What is Electrical Resistance? Definition and Unit of Resistance

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.

what is electrical resistance?

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.

electrical resistance

When the voltage is applied in a circuit, the current flow through the resistance. Let the voltage across resistance is V and current flowing in the circuit is I. The voltage across the resistance is proportional to the flow of electric current. According to Ohms’ Law ;
 
  I
  RI
R = V/I
Where,
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

If one volt of potential difference is applied across the resistance cause 1 ampere current to flow through it, then the resistance  is equal to 1 unit. The unit of resistance is volt per ampere called ohm named after georg Ohm, The symbol of Ohm is Ω. The Ohm is the SI unit of the resistance.
unit of electrical resistance
 
R = V/I   ( Ohm’s Law)
 
R = 1 Volt/ 1 ampere

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 

The resistance of the substance either decrease or increase with an rise in the temperature. In metallic substance- like aluminum, copper, silver etc.- the resistance increases with an increase in the temperature. 
The resistance of the metal substance increase with rise in temperature because of the increased collision of the electrons with the electrons and the atom.The collision between the electrons and atom cause hindrance of the electrons movement and thus the resistance gets increased.The temperature coefficient of resistance of the metallic substance is positive.

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.


effect of temperature on resistance

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. 

effect of temperature on conductor resistance

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.

effect of temperature on semiconductor material

Insulating material :
The materials which offer very high resistance and restrict the flow of electrons are called insulating materials. The insulating material is used in electrical application for preventing the leakage current.The examples of the insulating materials are- rubber, paper, glass, wood,plastic, mica,porcelain,polyester, SF6, mineral oil,nitrogen gas etc.
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