4.1.2: Mean drift velocity

Number density is the number of free charge carriers per unit volume. The higher the number density the better the electrical conductor. This is because the material has more free charge carriers per unit volume. 

Materials can be classified into insulators, semiconductors, and conductors depending on their number density.

Conductors have a number density of about 10^28 m^-3

Semiconductors have a number density of about 10^17 m^-3

Insulators have a number density of below around 10^17 m^-3

Semiconductors can carry the same current than conductors but the electrons need to move much faster. Computer semiconductors are made of silicon because electrons moving faster means the temperature increases

Mean drift velocity:

Mean drift velocity is the average velocity of electrons/charged particles in a  wire/medium. It can be calculated using the equation I = Anev

I = current

A = cross-sectional area

n = number density

e =elementary charge (1.6x10^-19 C)

v = mean drift velocity

From the above equation we can see that increasing the cross sectional area decreases the mean drift velocity. They are inversely proportional.