Question

What causes the polarity of the voltage to be reversed after some time in an AC source?

Answer 1

You can use electrical power to perform physical work, to transmit data signals from one point to another, or to convert it into other energy forms such as heat and light. The two basic types of electrical power are direct current and alternating current. Direct current, or DC, flows in one direction only and maintains the same polarity. Alternating current, or AC, reverses polarity at a specified period. This polarity switch is a product of the AC power generation process.

Answer 2

The generation of AC current is such that it corresponds to a sine or cosine wave, though not necessarily always. The reason is simply because the method to generate AC current is periodic in nature, i.e it repeats itself after certain amount of time.