What is a short sequence of characters that appears at the end of a filename and is preceded by a period called?

The short sequences of characters that appear and can be seen at the end of a filename preceded by a period is filename extensions. While a sequence structure is a set of statements that execute in the order they appear. 

the most common energy conversion is from kinetic energy to potential energy, and then from potential energy to kinetic energy.

later on comes the heat energy, mechanical energy, and other energy conversions.

this energy conversion is so common that we can find it in our day to day lives as well.

- one of the most common energy conversions involves the changing of potential energy to kinetic energy or kinetic energy to potential energy