It is that current whose direction changes with time. in order tousreverses continuously. The end A, of the resistor is earthed i.e. at zero and the voltage of end B, varies with time in a way represented in the fiegure(b) the electric current in the resisor R, then flows as shown in the figure, the shape of the graph remains the sameduring the time interval 0, to T & T to 2T etc. The interval of time after which the value of electric current repeats itselfis calledtime period T and the electric current is said to have completed one cycle. During the time interval 0 to T/2, the current increases from 0 to certain maximum value and then again falls to zero. Simmilarly during time interval T/2 to T, the electric current varries in the same manner but in opposite direction. Such a flow of current is called Alternating Current.