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

**This is proportional to t for steady current and the quotient, I = q/t***q = q+ – (q–).*
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

**[i.e., between times***∆t***and***t**(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.

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