I’m guessing that there’s a formula that approximates the time.
It has to be a low number, I would guess.
Through the air, copper wire, what?
http://www.blurtit.com/q536187.html
http://www.eskimo.com/~billb/miscon/speed.html
Depends on your definitions, but the general answer is it moves at the speed of light. And for just about every real-world application, over a span of 18,000 feet that’s essentially instantaneously.
Better link:
>Depends on your definitions, but the general answer is it moves at the speed of light. And for just about every real-world application, over a span of 18,000 feet that’s essentially instantaneously.
About right. It moves more slowly in a wire, at a speed like 0.6 c or 0.9 c, somewhere in there. In radio technology one might use different lengths of wire to alter phase relationships between parts of an antenna system. In computer technology, this speed limits how fast a computer can work given the physical separation between components.
It’s gonna be something like 20 or 30 microseconds. Your PC microprocessor could carry out thousands of instructions during this time, so whether this is instant or slow depends on your perspective.
To put the rule-of-thumb answer in this thread, rather than buried somewhere in the linked-to pages, figure 1 foot per nanosecond = 1000 ft per microsecond.
That’s close to a best-case number; depending on your transmission medium it may be as little as 2/3rds that fast.
And as Napier said, depending on what you’re trying to do, the delay over an 18,000 ft run may be meaninglessly short, or overwhelmingly huge. We are definitely well past the point in our technology where the speed of light is slow enough to be a significant barrier to progress.
Right. The speed of light in a vacuum is very close to 1 billion (10[sup]9[/sup]) feet per second. (The precise figure is about 1.6% below this, or 9.83571x10[sup]8[/sup]).
The propagation velocity of a signal along a conductor is limited by the speed of light through the the surrounding isulating medium. For a bare wire in total vacuum, this is equal to c. For bare wire in air at sea-level, it’s between 98 and 99% of c and for typical plastic insulated wire, it’s somewhere between 60% and 70% c.
(bolding mine) My layman’s guess would’ve been the signal’s speed would be the speed of light within the conductor, not the insulator. I always imagined current (once I learned it wasn’t actually the individual electrons moving) being the electromagnetic fields of the electrons exerting a force on the electrons further down the wire. Although the intuitive answer is more often wrong than right when dealing with sub-atomic phenomenon.
Why is the insulator the determining factor for signal speed rather than the conductor?
[slight hijack]
Can you explain how the insulator affects the speed through the medium? That’s (for me) counter-intuitive. Of course, just about everything about the freakin’ speed of light is counter-intuitive. 
[end hijack]
[another slight hijack]
What about a signal through fiber optics? It’s not electricity, but rather light itself. And I know light will propagate through different materials much slower than a vacuum. Is it negligible, or at least much faster than an electric signal through insulated wire?
[end of another slight hijack]
One quick explanation for the differences is that the speed of light is not a constant. The speed of light in a medium is a constant.
Light, for example, travels slower in water which is why you get that neat distortion viewing at an object at an angle through water and why prisms work.
What’ll really seem weird is once the light leaves the prism, is speeds back up!
Light is fastest through a vacuum, that’s the number typically used for “c”, 186,282 miles/sec, 3x10^8 meters/sec, etc.
The insulation on the wire is important because of inductance and capacitance effects.
On Edit: I see that I’m late to the “in a medium” part of the conversation…
Photons. Electrical signals are just another maifestation of our good friend, the electromagnetic force, which we all know is carried by photons. For various reasons I won’ t get into at this stage, photons don’t like to propagate through conductors, but dielectric materials (insulators) are no problem. So, the photons, and therefore the electric field travel just ouside the wire and whatever medium happens to be there is what determines its speed.
Not if you’re a chip designer. Things are so fast today that when you do placement of blocks on a die, you need to make sure that the blocks which talk to each other the most are close to each other. There are plenty of other things to worry about, but the length of the wire is important for both timing and signal quality reasons.
What I hope are pleasant and enlightening further notes:
>Light, for example, travels slower in water which is why you get that neat distortion viewing at an object at an angle through water and why prisms work. What’ll really seem weird is once the light leaves the prism, is speeds back up!
The nature of light is very different in water or glass than it is in a vacuum. In the vacuum you can think of photons as particles flying along unimpeded. But light passing through water or glass is actually forcing vibrations in the charge centers, and these vibrations propagate mechanically. These vibrations depend on how hard it is to distort the charge center locations (the dielectric constant) and also how many charge centers there are per volume (which depends on density). Therefore, the index of refraction is approximately some constant times the dielectric constant for the material times its density. This light traveling through the transparent substance is slower, hence the index of refraction. The mechanical vibrations have interesting consequences. One of them is the way you get a reflection off the surface of a window, and the way this is strongly polarized at one particular reflection angle because one component of the charge center vibration is arranged along the axis of the reflected beam, where it can contribute no perpendicular field. These charge centers re-radiate light coming out the other side of the window, or reradiate photons, and this light goes faster than the vibration process does.
>Light is fastest through a vacuum, that’s the number typically used for “c”, 186,282 miles/sec, 3x10^8 meters/sec, etc.
The physical constant c is by definition exactly 299,792,458 m/s. The second is defined by a certain atomic resonance, and the definition of the meter then comes from the second and this definition of c. There are no other values for c, except for antiquated values predating this definition.
This constant is more than just the speed of light in a vacuum. It is the limiting speed at which information or mass or energy (which three things are all actually versions of the same underlying agency) can travel. It is also the square root of the proportionality constant between mass and energy, per Einstein’s theory of special relativity.
If you call c “the speed of light”, and then look at the propagation of light through a solid medium, you could argue that c was lower there. However, it is better usage to say that light travels at c in a vacuum and at somewhat lower rates through solids and liquids. Said this way, it’s correct to think the speed of light is only a constant within one particular medium (and at one temperature and pressure), though of course “c” is always absolutely a constant.
The 1 foot per nanosecond (quoted upstream by LSLGuy) is a nice easy to remember number. Admiral Grace Hopper used to keep a 1 foot piece of wire in her office. When someone would ask what it was, she would say it was a nanosecond because that’s how long it took electricity to travel down that length of wire.
My understanding is that she used her nanoseconds to explain the latency in satellite communications.
One of the prototype products at my company is having a problem with its firmware. It a timing issue of some kind. Anyway, it’s a difference of around 25 picoseconds that is killing us. A picosecond is one trillionth of a second.
I was told, and I’m willing to be corrected, that physicists redefined the speed of light to be exactly 3x10^8 m/s. I suppose because the got tired of all the nasty arithmetic.
The did this by subtly redefining the length of the meter so 300 million of them fit precisely into a second’s worth of travel.
No?
Agreed, but I was directing the comment at the OP’s using 18,000 feet as a benchmark. Seems unlikely that a distance of that type would crop up in microchip design so I assumed he’s interested in some type of transmission over distance. In broadcast that type of distance would be negligible.