Definition of Resistance
The property of the substance to resist the flow of electric current through it is known as resistance. The conductors have free electrons moving randomly inside the metal, and when the voltage is applied the electrons starts moving from the lower potential point to the higher potential point.
During drifting of the electrons, the collision between the electrons and atom and molecule of the conductor takes place, and this collision impede the path of flow of electrons. This impediment of flow of electron caused by collision of electrons with atom and molecule is called resistance.
V ∝ RI
R = V/I
V – Voltage
I – Current
R – Resistance of the substance
Hence, the resistance of the substance is defined as ratio of applied voltage to the current through the substance.
Unit of Resistance
R =1 Ohm (Ω)
Different Resistance Units
The larger units of resistance is Kilo-Ohm, Mega-Ohm and Giga-Ohm. The smaller units of resistance are milli Ohm, Micro Ohm and nano Ohm.
The relationship between different resistance units is as given below.
1 Giga Ohm (GΩ) = 109 Ω
1 Mega Ohm (M Ω) = 106 Ω
1 Kilo Ohm (K Ω ) = 103 Ω
1 Milli Ohm (m Ω) = 10-3 Ω
1 Micro Ohm (μΩ ) = 10-6 Ω
1 Nano Ohm (nΩ ) = 10-9 Ω
Effect of Temperature on Resistance
In semiconductor like germanium and silicon, the numbers of free electron in the valence band increase because of breaking of covalent bond at increased temperature. Thus the more electrons from the valence band reach in the conduction band. As a result, the resistance of the semiconductor material decrease with an increase in the temperature.
Similarly the resistance of the insulating material decrease with increase in temperature. The below given graph shown the relationship between the change in the resistance and temperature rise for three categories of the material widely used for electrical application.
Resistance of the Different Materials
Substances can be divided into the following categories based on the resistance;
Electrical Resistance of Conducting material :
The metallic substances offers very little resistance to the current flowing through them and these substances are called conductors. The silver and copper has much less resistance than the resistance of the aluminum. The aluminum is used widely in the electrical applications because of its lower cost and lower specific weight. The metallic substances have positive temperature coefficient of resistance. The resistance of metals increase linearly with an increase in temperature as shown in the below given graph.
Semiconductor material :
The material offer moderate resistance is called semiconductor. The examples are germanium, silicon. The semiconductor material is negative temperature coefficient of resistance. The resistance of the semiconductor substances decrease with an increase in temperature. The resistivity of the semiconductor decrease exponentially with increase in temperature as shown in the below given graph.