SPEED OF "ELECTRICITY"
1996 Bill
Beaty
| La versión Español |
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 a flow of syrup. Even maple syrup moves too fast, so that's not a good analogy. Electric charges flow as slowly as a river of warm putty. And in AC circuits, it doesn't move forward at all, instead it sits in one place and vibrates. Energy can flow fast in an electric circuit because metals are already filled with this "putty." If you 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." Calling it "electricity" can be misleading because charge is not energy, yet many people think that electrical energy is the "electricity." It can be misleading because the Sea of Charge exists within in all metal objects, all the time, even when the metal has not 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 full of electricity. Better to call it by the name "charge-sea," and avoid the misleading word "electricity".
During an electric current, the 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 and 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 of 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 low-speed charge flow, large current means high speed.
Zero current means the charges have stopped. Note however that an
electric current does not have just one speed. Charges speed up when they
flow into a thinner wire. The high current in the lightbulb of a big
flash-lantern will
be much faster than the same current in the 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 *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 flow in 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. I know:
- 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
- Mobile electrons per cc (for copper, if 1 per atom): Q = 8.5*10^+22
- Charge per electron: e = 1.6*10^-19
The equation:
cm/sec = ________I_______ = .0023 cm/sec = 8.4 cm/hour
Q * e * R^2 * pi
This is for DC. Chris R. points out that for a particular value of
frequency of AC, the "skin effect" can cause the flow of charges in
the center of a wire to be reduced while the current on the surface
becomes
stronger. There are fewer charges flowing, and hence they must flow
faster. ("Skin Effect" is stronger at high frequencies and with thick
wires. The effect can USUALLY be ignored in thin wires at 60Hz
power-line frequencies.)
The size of the wiggle
And about AC... how far do the electrons move as they vibrate back and forth? Well, we know that a one-amp current in 1mm wire is moving at 8.4cm per hour, so in one second it moves:
8.4cm / 3600sec = .00233 cm per second
And in 1/60 of a second it will travel back and forth by
.00233cm/sec * (1/60) = .0000389cm, or around .00002"
This simple calculation is for a square wave. For a sine wave we'd
integrate the velocity to determine the width of electron travel.
So for a typical AC current in a typical lamp cord, the electrons don't
actually "flow," instead they vibrate back and forth by about a
hundred-thousandth of an inch.
The width of one Coulomb
On thinking along these lines I notice something interesting: in copper, one coulomb of movable electrons has a certain size! There are about 13,000 coulombs of free electrons per cubic centimeter of copper.8.5*10^+22 elect/cc * 1.6*10^-19 coul./elect = 13600 Coul./ccTherefore one coulomb would form a cube approximately 0.4mm across...
1/(13600cc^(1/3)) = 0.042 cmHA! A coulomb in copper is about the size of a grain of sand! We can now discuss electric current within wires as if it were cc per second of fluid flow inside of small hoses. If an Ampere is one coulomb per second, we're REALLY saying that an Ampere is "one saltgrain-sized blob, moving each second, squeezing itself into whatever sized wire." So, for the usual sizes of wires used in electric circuitry, if we deliver one salt-grain per second (one amp,) that's a very slow flow. The tiny saltgrains are going by: bip, bip, bip, once per second.. In 16-gauge wire the saltgrain blobs would be morphed to fill the cross-section, so they would resemble very thin stacked pancakes. In 30-gauge wire the saltgrains would be almost undistorted, and so the charges would move at about 0.4 mm/sec during a 1-amp current.
One thing's not certain in the above calculations: the charge density for copper. My above value for Q assumes that each copper atom donates a single movable electron. The email from the person below points out that this might not be true. For example, if only one in ten conduction electrons are movable, while the rest are "compensated" and frozen, then the speed of the charge flow will be ten times greater than 8.4cm/hour.
