There are four factors that are affecting resistance
- Length of Material
- Cross-sectional Area
- Type of Material
Length of Material
The resistance of a material is directly proportional to its length. The longer the material the more resistance it has.
The resistance of a material is indirectly proportional to its width. The wider or thicker the material is the less resistance it has allowing more free electrons to flow.
Type of Material
The type of material affects the amount of free electrons able to flow through it. A material which is a conductor has less resistance while a material which is an insulator has more resistance.
The temperature of the material affects its resistance. Some materials such as thermocouples and thermistors are design to change their resistance with temperature.
Then the resistance of any material which has a uniform cross-sectional area, A and length, ℓ can be represented in mathematical form as:
Where: r is known as the resistivity of the material in ohm-meters
The circuit element used to model this current resisting behaviour of a material is called a resistor. The resistor is the simplest passive element used in Electrical and Electronic circuits that is they contain no source of power or amplification but only attenuate or reduce the voltage or current signal passing through them. The circuit symbols used to show a resistor in schematic diagrams are given below, where R stands for the resistance of the resistor, in this case 100W’s.
The symbol used in schematic and electrical drawings for a Resistor can either be a “zig-zag” type line or a rectangular box.
A resistor can either be fixed or variable. Most resistors are of the fixed type, meaning their resistance remains
The two most common types of fixed resistors are wire-wound and carbon composition Carbon composition resistors are used when large resistance is needed while wire-wound resistors with their metal finned body are used for very high wattage applications. Variable resistors, called potentio-meters or rheostats can be either linear or logarithmic types having an adjustable resistance value from zero ohms to their maximum resistance.