Electrical Resistance- Definition, Unit, Formula

The property of the material that opposes the flow of electric current is called electrical resistance. When voltage is applied to the material, the flow of electrons through the material takes place. The number of electrons passing through the material varies and depends on the material’s electrical conductivity.

The metals have lesser resistance towards the flow of electrons compared to insulating material. The electrons in the metals are loosely bound, and they get released easily with the application of low voltage. In an insulator, these are tightly bound, and a lot of energy is required to release the electrons.

How does Electrical Resistance Oppose Electric Current?

When voltage is applied, the free electrons in the metal start moving from lower potential to higher potential, and during drifting motion, electrons collide with each other and with the atom of the substance. This collision causes hindrance in the free movement of electrons. The impediment in the flow of electrons causes resistance.

Thus, the resistance can be defined as a material property that opposes the flow of electrons. The flow of current is just opposite to the flow of electrons. In other words, we can say the material property that opposes the flow of electric current is called the resistance.


Greek letter(Ω). The symbol Ohms is named after Georg Simon Ohm (1784-1854), a German physicist who formulated Ohm’s law after studying the relationship between voltage, current, and resistance.

Metals like silver, gold, aluminum, and copper are good conductors of electricity because they offer less resistance. Insulators like rubber, paper, and glass are bad conductors of electricity because of higher resistance. The lower the resistance, the higher the current flow, and the higher the resistance lower the current flow. The current flowing through the material is inversely proportional to its resistance.

Unit of Electrical Resistance

The unit of resistance is Ohm(Ω). According to Ohm’s Law;

R = V/I

When 1 volt is applied to the circuit, it causes 1-ampere current to flow in the circuit, then the resistance of the circuit is 1Ω.

R = V/I
= 1/1
R= 1Ω

The resistance can also be expressed in the lower and bigger units of Ohm. These are milli-ohm, micro-ohm, kilo-ohm, mega-ohm, giga-ohm and tera-ohm.

UnitAbbreviationValue in Ohm(Ω)
Nano-Ohm10-9 Ω
Micro-OhmμΩ10-6 Ω
Milli-Ohm10-3 Ω
Kilo-Ohm103 Ω
Mega-Ohm106 Ω
Giga-Ohm109 Ω
Tera-Ohm1012 Ω

Resistance of different materials is unlike and depends upon the shape and material properties.

Resistance of Different Materials

The materials are divided into three categories depending on the electrical resistance value.

  1. Conducting Material: Metals like silver, gold, aluminum, and copper offer very low resistance and are good conductors of electricity. The silver is the best conductor, but it is not used because of its higher cost. Aluminum is widely used as a conductor because of its lower cost and availability. For transmission lines, aluminum is used because of its lower specific weight. Copper is mainly used for winding electrical machines and for electric bus bars. The copper is costlier than aluminum.
  2. Semiconductor Materials: The other class of material is semiconductors. The semiconductor has higher electrical resistance than the conductor and lower resistance than the insulator. The semiconductor material is widely used in electronics, power, and communication circuits.
  3. Insulating Materials: The third category of material is insulators. Insulating materials have very high resistance, and materials like paper, wood, glass, PVC, XLPE, SF6, Vacuum, etc., are bad conductors of electricity. The insulators are used in electrical networks to impede the flow of electric current.

Effect of Temperature on Electrical Resistance

The different materials, conductors, semiconductors, and insulators have different electrical resistance behaviors with increased temperature.

The resistance of metals increased with an increase in the temperature. With increased temperature, the collisions of the electrons with the atom increase and offers more resistance at higher temperature. The conducting material has a positive temperature coefficient of resistance. The losses in the conductor increase with an increase in temperature.

The resistance of the semiconductor material decreases with an increase in the temperature. At increased temperatures, the number of free carriers increases, and as a result, the resistance decreases. The semiconductor material has a negative temperature coefficient of resistance. The reliable working of the semiconductor devices can be ensured by maintaining the lower surrounding temperature.

The electrical resistance of the insulating material decreases with an increase in temperature. The insulating material has a negative temperature coefficient of resistance.

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