4.1.3 - electron drift velocity

what are conduction electrons

• delocalised electrons

• when conducting electricity they drift slowly along the wire

what is the movement of metals through a metal like

• random when there is no potential difference across the ends of the wire

• therefore no overall net movement of the electrons

what does adding a potential difference across the ends of a wire cause

• cause current to flow in the wire

• net movement of electrons in one direction (random motion still present)

• from negative potential to positive potential

what do you need to calculate the mean drift velocity of electrons

• number of free electrons per unit volume in m^-3 (n) - can be calculated for the density of copper and its atomic mass

• current flowing through conductor in amps (I)

• length of conductor in meters (L)

• cress sectional area of wire in m² (A)

• charge on the electron - 1.602×10^-19 in coulombs (e)

• the mean drift velocity of the electron in ms^-1 (v)

• volume of wire section = cross sectional area x length - V=AL

• number of free electrons available in this area - nAL

• the total amount of charge available to move in theis volume - nALe

• time = distance/velocity the time taken for all the changes to travel length (L) will be L/v

• rate of flow of charge will be amount of charge in volume of wire/time taken for charge to travel the length (L) - nALe/(L/v) which simplifies to nAev

• rate of flow of charge is current so end up with - I=nAev

why are metals good conductors

contain very large number of free conduction electrons per unit volume (large value of n)

why are insulators bed conductors

few or no conduction electrons (low value of n)

what are semiconductors

• neither very good nor very poor in their conducting properties

• intermediate value of conduction electrons

how does the thickness of wire effect the mean drift velocity of electrons

• wires have same current

• thick wire

  • big n, big A

  • v=I/nAe

• thin wire

  • big n, little A

  • v=I/nAe

  • if A smaller V is higher (number the current is divided by will be less)

how does a semiconductor effect the drift velocity of a section of wire

• thin wire

  • little n, little A

  • v=I/nAe

  • if A is small and v is small v will be even higher (current is divided by even less)