What is the scientific basis of the separation achieved by electrophoresis?

The movement of charged particles in an electric field

Gel electrophoresis is used to sort DNA strands by size. DNA solution is placed in small well at one end. On the opposite end, is the positive terminal. A dye or indicator is added to the DNA, and after placing the DNA in the wells, the terminal is switched on. As DNA is slightly negatively charged, it moves through the gel towards the positive end. The gel acts as a buffer, which means that larger strands move slower than small ones. After the time is up, the DNA strands will be sorted by size, the smallest being the closest to the terminal, and the largest being the furthest away. The dye allows the DNA strands to be visible to us.