Elektronin v Aineessa?

Elektronien terminen nopeus huoneenlämmössä on noin 10^7cm/s, mutta koska liike on satunnaista ja siroamisia tapahtuu usein, niin keskimääräiseksi nopeudeksi tulee 0

Elektronien nopeus:

(lainaus alkaa)

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 x 10^ 22
* Charge per electron: e = 1.6 x 10^-19

The equation:


________I_______ = .0023 cm/sec = 8.4 cm/hour
Q x e x R^2 x 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 x 10^ 22 electrons/cc x 1.6 x 10^-19 coul./electron = 13600 Coul./cc

Therefore one coulomb would form a cube approximately 0.4mm across...

1/(13600cc^(1/3)) = 0.042 cm

HA! 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 in electric circuitry, if we deliver one salt-grain per second (one amp,) that's a very slow flow. In 16-gauge wire the saltgrain blobs would resemble very thin stacked pancakes. In 30-gauge wire the saltgrains would be almost undistorted, and charges would move at about 0.4 mm/sec during a 1-amp current.

(lainaus loppuu)