While browsing the net looking for trivia questions, I came across a little gem. Electricity travels at the speed of light. The question gave an example of a switch being turned on in New York and the lights going on in LA in something like 0.001 seconds using the speed of light as a velocity.
If electricity is the movement of electrons and no matter can travel at the speed of light then how can electricity travel at the speed of light?
The electric and magnetic fields travels at the speed of light within the material.
Which turns out to be about 8 or 9 inches (20 to 23 cm) per nanosecond for most of the materials used in conductors.
Electrons themselves do not travel the length of the wire. They pass energy between themselves down the length instead.
Someone explained this to me as being a bit like having a tube full of marbles; you push one in at one end and one pops out at the other, but not the same one, with the speed of transmission (in the case of the analogy) being limited to the speed of sound through glass.
The electrons in a wire actually travel quite slowly. This is referred to as the drift speed of the electrons, and is typically only a few centimeters per second.
The energy-carrying electromagnetic field that is exists within and surrounding the wire moves at something like 1/3 to 2/3 of the speed of light in a vacuum, depending on the conductor in question.
I suppose I should mention that the speed of light in a vacuum is about a foot (~30 cm) per nanosecond.
But they do move.
1 amp = 1 Coulomb traveling past a point in 1 seconds.
They question remains, how fast are they moving?
It’s already been anwered by robby. They typically move at a few cm per second; the exact velocity depends on the current.
Remember that c (186,282 MPS/not quite 300,000,000 CPS) is the speed of light in a vacuum – energy typically travels through a medium at a significantly slower (though still extremely high) speed.
Or think of a whip; you jerk the handle and the energy (mechanical, in this case) transits down the length until imparted by the tip.
Direct current (DC) has an electron current or “drift speed”, as robby indicates, but it doesn’t even come close to equalling the transmission speed of electrical energy. The energy is in the form of electromagnetic waves that are actually mostly outside the wire. The metal lattice of the wire simply acts as a highly conductive medium. With alternating current (AC) the net electron current is actually zero (or very close to it) with the electrons just jumping up and down like kids in the mosh pit.
Stranger
Not conductors, but the insulators that surround the conductors. The electic and magnetic fields which propagate, do so in the insulators which separate the conductors. For example, with a coaxial cable, the fields are contained within the cable, so it’s nice, well-behaved, and the speed is dependent on the particulars of the mushy stuff in-between.
For wires, the field spills over into the surrounding space, but it’s still the properties of the surrounding material that determines propagation speed, not the conductors.
Right. My terminology was a little sloppy.
That should be per* hour* It’s sub mm/second. Bill Beaty has written a lot on this:
He works an example of 1 Ampere flowing in a 2mm diameter wire, and that works out to 8.4cm/hour.
To put this into context, if the electron flow were as fast as the end of the minute hand on a wall clock, the wire will go up in flames.
If you want to really understand things electrical, I can’t recommend these articles enough:
Electricity explained
Since we’re discussing the speed of the electrons through a wire, I’ll throw in one of my favorite tidbits. You’ve heard how, for objects travelling fast enough, lengths contract? Well, “fast enough” is usually considered to be up somewhere near the speed of light. But if you put a compass next to a current-carrying wire, you’ll see that the wire produces an easily-measured magnetic field… And that magnetic field is actually due entirely to the relativistic length contraction effects of those electrons moving at a few centimeters per hour.
Do I have this right then,
- The electrons aren’t moving very fast in DC and hardly at all in AC.
- The energy in AC is tranmitted around the conductor in electromagnetic waves travelling at the speed of light.
- The electrons in AC are moving up and down, not back and forth.
- The answer to the trivia question is the OP is true. Electricity travels at the speed of light.
In an electric circuit connected to a voltage one end is positive with respect to the other end. The electrons move toward the positive end. In alternating current circuits the ends alternate in being positive so the electrons move first one way and then the other always going toward the positive end of the circuit.
Yes but electricity doesn’t always move at c, c being the velocity of light in a vacuum. Sometimes the velocity of light is less than c, depending upon the characteristics of the medium in which the light, or other electromagnetic wave, is traveling.
[ol][li]True.[/li][li]True for AC and DC. The conductor is just a way to keep the waves “anchored” in a sense.[/li][li]“Up and down” versus “back and forth” is a bit of a misnomer. (I fear I contributed to this with my analogy.) The electrons jump back and forth between positions in the metal lattice of the conductor but there’s no net travel of electrons down the wire. [/li][li]The answer to the OP is quasi-true, that is to say, “It depends on what the definitiion of ‘is’ is.” EM waves will certainly travel at the speed of light in the medium in which they’re moving. However, I suspect you’ll see additional (if slight) lag, and a thundering herd of impedence from trying to power a light bulb on a circuit that stretches 3000 miles. Seriously, are electric rates that out of control in California? ;)[/ol][/li]
Stranger
And the electrons move at the same speed in DC and AC, given the same wires and amount of current. It’s just that in AC, they hardly get a chance to move any distance at all before they have to turn around and go back the other way. So a given electron in, say, the cord of your lamp might never move more than a micron away from where it starts off.
[QUOTE=Stranger On A TrainThe answer to the OP is quasi-true, that is to say, “It depends on what the definitiion of ‘is’ is.” EM waves will certainly travel at the speed of light in the medium in which they’re moving. However, I suspect you’ll see additional (if slight) lag, and a thundering herd of impedence from trying to power a light bulb on a circuit that stretches 3000 miles.
Stranger
[/QUOTE]
Now I’m a little confused. “Impedance” is a circuit concept and doesn’t appear in EM theory that I’m aware of, or does it?