In a given time interval t, let q+ be the net amount of positive charge that flows in the forward direction across the area. Similarly, let q– be the net amount of negative charge flowing across the area in the forward direction.
The net amount of charge flowing across the area in the forward direction in the time interval t, then, is q = q+ – (q–). This is proportional to t for steady current and the quotient, I = q/t
I is defined to be the current across the area in the forward direction. (If it turns out to be a negative number, it implies a current in the backward direction.)
we define the current as follows.
Let ∆Q be the net charge flowing across a cross-section of a conductor during the time interval ∆t [i.e., between times t and (t + ∆t)].
Then, the current at time t across the cross-section of the conductor is defined as the value of the ratio of ∆Q to ∆t in the limit of ∆t tending to zero.
In SI units, the unit of current is Ampere.