Electrical Engineering ⇒ Topic : Alternating current (A.C.)

Alternating Voltage and Current

A voltage which changes its polarity at regular intervals of time is called an alternating voltage. When an alternating voltage is applied in a circuit, the current flows first in one direction and then in the opposite direction; the direction of current at any instant depends upon the polarity of the voltage. Fig. A. shows an alternating voltage source connected to a resistor R. In Fig. A  the upper terminal of alternating voltage source is positive and lower terminal negative so that current flows in the circuit as shown in Fig. A.

   After some time (a fraction of a second), the polarities of   the voltage source are reversed in figure B so that current now flows in the opposite direction. This is called alternating current because the  current flows in alternate directions in the circuit.                                                                                        

An alternating current or voltage is one which changes its magnitude and direction regularly with the time. The wave shapes of such a current are shown in figures