__Mean drift velocity__ is the average velocity of electrons as they move through a wire. Its symbol is v and its unit is ms^{-1}. Charge carriers actually move slowly as free electrons repeatedly collide with the positive metal ions as they drift through the wire towards the positive terminal; the lights turn on quickly because all the free electrons in the wire start moving at once.

In this equation, I is the current, A is the cross-sectional area of the conductor in m^{2}, e is the elementary charge (1.6 × 10^{−19}), n is the number density of charge carriers (i.e. number of free electrons per m^{3}), and v is the mean drift velocity of the charge carriers.

The __number density__ determines the number of free electrons per m^{3} and therefore how good the electrical conductor is. Conductors have a very high number density (order of 10^{28}m^{-3}), insulators have a much lower value and semiconductors have a value between the two (order of 10^{17}m^{-3}).