Direct Current and Alternating Current: You have studied that if positive and negative terminals of a battery are connected to the ends of a conductor, current begins to flow through it and the direction of its flow is from the positive end of the conductor towards its negative end.
Such a current which always flows only in one direction is called direct current or d.c. on contrary to it, there is also a current also whose direction changes again and again. This current is called alternating current or a.c. we will explain alternating current from Fig. 16. 19.
Fig. 16. 19
Suppose a resistance R has been connected with a source of voltage which is such that the voltage of its end a changes with respect to the end B in accordance with the graph shown in Fig. 16.20 here you can see the shape of graph between the time intervals 0-T, T-2Tand 2T-3T is exactly the same. The shape of the graph remains the same is exactly the same? The shape of the graph remains the same after the time interval T.
Fig. 16 .20: the graph shows the variation of voltage with the variation of voltage with time.
This time interval after which the voltage repeats its value is known as time period. Now let us examine the type of current which passes through the resistance R due to such a Voltages. During the time interval 0 to T/2, the potential of the end A is positive with respect to end B, so the current in this interval flows from a to B. it starts from zero, gradually increases to its maximum value and then finally decreases to zero. Hence forth during the interval T/2 to T, the potential of end a gets negative with respect to B, so now the current flows in the opposite direction from B to A. in this interval also, the current starts from zero, gradually increases to the maximum value in the opposite direction and finally decreases to zero (fig 16.21). In this figure positive values of current indicate current flowing from A to B and the negative values from B to A.
Fig. 16. 21: the graph showing variation of current with time T.
After the period 0 – T, the next period starts. Again in this period from time T to 3T/2, the current flows from A to B and from 3T.2 to T from B to A. this continues in subsequent periods. Such a current is known as alternating current or a.c. the set of all the values of current during one period is known as one cycle. The number of cycles which the current completes in one second is known as its frequency. The frequency of a.c. used in our houses is 50 cycles per second.