Alternating Quantity: Any quantity that changes its magnitude with respect to time is called alternating quantity.In electrical engineering it is known as alternating voltage or current and is defined as below "If the polarity of the voltage changes with time, it is known as alternating voltage and the current caused by such a voltage is called an alternating current." Consider the following figure. The above figure explains that voltage or current increases with time to a certain specific value which is called peak or maximum value and after that it decreases even to reach starting position i.e 0 in our case. As time goes on the quantity reverses its direction and increases to again that maximum value which it reached out in the positive half cycle but with negative sign. After that it again follow decreasing and finally reaches the starting position again.This is the way that alternating quantity repeat itself again and again. Mathematically, v= V m Sin