Definition: Resistance (Represented by R) offers hindrance to the flow of current in an electrical circuit. It is measured in ohms, which is named after its developer George Simon Ohm. He is a German physicist, who has studied the relationship between voltage, current and resistance and tells us that when one volt of voltage is applied across the component and one ampere of current is passed through it, then the resistance offered by that component is one ohm. The symbol used for resistance is the Greek letter omega (Ω).
The resistance of an object can be stated as the ratio of the potential difference applied across the terminal to the current applied.
Here you can also say that current is inversely proportional to the resistance, i.e. when the current in the circuit gets doubled, then the resistance offered is half and if the resistance is doubled, then the current applied is half.
All materials resist the flow of current. So they can be categorized into two broad terms:
Conductors: These are those materials which offer very less resistance and electrons can move easily in the substance.
Ex:- copper (Co), silver (Ag), Gold (Au) etc.
Insulators: these are those materials that offer high resistance to the substance and electrons do not able to move easily or it resists the flow of electrons.
Ex: rubber, wood, plastic etc.
Laws of Resistance
The resistance of a substance depends upon the four factors- length, cross-sectional area, the nature of the material and the temperature of the substance.
The resistivity or specific resistance can be measured from the 4 laws of resistance which are as follows:
1st law of Resistance
This law states that resistance is directly proportional to the length of the substance or you can say that when the length of the substance increases, then its resistance also increases and when the length of the substance decreases, then its resistance also decreases. The reason behind this is when the length gets increased, then the electrons will travel more paths and the chances of collision get increased and as a result, the number of electron passing through the conductor get reduced, thus, the current is reduced. It can also be written as
Where L is the length of the conductor
2nd law of Resistance
This law states that the resistance of a conductor is inversely proportional to the area of cross-section. It can also be written as
Where ‘A’ is the area of cross-section.
You can say that if the area of cross-section is more, then more electrons will pass, which causes more current through the conductor which means that less resistance is offered. Thus, when an area of cross-section is more, then there is less resistance and vice-versa.
After combining 1st two laws, the resistance can be written as
Where ρ is the constant of proportionality and is called as specific resistance or resistivity. The letter ρ is pronounced as rho.
3rd law of Resistance
The resistivity of all materials is not same. The relationship between resistivity and resistance is direct proportionality, i.e. if the resistivity of the material is large, then the resistance offered by the material is also more and vice-versa is also true.
4th law of Resistance
This law also states that the temperature of the substance will affect the resistance offered by the substance. As we know that, more temperature causes more vibration between electrons because the electrons will gain energy and become excited, which leads to the great obstruction among electrons? So here we can say that as the temperature increases, the resistance offered by the substance also increases and vice-versa.