Define electric current when there is no steady flow of charge.

(N/A) When the flow of charge is not steady,the electric current at any instant $t$ is defined as the rate of flow of charge through a cross-section of the conductor at that specific instant.
Mathematically,it is expressed as the limit of the ratio of the small amount of charge $\Delta q$ passing through a cross-section in a small time interval $\Delta t$ as $\Delta t$ approaches zero:
$I(t) = \lim_{\Delta t \to 0} \frac{\Delta q}{\Delta t} = \frac{dq}{dt}$
Here,$I(t)$ represents the instantaneous current,and $\frac{dq}{dt}$ is the time derivative of the charge $q$.