Current and Signal Velocity

Part 1: Fundamental Concepts: Current and Signal Velocity

In the previous section, we discussed the concept that the current in a circuit is due to the electrons (or holes) moving through the circuit, and the voltage present in the circuit provides the force that causes them to move.  One question that might come to mind may be, how fast do they move?  The answer might surprise you.

From first hand experience, you get the impression that current moves very quickly.  When you flip on a light switch, the light bulb seems to respond immediately.  When you watch a television program, you don’t adjust the time you expect to watch a program based on how far away you are from the cable company.  The signal that moves through a wire, moves at very close to the speed of light.  Without getting into too many details, the speed of light in a vacuum, well away from other influence (such as gravity), is a constant.  When light passes through a material, such as glass, it slows down.  Our electrical signal, the TV program for instance, is not traveling in a vacuum (it is typically in copper wire) so it also slows down a bit.  In straight, solid copper wire that is not particularly close to anything else, the signal travels at about 96% of the speed of light.  In a coaxial cable of the sort connected to most televisions, the signal travels at about 70% of the speed of light.  The speed of light is very fast.  It travels at about 300 million meters per second or roughly 670 million miles per hour.

Did you notice how things were carefully worded in the previous paragraph to use “signal” in lieu of electron speed?  Let’s return to our analogy of electric current as water flowing in a hose.  Assuming our hose is full of water, when you turn on the faucet water begins to flow out of the hose immediately.   There is no delay in the water coming out of the hose when the faucet is turned on.   When the faucet is turned on, the water that comes out of the end of the hose is not the same water that went into the hose.  The water that came out of the hose did so because the water that came in at the faucet end pushed the water in the hose forward.  This caused water to come out at the end of the hose.  You experience this most days as you wait for the hot water to get to your shower.  When you turn on the faucet, water comes out immediately and, after a while, the water from the hot water tank comes out at the shower head.

The situation with electrical current is much the same.  The wire conductor that carries our current is “full” of electrons.  When a potential difference (a voltage) is applied to the wire, all of the electrons begin to move at roughly the same time.  This is not an instantaneous effect — it takes a small amount of time for the force from the voltage to be felt, and the further away the electron is away from the voltage, the longer it takes for the force to be felt.  The time it takes from applying the potential to the end of the wire and getting current flow out is referred to as the signal propagation delay, which is near the speed of light.  How long does it take for the electrons entering one end of the wire to come out the other end?  This depends on the cross sectional area of the wire, how big around it is, and the magnitude of the current flow.

Let’s assume that there is 0.833 amps flowing in a 12 gauge wire (this the amount of current it takes to light a 100W light bulb at 120V using wire which is standard in most homes US homes).  The average drift velocity, or the average speed of an electron, under these conditions is 2.6 inches per hour (or 6.65 cm per hour).  Electrons moving in electrical circuits do so very slowly.  It is comparable to watching an hour hand move on a clock.  If you were able to observe an individual electron, you would see that it not only moves very slowly, but it also does not move in a straight line and, on occasion, it moves in the opposite direction of the current flow.  This happens because there are other factors, such as heat,  that influences the electron’s motion.

In the next chapter, we will discuss the difference between AC (alternating current or a current that periodically changes polarity) and DC (direct current which does not change polarity).  The power supplied to your home is AC, which means in this case, the electron is changing direction 50 or 60 times a second depending on the country you live in.  Given how slowly they move, this means that the electrons are wiggling back and forth in the wire and are not really going anywhere.

Although it is of no value whatsoever in understanding how a circuit works, it is interesting to consider the mass of the current flow.  It is a somewhat unusual way to look at things, but when an electrical current flows through a wire there is a movement of material (the electrons) and, therefore, there is mass moving in a circuit, albeit a very small one (about 5 nano grams for the 0.883A current discussed above).  The distribution of the mass in a conductor does not change, however.  The same number of electrons exist in the conductor whether or not current is flowing.  The difference is they are moving when current is flowing, and they are essentially stationary when it isn’t. 

In summary, signals move very fast and the electrons move very slowly.  When current flows, mass is moving.  Electronics is interesting stuff.