RESISTIVITY OF METAL

Resistivity of Metal

Resistivity of Metal

Introduction

The opposition to the flow of electric current in an electrical circuit is called Resistance. However, resistance is not the same for all types of materials as some will have a very high value of resistance making them insulators, while others will have a very low value of resistance making them conductors. The resistance of any material is determined by the following factors:

The type of material from which it is made - wood, plastic, steel, copper etc

The cross-sectional area of the material - the larger its diameter the lower the resistance

The length of the material - the longer it is the higher is its resistance

The temperature of the material - with a positive temperature coefficient, the resistance will increase as the temperature increases

The amount of resistance, (R) that a material has to the flow of electric current is called resistivity and this is measured in Ohms per metre, (O/m). The inverse of resistivity is called conductance. The resistance of a material is directly proportional to its length, and inversely proportional to its cross-sectional area. So if the materials length is doubled its resistance doubles and if the materials diameter doubles its resistance halves.

A good conductor such as copper, aluminum or silver contains an abundance of free electrons within its lattice structure so current flows freely through them as these materials offer very little resistance, while other materials that oppose current flow have a higher resistance. Either way, the number of free electrons present in a materials crystalline structure determines the amount of opposition to current flow and therefore its electrical resistance.

Electrical Resistance and Resistors

Electrical resistance in a D.C. circuit is measured in OHMS (O) and a material will have a value of one ohm when a potential of one volt produces a current of one ampere. Then Ohms can also be considered as volts/amps. The more resistance a material or circuit possesses the less current will flow. Opposition to the flow of current in the form of resistance can sometimes be a bad thing as wires, cables and components become hot. This is because the current flowing through the resistive material produces a form of friction which in turn causes its energy to be dissipated in the form of heat.

Ohm's Law

Electrical components that use their resistivity to control the amount of electric current that flows through them are called Resistors. An ideal resistor in an electrical circuit is a device which produces a linear relationship between the voltage applied across it and the current flowing through it. These types of devices are classed as Ohmic as they produce a proportional relationship according to Ohm's law.

Ohm's law defines the fundamental relationship between voltage, current and resistance and this is given as: V = I x R

As well as limiting the flow of current through a circuit, there are many different uses for the humble resistor. Resistors can be used to set accurate operating current conditions or provide a voltage drop across themselves when a current ...