6 ms·

I thought that the speed of electricity, actually electrons in a cable for a consumer use was around 1cm/s.

ColinWright 12y ago
Signals travel at about 30 cm/nsec, in coax about 2/3 of that. The actual electrons themselves travel about 1 mm/sec. Sort of. You can find out more with web searches, but those are broadly the numbers. Broadly. http://www.physlink.com/education/askexperts/ae69.cfm http://www.physlink.com/education/askexperts/ae69.cfm https://uk.answers.yahoo.com/question/index?qid=20130416131646AASIrg8 https://uk.answers.yahoo.com/question/index?qid=201304161316...
- Retr0spectrum 12y ago
  Why is it slower in coax?
  - ColinWright 12y ago
    Broadly speaking, as it goes down a conductor it has to build a magnetic field, and as it does so the increasing magnetic field creates an electric field that opposes the motion of the signal. Roughly.
  - marcosdumay 12y ago
    30cm/ns is the speed of light in vacuum. Coax has a hight refraction index, thus electricity moves slower in it.
  - zackmorris 12y ago
    One more way to think about it is the propagation delay of a transmission line, or actually the opposite of that, which is the velocity factor, proportional to 1/sqrt(LC): https://en.wikipedia.org/wiki/Wave_propagation_speed https://en.wikipedia.org/wiki/Wave_propagation_speed Coaxial cables are great for blocking interference because one conductor is inside the other, so external electric fields can't penetrate. But they have a high parasitic capacitance because the outside conductor has a large surface area. So if capacitance is causing too much propagation delay, you can use twisted pair. It also works to avoid interference, but because the wires constantly change position in relation to electric fields: https://en.wikipedia.org/wiki/Twisted_pair https://en.wikipedia.org/wiki/Twisted_pair The problem with coax for cable TV though is that for high frequencies, losses can be high because the capacitor begins to approximate a conductor (and TV uses really high frequencies!). So cable length becomes a limitation. Then you use fiber optic cables (I find total internal reflection somewhat miraculous, like an optical analog to superconductors). Also in computer chips, parasitic capacitance is a problem because capacitance grows with area and shrinks with distance. But the conductors are so close to one another that even a small area results in a large capacitance. That's why the latency to memory can be orders of magnitude higher than say, cache. Optical interconnects would help here as well. It's been 15 years since I studied this stuff though, so if I misremembered, somebody please correct me!
    - jpmattia 12y ago
      > * But they have a high parasitic capacitance because the outside conductor has a large surface area. > So if capacitance is causing too much propagation delay, you can use twisted pair.* The capacitance is not really "parasitic", it is the circuit element representation of the electric field which is propagating. The dynamic electric field gives rise to a magnetic field, which is the inductive element in the expression 1/sqrt(LC) that you listed. Twisted pair is no different: There is a capacitive element and inductive element per unit length. As it turns out, the capacitance varies as the log of the distance between the wires, so you can twist the wires and not change the capacitance very much. There used to be lots of "twinax", which kept the two wires at a fixed distance, but the separation plastic was mostly a PITA. (The pics I can find on google don't show the old flat plastic twinax that folks used to use. You can still find that stuff hanging off old television antennas.)
jokoon 12y ago
make the difference between electricity and a signal. a signal is information, electrons are just particle which flows through a wire. if you slowly push a train, at the other end it's still going at the exact same speed at the exact same time (well in theory, it still depends on a certain fraction of the speed of light).
MereInterest 12y ago
Actually much, much less than that. http://en.wikipedia.org/wiki/Drift_velocity#Numerical_example http://en.wikipedia.org/wiki/Drift_velocity#Numerical_exampl... Even for a large current in a small wire, you only get average speeds of about 1cm every 40 seconds. Luckily, that isn't the speed at which signals move. Imagine that each of us is holding one end of a 10 foot metal pole. I transfer a message to you by pushing on the bar. You don't need to wait for my end to reach you before you can tell that it has moved, because you can feel the entire rod moving. In the same way, the receiver of a signal doesn't need to wait for the electrons to move all the way, because pushing on some of the electrons makes the entire mass of electrons move.
jpmattia 12y ago
> I thought that the speed of electricity, actually electrons in a cable for a consumer use was around 1cm/s. Folks who worry about the velocity of electrons when discussing the velocity of signal propagation in a coax cable have not fully understood Maxwell's Equations. The fields are responsible for the propagation, and the geometry + dielectric determine the speed of the fields in the cable to first order. If those fields were in the visible, as they are for some fiber, then folks would talk about the speed of light in the coax. Since they are not in the visible, we talk about the speed of the TEM mode instead. EDIT: Some other folks are pointing out that the speed of propagation in coax is about 2/3 the speed of light. This has nothing to do with the velocity of electrons in the metal. Most coax has a central conductor, an insulator (made of teflon PTFE), and a outer conductor (the shield). The speed is almost entirely determined by the index-of-refraction of the teflon, which is approximately 1.5. The propagation speed is 1/index (approximately), so the the reciprocal of 3/2 gets you to 2/3.
ajarmst 12y ago
I've always felt that what we used to call the Shannon Limit (more accurately, the Shannon-Hartley Theorem: https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...) was more useful in conversations like this than any reference to the speed-of-light or motion of particles in a medium.
snarfy 12y ago
The speed water flows through a pipe makes little difference to the speed of sound in water.

