Editing Electric current (section)

==Drift speed==
The mobile charged particles within a conductor move constantly in random directions, like the particles of a [[gas]].  (More accurately, a [[Fermi gas]].) To create a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in most [[metal]]s and they follow an erratic path, bouncing from atom to atom, but generally drifting in the opposite direction of the electric field. The speed they drift at can be calculated from the equation:
<math display=block>I=nAvQ \, ,</math>
where
* <math>I</math> is the electric current
* <math>n</math> is number of charged particles per unit volume (or charge carrier density)
* <math>A</math> is the cross-sectional area of the conductor
* <math>v</math> is the [[drift velocity]], and
* <math>Q</math> is the charge on each particle.
Typically, electric charges in solids flow slowly.  For example, in a [[copper]] wire of cross-section 0.5&nbsp;mm<sup>2</sup>, carrying a current of 5&nbsp;A, the [[drift velocity]] of the electrons is on the order of a millimetre per second. To take a different example, in the near-vacuum inside a [[cathode-ray tube]], the electrons travel in near-straight lines at about a tenth of the [[speed of light]].

Any accelerating electric charge, and therefore any changing electric current, gives rise to an [[Electromagnetism|electromagnetic]] wave that propagates at very high speed outside the surface of the conductor.  This speed is usually a significant fraction of the speed of light, as can be deduced from [[Maxwell's equations]], and is therefore many times faster than the drift velocity of the electrons. For example, in [[electric power transmission|AC power lines]], the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant [[external electric load|load]], even though the electrons in the wires only move back and forth over a tiny distance.

The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the [[velocity factor]], and depends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on their shape and size.

The magnitudes (not the natures) of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases. (See also [[hydraulic analogy]].)
* The low drift velocity of charge carriers is analogous to air motion; in other words, winds.
* The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas (sound waves move through air much faster than large-scale motions such as [[convection]])
* The random motion of charges is analogous to heat{{snd}}the thermal velocity of randomly vibrating gas particles.