The speed of electricity

Electricity travels a the speed of light.

One ampere of current consists of approximately 6,240,000,000,000,000,000 electrons moving past a point in one second. In a 12 gauge wire (the typical size for copper household wiring in the U.S., about 0.75 mm2) that number of free electrons are packed into a length of 1/1000 of an inch (25.4 micrometers). Therefore, it will take 1,000 seconds to move one ampere's worth of electrons a distance of one inch through a 12 gauge wire. That's about 16 minutes to move one inch.  However, the “information” or “signal” travels much faster.

How to make a marble travel faster than light

Let me play magician for a moment. Imagine a one-kilometer-long pipe. I'm going to put a marble into one end. Watch as the marble instantly comes out the other end. I made a marble travel one kilometer in the blink of an eye. Not only faster than a speeding bullet, but faster than light itself. What you don't know is that the pipe is already full of marbles. When I put a marble in one end, another marble is pushed out the other end by the rest of the marbles inside the pipe.

Not quite faster than light

Actually, this doesn't happen faster than the speed of light. Glass may seem solid, but like anything else it has some elasticity. When I push a marble into the pipe, it and the first marble in the pipe deform a tiny bit. It takes a tiny moment for them to recover their original shape. Then the first marble pushes on the second, which continues the process from marble to marble. This results in a pressure wave (a sound wave) that propagates from marble to marble down the pipe. When this pressure wave reaches the last marble, finally all the marbles are in motion. Assuming all the marbles are already in firm contact with each other this pressure wave travels down the line of marbles at about 4,500 meters per second (the speed of sound in glass). Therefore, in this one-kilometer-long pipe, it takes about 1/4 of a second from the time my marble contacts the first marble in the pipe before all the marbles are moving.

The information or signal vs. the medium

The marbles don't travel faster than the speed of light. Neither do they move at the speed of sound. However, the pressure wave that initiates the motion does move at the speed of sound (which is much faster in glass than air). This means that the last marble is induced to move long before the first marble could travel all the way down the pipe by itself and cause is to move. The marbles themselves move very slowly, but the signal—the pressure wave that initiates the movement—moves through the marbles at the speed of sound. In electric circuits you have a similar situation. Electrons move very slowly through circuits. However, when there is a change in voltage or current, that change propagates as a wave through the mass of electrons much faster.

Let's say you have a six-inch-long 12 gauge wire between a switch and a light bulb. Let's also say that the light bulb uses one ampere of current. When you flip the switch the electrons begin moving at 1/1000 inch per second. It will take 96 minutes before the electrons at the switch reach the light bulb. Does it take an hour and a half for the bulb to light? No. The electrons at the bulb begin moving the instant you flip the switch. The electrons don't move very fast at all, but the signal or information, moving as a wave, arrives almost instantly. It's like pushing the marbles in the pipe. The electrons don't travel lightning fast, but the wave initiated when you throw the switch does. All the electrons in the wire begin moving instantly.

Still not the speed of light

Electrical signals still don't travel at the speed of light. Inherent capacitance and inductance limit the speed to anywhere from 50% to 98% of the speed that light travels in a vacuum. Take the 6-inch-long of wire. There is another wire carrying the electricity back to the battery. These two wires are conductors separated by insulators (the wire insulation and the air between them). They constitute a capacitor (see Capacitors above). It's a capacitor of tiny value but nevertheless there is significant capacitance between them. They also have resistance. Therefore we have an RC time constant to consider (see RC Time Constants). The wave that travels from the switch to the light bulb should travel at near the speed of light, but it takes time for the inherent capacitance to charge. This increases the time it takes for the voltage at the light bulb to rise to the battery voltage. A wire, even a straight one, has inductance. This inductance means that the wire will push back on the electric current until the current stabilizes. This further delays the electric wave from reaching the light bulb. Electric signals are very fast, but they don't travel at the speed of light.[1]

The Speed of Electricity

1The highest speeds are attained in open dipole antennas. An open dipole antenna is essentially a single straight wire or rod cut in the middle, making two wires or rods along a straight line. Being straight, the two wires or rods have the least possible inductance. Arranged along a single axis each wire or rod has its mass as far from the other as possible, therefore having the least possible capacitance. Having the lowest possible inductance and capacitance to slow the electrical waves, these waves travel from 95% to 98% of the speed of light in a vacuum. The lowest speeds tend to be in twisted-pair cables (such as computer network cables). The pairs of wires are twisted together putting them as close to each other as possible, giving them the highest possible capacitance. They are twisted, increasing their inductance. Signals travel in network cables at approximately 50% of the speed of light in a vacuum. For more about antennas refer to the class on electronic communications. For more on networking cables see the computer networking course.