Signals travel at about 30 cm/nsec, in coax about 2/3 of that. The actual electrons themselves travel about 1 mm/sec. Sort of. You can find out more with web searches, but those are broadly the numbers. Broadly. http://www.physlink.com/education/askexperts/ae69.cfm http://www.physlink.com/education/askexperts/ae69.cfm https://uk.answers.yahoo.com/question/index?qid=20130416131646AASIrg8 https://uk.answers.yahoo.com/question/index?qid=201304161316...

Retr0spectrum 12y ago
Why is it slower in coax?
- ColinWright 12y ago
  Broadly speaking, as it goes down a conductor it has to build a magnetic field, and as it does so the increasing magnetic field creates an electric field that opposes the motion of the signal. Roughly.
- marcosdumay 12y ago
  30cm/ns is the speed of light in vacuum. Coax has a hight refraction index, thus electricity moves slower in it.
- zackmorris 12y ago
  One more way to think about it is the propagation delay of a transmission line, or actually the opposite of that, which is the velocity factor, proportional to 1/sqrt(LC): https://en.wikipedia.org/wiki/Wave_propagation_speed https://en.wikipedia.org/wiki/Wave_propagation_speed Coaxial cables are great for blocking interference because one conductor is inside the other, so external electric fields can't penetrate. But they have a high parasitic capacitance because the outside conductor has a large surface area. So if capacitance is causing too much propagation delay, you can use twisted pair. It also works to avoid interference, but because the wires constantly change position in relation to electric fields: https://en.wikipedia.org/wiki/Twisted_pair https://en.wikipedia.org/wiki/Twisted_pair The problem with coax for cable TV though is that for high frequencies, losses can be high because the capacitor begins to approximate a conductor (and TV uses really high frequencies!). So cable length becomes a limitation. Then you use fiber optic cables (I find total internal reflection somewhat miraculous, like an optical analog to superconductors). Also in computer chips, parasitic capacitance is a problem because capacitance grows with area and shrinks with distance. But the conductors are so close to one another that even a small area results in a large capacitance. That's why the latency to memory can be orders of magnitude higher than say, cache. Optical interconnects would help here as well. It's been 15 years since I studied this stuff though, so if I misremembered, somebody please correct me!
  - jpmattia 12y ago
    > * But they have a high parasitic capacitance because the outside conductor has a large surface area. > So if capacitance is causing too much propagation delay, you can use twisted pair.* The capacitance is not really "parasitic", it is the circuit element representation of the electric field which is propagating. The dynamic electric field gives rise to a magnetic field, which is the inductive element in the expression 1/sqrt(LC) that you listed. Twisted pair is no different: There is a capacitive element and inductive element per unit length. As it turns out, the capacitance varies as the log of the distance between the wires, so you can twist the wires and not change the capacitance very much. There used to be lots of "twinax", which kept the two wires at a fixed distance, but the separation plastic was mostly a PITA. (The pics I can find on google don't show the old flat plastic twinax that folks used to use. You can still find that stuff hanging off old television antennas.)

jokoon 12y ago

make the difference between electricity and a signal. a signal is information, electrons are just particle which flows through a wire. if you slowly push a train, at the other end it's still going at the exact same speed at the exact same time (well in theory, it still depends on a certain fraction of the speed of light).

MereInterest 12y ago

Actually much, much less than that. http://en.wikipedia.org/wiki/Drift_velocity#Numerical_example http://en.wikipedia.org/wiki/Drift_velocity#Numerical_exampl... Even for a large current in a small wire, you only get average speeds of about 1cm every 40 seconds. Luckily, that isn't the speed at which signals move. Imagine that each of us is holding one end of a 10 foot metal pole. I transfer a message to you by pushing on the bar. You don't need to wait for my end to reach you before you can tell that it has moved, because you can feel the entire rod moving. In the same way, the receiver of a signal doesn't need to wait for the electrons to move all the way, because pushing on some of the electrons makes the entire mass of electrons move.

jpmattia 12y ago

> I thought that the speed of electricity, actually electrons in a cable for a consumer use was around 1cm/s. Folks who worry about the velocity of electrons when discussing the velocity of signal propagation in a coax cable have not fully understood Maxwell's Equations. The fields are responsible for the propagation, and the geometry + dielectric determine the speed of the fields in the cable to first order. If those fields were in the visible, as they are for some fiber, then folks would talk about the speed of light in the coax. Since they are not in the visible, we talk about the speed of the TEM mode instead. EDIT: Some other folks are pointing out that the speed of propagation in coax is about 2/3 the speed of light. This has nothing to do with the velocity of electrons in the metal. Most coax has a central conductor, an insulator (made of teflon PTFE), and a outer conductor (the shield). The speed is almost entirely determined by the index-of-refraction of the teflon, which is approximately 1.5. The propagation speed is 1/index (approximately), so the the reciprocal of 3/2 gets you to 2/3.

ajarmst 12y ago

I've always felt that what we used to call the Shannon Limit (more accurately, the Shannon-Hartley Theorem: https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...) was more useful in conversations like this than any reference to the speed-of-light or motion of particles in a medium.

snarfy 12y ago

The speed water flows through a pipe makes little difference to the speed of sound in water.