What are the Laws of Resistance

In this article, we are going to discuss laws of resistance, factors affecting resistance, resistivity or specific resistance, and the unit of resistivity.

 Laws of Resistance

As we know resistance is the opposition to the flow of current.

There are four laws of resistance,

First Law

This law states that the resistance of a circuit is directly proportional to the length of the material. So, when the length of the material or conductor is increased, the resistance is also increased and when the length of that material has decreased the resistance is also decreased.

Mathematically,

Resistance ∝ Length

R ∝ L

 

Second Law

This law states that resistance is inversely proportional to the cross-sectional area of the conductor. It means that when the cross-sectional area of the conductor having the same length is increased, the resistance is decreased and when the cross-sectional area of the same conductor or material has decreased the resistance is increased.

Mathematically,

Resistance ∝ 1 / Cross-sectional area

R ∝ 1 / A

 

Third Law

This law states that the resistance of the conductor depends on the material of the conductor. It means that conductors made of copper, aluminum, and iron of the same length and cross-sectional will have different resistances as they are made of various substances.

 

Fourth Law

This law states that the temperature of the conductor also affects its resistance. Some conductor materials have a positive effect while others have a negative effect in their resistances with respect to temperature. Meaning that the resistance of some materials increases with the increase in temperature and the resistance of some materials decreases with the increase in temperature.

Note: In some cases, some other factors like tension and pressure also have an effect on resistance.    

 

Resistivity

By combining the first and second laws we get,

R ∝ L

R ∝ 1 / A

So,

R ∝ L / A

R = ρ L / A 

 

Where ρ (rho) is the proportionality constant and it is known as resistivity.

 

Resistivity is the specific resistance of the conductor’s material or substance. It is also known as the coefficient of resistance. The resistivity of a material can also be defined as the electrical resistance between opposite sides of a cube of a unit volume of that material.

 

Unit of Resistivity

The unit of resistivity can be easily determined,

Mathematically,

                                    Resistance ∝ Length / Cross-sectional Area

                                                  R ∝ L / A

                                                   R = ρ L / A

                                                   ρ = R A / L

                                                   ρ = Ω m2 / m

                                                   ρ = Ω m

So, the Ohm-meter is the unit of resistivity.

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