Current and charge

Question 1

Millikan’s experiment - the discovery of the quantisation of charge

Answer 1

In 1909 Robert Millikan, helped by Harvey Fletcher, carried out one of the most important physics experiments of the 20th century, now simply called ‘the oil-drop experiment’. He analysed the motion of electrically charged oil droplets between two oppositely charged parallel plates. Oil droplets falling through the air experienced gravitational force, air resistance and upthrust.

Question 2

Classification of materials

Answer 2

The number density is the number of free electrons per cubic metre
of material. The higher the number density, the greater the number of free electrons perm? and so the better the electrical conductor.

Conductors have a very high number density (of the order of 1028 m-3)

semiconductor is in between insulators and consudcters having around 10^17 free electrons per cumic meter

Question 3

drift velocity equation

Answer 3

I = nAvq

I = nAve

where I is the electric current in the conductor in amperes, A is the cross-sectional area of the conductor in m2, e is the elementary charge
(1.60 × 10-19 C), n si the number density, and vsi the mean drift velocity of the charge carriers in ms-)