Electrical Engineering ⇒ Topic : Relation Between Current and Drift Velocity
Relation Between Current and Drift Velocity
Consider a portion of a copper wire through which current I is flowing as shown in Fig.(a).Clearly, copper wire is under the influence of electric field.
Let A = area of X-section of the wire
n = electron density, i.e., number of free electrons per unit volume
e = charge on each electron
Vd = drift velocity of free electrons
In one second, all those free electrons within a distance vd to the right of cross-section at P (i.e., in a volume Avd) will flow through the cross-section at P as shown in Fig.(a). This volume contains n Avd electrons and, hence, a charge (nAvd)e . Therefore, a charge of neAvd per second passes the crosssection at P.
I = n e A vd.
Since A, n and e are constant, I α Vd
Hence, current flowing through a conductor is directly proportional to the drift velocity of free electrons.
The drift velocity of free electrons is very small. Since the number of free electrons in a metallic conductor is very large, even small drift velocity of free electrons gives rise to sufficient current.
The current density J is defined as current per unit area and is given by
The SI unit of current density is amperes/m2
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