In the Charge And Charge Motion page, we note that current is the result of transferring electrons from atom to atom in a material. In almost all cases, there is some opposition to the motion of electrons in a material. This results in some loss of energy when transferring charges through a material—in general, this energy loss results from conversion of electrical energy to heat 1.
Resistance characterizes the loss of energy associated with passing an electrical current through a conductive element. Units of resistance are volts per ampere, or Ohms, and are represented by the greek symbol Ω. A high resistance corresponds to a large energy loss associated with current passage through a material, while low resistance corresponds to small energy loss associated with current passage through a material. As extreme examples, perfect conductors have zero resistance and perfect insulators have infinite resistance.
Resistors are circuit elements whose characteristics are dominated by this energy loss. The circuit symbol for a resistor is shown in Fig. 1. Resistors are probably the simplest and most commonly used circuit elements. Common uses for resistors are to limit currents to acceptable levels or achieve desired voltage levels in a circuit. Currents may need to be limited for a couple of reasons: