Alternating current will start like suppose we remove the commentator from a DC generator and make an arrangement so that the arm of the coil is always in electrical contact with the terminal, and the other arm, with the another terminal, then with each half-rotation, as the direction of the current in the coil changes, the direction of the current in the bulb connected across the terminal of the generator will also change. If the coil rotates at a fixed speed, the direction of the current will change after fixed intervals. An electric current that reverse its direction with the time is called an alternating current. The term AC is not confined to current only and is generally used to refer an alternating electrical quantity. Thus we use terms like AC voltage and AC power.
The house hold power supply is AC. Suppose you connect a bulb to an AC source such as the AC supply of your house. What is the potential difference across the bulb? It keeps changing with time. When the current flows from point A to B in the bulb Va>Vb . When the current flows from B to A in the bulb, it will be vice-versa. When the current becomes momentarily zero before changing direction at that time both voltages are equal. In fact, potential difference across the bulb changes gradually from a peak value to zero and then to the minimum value with negative sign. From minimum value it will increase to zero before reaching to the maximum value.
Since the value of potential difference across a device such as a heater or a bulb does not remain constant, what is the effect on the amount of heat produced , this we can easily calculate. It turns out that the heating effect of an AC voltage, whose peak value is vo, is the same as that produced by a constant voltage that will be division of peak value with square root of 2. This value is called RMS voltage. When we say that AC supply is at 220V, we mean its RMS voltage is 220V.