Version en Español SPEED OF "ELECTRICITY"

1996 Bill Beaty How fast does electricity flow? Well, it depends on what you mean by "electricity." The word Electricity has more than one contradictory meaning, so before we can talk about its flow, we have to decide on which of several "electricities" we really mean. For a discussion of electric current, see below. But for articles about fast-flowing electromagnetic energy, see the FAQ, this energy article, or an old email.

OK, then how about this. When we turn on a flashlight, something called an "electric current" begins to happen. Inside the flashlight bulb, the thin filament-wire gets hot because there is electric current in the metal. This current is a motion of something. How fast does this "something" move? This question can be answered.

The quick answer Inside the wires, the "something" moves very, very slowly, almost as slowly as the minute hand on a clock. Electric current is like slowly flowing water inside a hose. Very slow, so perhaps a flow of syrup. Even maple syrup moves too fast, so that's not a good analogy. Electric charges typically flow as slowly as a river of warm putty. And in AC circuits, the moving charges don't move forward at all, instead they sit in one place and vibrate. Energy can only flow rapidly in an electric circuit because metals are already filled with this "putty." If we push on one end of a column of putty, the far end moves almost instantly. Energy flows fast, yet an electric current is a very slow flow.

The complicated answer Within all metals there is a substance which can move. This stuff has several different names: the Sea of Charge, or the Electron Sea, or the Electron Gas, or "charge." We often call it "electricity," and state that electric currents are flows of electricity. Calling it "electricity" can be misleading because many people believe that electricity is a form of energy, yet charge is not energy, and currents are not flows of energy. Also it can be misleading because the Sea of Charge exists within in all metal objects, all the time, even when the metal hasn't been made into a wire and is not part of an electric device. If the Electron Sea is "electricity," then we must say that all metals are always full of electricity, and that batteries are simply electricity-pumps. Better to call it by the name "charge-sea," and avoid the misleading word "electricity" entirely.

During an electric current, the metal wire stays still and the sea of charge flows along through it. When the flashlight switch is turned off and the lightbulb goes dark, the charge-sea stops moving forward. Even though it stops moving, the charge-sea is still inside of that wire. If the flashlight is again turned on, but then two light bulbs are connected in parallel instead of one, the electric current will have twice as large a value, and twice as much light will be created. And most important, the charge-sea within the battery's wires will flow twice as fast. In other words, the speed of the charges is proportional to the value of electric current; small current means slow charge-flow, large current means high speed. Zero current means the charges have stopped in place. Note however that an electric current does not have just one speed within any circuit. Charges speed up whenever they flow into a thinner wire. The high current in a large flash-lantern's lightbulb will be much faster than the same current in the other conductors in the lantern. Even though an electric current is a very slow flow of charges, we can't know the actual speed of flow unless first we know the thickness of the wires, as well as the *value* (the amperes) of the current in the wires.

If a thin wire is connected in a circuit end to end with a thick wire, it turns out that the charges in the thin wire move faster. This makes sense: it works just like water in rivers. If a huge wide river moves into a narrow channel, the water speeds up. When the channel opens out again downstream, the river slows down again. The electric current inside a very thin wire will be tend to be fast, even if the value of current is fairly low. This means that we can't know the speed of the flowing charge-sea unless we know how thick the wires are.

If a copper wire is connected into a series circuit with an aluminum wire of the same diameter, the charges in the copper will flow slower. This occurs because there is one movable charge per each atom in the metals, but there are more atoms packed into the copper than into the aluminum, so there is more charge in each bit of copper. When the charge-sea flows into the copper, it gets packed together and slows down. When it flows out into the aluminum, it spreads out a bit and speeds up. This means that we cannot know how fast the charges flow unless we know how dense the charge-sea is within the metal.

The speed of electric current Since nothing visibly moves when the charge-sea flows, we cannot measure the speed of its flow by eye. Instead we do it by making some assumptions and doing a calculation. Let's say we have an electric current in normal lamp-cord connected to bright light bulb. The electric current works out to be a flow of approximatly 3 inches per hour. Very slow!



Here's how I worked out that value. First, I know that: Bulb power: about 100 watts, about 100V at 1A

Value for electric current: I = 1 ampere

Wire diameter: D = 2/10 cm, radius R=.1cm (that's #12 gauge AWG)

Mobile electrons per cc (for copper, if 1 per atom): Q = 8.5*10^+22

Charge per electron: e = 1.6*10^-19

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