Electric potential is similar to gravitational potential. When an electric charge moves from a higher electric potential to a lower one, it cause a flow of charge, an electric current. An electric potential difference is therefore needed to cause an electric current. In electrical circuits the symbol represents a resistor. Its unit, the ohm, is given the symbol W , the Greek letter omega .
Resistance is the ratio of voltage to current.
The resistance of any material depends not only on its atomic or molecular structure but on its physical dimensions. For a piece of wire, for example, its resistance depends on the metal, its length, and its cross-sectional area. The longer the wire, the more resistance to flow of current, so the current is reduced. On the other hand the larger the cross-sectional area, the larger the current flow and the resistance is reduced. The symbol r is called the resistivity of the material.