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]
