Acurrent exists whenever electric charges move. if δq is the net charge that passes through a surface during a time period δt, then the average current during this time interval is defined asaverage current = δqδt = q2 − q1t2 − t1.if we take the limit of this average current over smaller and smaller time intervals, we get what is called the current i at a given time t1: i = lim δt→0 δqδt = dqdt. thus the current is the rate at which charge flows through a surface. the current in a wire is defined as the derivative of the charge: i(t) = q'(t). what does bi(t)a dt represent?