mean drift velocity

• mean drift velocity is the average distance travelled by a charge carrier per unit time
• with a current of a few amps , the average or mean drift velocity of electrons will be of the order of 0.3mm per second

  A high current is possible because there are many, closely packed charges
• we can distinguish between classes of electrical conductor from the density charge carriers they contain
• resistivity

  • The ability to carry current increases with an increase in number of electrons
  • the resistivity of a material is directly proportional to number of electrons
• Conductors have a high number density

  Semiconductors have an electron density of about 10^19

  Insulators have a much lower electron density of about 10^9