So far, in previous chapters, we considered dc sources that do not vary with time. In this chapter, we discuss the voltages and currents that vary with time known as alternating voltage. Today, most of the electrical appliances make use of AC voltage including the electric mains supply in our homes and offices. An AC voltage is preferred over a DC voltage because an AC voltage can be efficiently and easily converted from one voltage level to another by means of transformers. And also it can be transmitted over long distances economically. In chapter 7 Alternating current of CBSE class 12 physics, learn about AC voltages in detail and how characteristics exhibited by ac circuits are exploited by devices of daily use.