26.2: Drift Velocity
The high speed of electrical signals results from the fact that the force between charges acts rapidly at a distance. Thus, when a free charge is forced into a wire, the incoming charge pushes other charges ahead due to the repulsive force between like charges. These moving charges move the charges farther down the line. The density of charge in a system cannot easily be increased, so the signal is passed on rapidly. The resulting electrical shock wave moves through the system at nearly the speed of light. To be precise, this fast-moving signal, or shock wave, is a rapidly propagating change in the electrical field. Good conductors have large numbers of free charges. In metals, the free charges are free electrons. The distance that an individual electron can move between collisions with atoms or other electrons is quite small. The electron paths thus appear nearly random, like the motion of atoms in a gas. But there is an electrical field in the conductor that causes the electrons to drift in the opposite direction (opposite to the field, since they are negatively charged). The average velocity of the free charges is known as the drift velocity and is represented in meters per second. Drift velocity is quite small, in the order of 10−4 m/s, since there are so many free charges. Free-electron collisions transfer energy to the atoms of the conductor. The electrical field does work in moving the electrons over a distance, but that work does not increase the kinetic energy of the electrons.
The charges of the moving particles may be positive or negative depending on the type of material. In metals, the moving charges are negative (electrons), while in an ionized gas (plasma), the moving charges may include both electrons and positively charged ions. In the case of a semiconductor like silicon, conduction is partly by electrons and holes. Holes are some vacancy sites of missing electrons, which act like positive charges. The conventional current is treated as a flow of positive charges, regardless of whether the free charges in the conductor are positive, negative, or both; whereas, in a metallic conductor, the moving charges are electrons, but the current still points in the direction of positive charges would flow.