One final point. Electrons in metals do not hold still. They wiggle around constantly even when there is zero electric current. However, this movement is not really a flow, it is more like a vibration, or like a high-speed wandering movement. How should we picture this? Well, remember that we can speak of moving wind and flowing water as if they had a genuine velocity... yet a similar type of rapid wandering motion is found in the atoms of all normal liquids and gases. Even when the wind is less than one MPH, the air molecules are zooming around at hundreds of MPH. Even when there is no wind at all, the air molecules still wiggle around at the same high speeds. We usually ignore this when discussing wind, and instead take the average velocity of all molecules in a certain small volume. We call it "thermal vibration," and we see the fast movements as a separate issue. Therefore we should do the same with circuitry: the electric current is akin to wind, while the high speed wandering motions of individual electrons is akin to thermal vibrations of the air. In the above article I concentrate on the slow "electron wind" which is measured by electric current meters, and I ignore the electrons' high speed "thermal vibration."
LINKS
- Copper wire at microscopic scale
- "Drift Velocity"
- Unifying electrostatics and electric circuits Chabay & Sherwood 1999 (.PDF)
I've seen one way to directly measure the drift velocity of charges in a conductor. Connect metal electrodes to the ends of a large salt crystal (NaCl), then heat it to 700 degrees C and apply high voltage to the electrodes. At this temperature the salt becomes conductive, but as electrons flow through it they discolor the crystal, and a wave of darkness moves across the clear crystal. The velocity of this slow-moving wave can be measured. (And if you double the current, the speed of the wave doubles.) This demonstration appears in:
Physics Demonstration Experiments (two volumes)
H. F. Meiners, ed. Ronald Press Co 1970
========================================================================== Date: Tue, 17 Oct 95 09:53:00 PDT From: O. Quist Subject: Re: your mail On Fri. 13 Oct 1995 Bill Beaty Wrote: > Very interesting! All the sources I've encountered state that each atom > in a conductor contributes one (or two?) electrons to the conduction > band. Might you know a rough figure for the actual number of > electrons/atom in a copper lattice? How much smaller is it than 1.0? The number of electrons in the conduction band is indeed as you say. But, that is not what I was saying (below). The actual number of electrons which contribute to the electrical current is not equal to the number of electrons in the conduction band. The electrons which contribute to electrical conduction are those electrons within the Fermi Surface which are "uncompensated." From symmetry, these electrons lie on, or near the surface, and result as the Fermi Surface is "shifted" by the electric field. The fraction of electrons that remain uncompensated is approximately given by the ratio (drift velocity)/(Fermi velocity). The result is the number of electrons which produce an observed current being considerably less than Avagadro's number. The number of electrons producing current being thus reduced, produces an increase in their average velocity. Average electron velocities are more probably in the meters/sec range rather than the 10ths of a millimeter/sec as is predicted by the free-electron theory. ======================================================================== Date: Tue, 16 Jun 1998 00:31:01 -0500 From: Roy M. To: William Beaty <> Subject: Re: Electron drift velocity in metals Newsgroups: sci.physics.electromag Its a minor point, but, drift velocity is an average. If some of those conduction electrons are "stuck", they still contribute to the average. If you want to exclude the slowest 99% then the average of those you do allow will be higher. But, its probably an unnecessary refinement in this context, which is to treat electrons like classical particles and calculate average drift velocities. Anyway, the effect of which you refer involves the fermi theory, Pauli exclusion and conservation of energy. In effect fewer electrons participate in conduction, but their mean free path is longer. The explanation is something like: no more than two (with opposite spins) electrons can occupy a given state. When two electrons collide, their final states must have the same total energy and the final states must have been vacant. Thus, if all the states which can be reached at a given energy level are already filled, then the two electrons cannot collide. Net result is that electrons in low energy states are "stuck" in those states. So only the relatively few electrons in high energy states are really available to participate, but most of the other electrons are not available to collide with the high energy electrons so that those electrons that do participate go futher (mean free path) than you might expect. ======================================================================== Subject: Re: Electron drift velocity in metals Newsgroups: sci.physics.electromag From: