To better understand what resistance is, you must first get an idea of how electrons flow. When an electron is knocked out of an atom, it will fly off and hit another atom. If the electron strikes the atom with enough force, it will knock off another electron. The atom that was just knocked off will hit another atom and so forth. Note that every time an electron strikes another, it is transferring its energy. Some of the energy is converted into heat every time it is transferred. The voltage will drop as the energy is transferred over long distances. Thus a long wire has a higher resistance than a short wire. Some materials - such as copper and silver - does not hold on to its electrons very tightly. Therefore it doesn't require much energy to knock off an electron. These materials are called conductors and has a very low resistance to electron flow. Materials such as clay and plastics hold on to their electrons more tightly than conductors. It takes more energy to knock off an electron from these materials. These materials are called insulators and has a high resistance to electron flow. Now, you must understand that this is NOT how electrons really flow; It serves only as something for you to work with. To really know how electrons flow, which we will not get into, you will need to study quantum physics. Resistance is represented by the letter R. The basic unit of measure is ohm or the symbol (Greek omega). In the next section (Ohm's Law), we will show you the relationship between Current, Voltage, and Resistance. Resistance will also be further discussed as we introduce the resistor.