1. Consider a conductor with cross sectional area A. Assume that the two ends of the conductor are connected to a battery to make the current flow through it.
2. Let ‘vd ‘ be the drift speed of the charges and ‘n’ be the number of charges present in the conductor in a unit volume.
3. The distance covered by each charge in one second is ‘vd‘
4. Then the volume of the conductor for this distance = Avd
∴ The number of charges contained in that volume = n.Avd
5. Let q be the charge of each carrier.
6. Then the total charge crossing the cross sectional area at position D in one second is ‘n q Ad ‘. This is equal to electric current.
∴ Electric current I = n q Avd.
∴ v = 1/nqA
