Resistance is the capacity of materials to impede the flow of current, or more specifically, the flow of electrical charge. The circuit element used to model this behavior is the resistor. The following figure shows the circuit symbol for the resistor, with R denoting the resistance value of the resistor.
Conceptually, resistance can be understood by thinking about the moving electrons that make up electric current interacting with and being resisted by the atomic structure of the material through which they are moving. In the course of interactions, some amount of electric energy is converted to thermal energy and dissipated in the form of heat. This effect may be undesireable. However, many useful electrical devices take advantage of resistance heating, including stoves, toasters, irons, and space heaters.
Most materials exhibit measurable resistance to current. The amount of resistance depends on the material. Metals such as copper and aluminum have small values of resistance, making them good choices for wiring used to conduct electric current. In fact, when represented in a circuit diagram, copper or aluminum wiring isn't usually modeled as a resistor; the resistance of the wire is so small compared to the resistance of other elements in the circuit that the wiring resistance can be neglected to simplify the diagram.
Every material offers some resistance, or opposition, to the flow of electric current through it. Good conductors, such as copper, silver, and aluminum, offer very little resistance. Poor conductors, or insulators, such as glass, wood and paper, offer a high resistance to current flow.
The size and type of material of the wires in an electric circuit are chosen so as to keep the electrical resistance as low as possible.
In the electric circuit, the larger the diameter of the wires, the lower will be their electrical resistance (opposition) to the flow of current through them. The electrical resistance of the conductors depends upon the length of the wires, the diameter of the wires, and the material of the wires.
Temperature also affects the resistance of electrical conductors to some extent. In most conductors (copper, aluminum, iron, etc.) the resistance increases with temperature. Carbon is an exception. In carbon the resistance decreases as temperature increases. Certain alloys of metals (manganin and constantan) have resistance that does not change appreciably with temperature.