would turn on in 1/c seconds. But this would imply that the information travel is not along the wires, but straight from the switch to the light bulb? [1] https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY
- Linosaurus 2y ago
  Iirc I thought they were a bit disingenuous. In theory the change in electric field will induce a small current in the other wire, and their magical science lamp turns on at any non-zero electricity. Whether the wires are connected or not at the far end doesn’t matter. They never clarified how strong the other current would be.
- tsimionescu 2y ago
  Information is a bit weird as a way to put it, but it may be correct. Still, in case this is very intentional wording, the information about the electrical circuit being lit up will (that is, any photon emitted at the moment and location that the voltage appeared, such as hitting a switch to close a circuit) still travels at c in all directions. It's only the (main) electric current that travels at 60-80% of c and along the wire. Also, the phenomenon being discussed in the Veritasium video is slightly different, and that video sort of mixes up two things to make things more mysterious. The simple explanation for what is going on in the video is that whenever you have a voltage change in a circuit, such as closing it with a switch, you get a tiny bit of radio waves being emitted radially outwards from that point out *. In the Veritasium experiment, with a very precise measurement tool, they could detect the tiny radio pulse emitted by the switch at the moment it was closed, far before the main current reached the same point moving along the long wires. The other thing he was mixing this up with is the Poynting vector, which is a mathematical object that represents the direction along which power (as in, worl, the thing you measure in Watts, not the actual current which you'd measure in Amperes) propagates in an electrical circuit. It's not clear at all that the Poyinting vector is a meaningful aspect of reality and not just some mathematical tool. It does happen to coincide with the direction that the radio waves propagate as described earlier, so some physicists do like to interpret it as a deeper, physically real, thing that is related, but you can just as easily ignore it. Note that when I say that it's "not real", this is similar to how I would describe the Lagrangian and Hamiltonian of a classical mechanics system. It's an extremely useful mathematical tool and it predicts the behavior of the system perfectly well, and is mathematically equivalent to Newton's laws of motion. But while the laws of motion describe real direct aspects of reality, such as objects having mass and momentum, the Lagrangian doesn't really represent an actual thing in physical reality. The above comes with a gigantic caveat: in QM, the Lagrangian and Hamiltonian correspond to the wave function, which is, to the best of our current understanding, the real underlying thing behind reality, with particles and waves and things localized being the artifical objects we introduce merely for convenience. * This is normally completely negligible for DC current, but for AC current, where the current switches direction all the time, a relatively high amount of radio waves get generated radically put from the wires along their whole direction. This can lead to problems such as AC power lines causing significant radio interference. This is also normally described as "parasitic", since the power those waves carry is lost from the transmission line, which is designed to carry as much as possible of the power generated at the source towards the destination.

fransje26 2y ago

So if I understand this correctly, the information is moving along the wire, at a speed between 60 to 100% of c. This brings me to a Veritasium video of a few years back that I didn't quite understand at the time. [1] The claim being made was that if you connected a light bulb to a switch, with 300'000 km of wire left and right, and if the switch and the light bulb were 1 m away, the light would turn on in 1/c seconds. But this would imply that the information travel is not along the wires, but straight from the switch to the light bulb? [1] https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY

Linosaurus 2y ago
Iirc I thought they were a bit disingenuous. In theory the change in electric field will induce a small current in the other wire, and their magical science lamp turns on at any non-zero electricity. Whether the wires are connected or not at the far end doesn’t matter. They never clarified how strong the other current would be.
tsimionescu 2y ago
Information is a bit weird as a way to put it, but it may be correct. Still, in case this is very intentional wording, the information about the electrical circuit being lit up will (that is, any photon emitted at the moment and location that the voltage appeared, such as hitting a switch to close a circuit) still travels at c in all directions. It's only the (main) electric current that travels at 60-80% of c and along the wire. Also, the phenomenon being discussed in the Veritasium video is slightly different, and that video sort of mixes up two things to make things more mysterious. The simple explanation for what is going on in the video is that whenever you have a voltage change in a circuit, such as closing it with a switch, you get a tiny bit of radio waves being emitted radially outwards from that point out *. In the Veritasium experiment, with a very precise measurement tool, they could detect the tiny radio pulse emitted by the switch at the moment it was closed, far before the main current reached the same point moving along the long wires. The other thing he was mixing this up with is the Poynting vector, which is a mathematical object that represents the direction along which power (as in, worl, the thing you measure in Watts, not the actual current which you'd measure in Amperes) propagates in an electrical circuit. It's not clear at all that the Poyinting vector is a meaningful aspect of reality and not just some mathematical tool. It does happen to coincide with the direction that the radio waves propagate as described earlier, so some physicists do like to interpret it as a deeper, physically real, thing that is related, but you can just as easily ignore it. Note that when I say that it's "not real", this is similar to how I would describe the Lagrangian and Hamiltonian of a classical mechanics system. It's an extremely useful mathematical tool and it predicts the behavior of the system perfectly well, and is mathematically equivalent to Newton's laws of motion. But while the laws of motion describe real direct aspects of reality, such as objects having mass and momentum, the Lagrangian doesn't really represent an actual thing in physical reality. The above comes with a gigantic caveat: in QM, the Lagrangian and Hamiltonian correspond to the wave function, which is, to the best of our current understanding, the real underlying thing behind reality, with particles and waves and things localized being the artifical objects we introduce merely for convenience. * This is normally completely negligible for DC current, but for AC current, where the current switches direction all the time, a relatively high amount of radio waves get generated radically put from the wires along their whole direction. This can lead to problems such as AC power lines causing significant radio interference. This is also normally described as "parasitic", since the power those waves carry is lost from the transmission line, which is designed to carry as much as possible of the power generated at the source towards the destination.