What is Resistance : Definition, Formula and Laws

The flow of electrons in a material produces electricity. These electrons do not travel in a straight path but have to undergo collisions. Based on the amount of electricity the material allows to pass, all materials are categorized as Conductors, Semiconductors, and Insulators. Conductors allow the free flow of electricity. But in materials such as semiconductors and insulators, the electricity experiences a certain force that opposes the free flow of electrons. This force is named as resistance. There are different laws. The material whose property is used in a circuit is known as a Resistor. Resistors come in the form of various types and various materials. Various environmental factors also affect the resistance of the materials.

What is Resistance?

Definition: It is the opposition force experienced by the flowing electrons in some substances. this opposes the flow of electricity in a material. When a current of one ampere flows through a material that has a potential difference of one volt across it, then the resistance of that material is said to be one Ohm.

The basic law for measuring for this is Ohm’s Law. According to this law, the current flowing in a material is inversely proportional to its material when the voltage is constant. This law is expressed as V=IR, where V is the voltage or potential difference across the material, I is the current flowing through the material and R is the resistance offered by the material.

The SI unit of resistance is represented by a greek symbol Ω. Some materials with its properties are used in electrical circuits. These materials are known as Resistors. Resistors are available in different shapes and values. The resistance symbol of a resistor is given below.

Resistance Symbol
Resistance Symbol

The Resistance Formula to calculate the material can be derived from Ohm’s Law. As the electrical resistance of a material depends on the voltage across the material and the current flowing through the material, the formula for this can be given as the voltage drop across the material per unit ampere current flowing through it. i.e. R = V/I.

In DC electrical circuits when the current is doubled the resistance is halved and if this is doubled then the current is cut in half. This rule can also be seen in the low-frequency AC electrical circuitry such as our household systems. An increase in its value generates the heat thereby heating up the system and leading to damage if not checked regularly.


In electrical circuits when the resistors are connected in series the total resistance is calculated as the sum of all the individual resistors. For example when the three resistors with R1, R2, and R3 are connected in series then the total resistance of the circuit is given as  R = R1+R2+R3.

When resistors are connected in parallel then the total resistance is given as the sum of the reciprocals of the resistances. For example, when the three resistors with R1, R2 values, and R3 are connected in parallel the total resistance in the circuit is given as 1/R = 1/R1 +1/R2 +1/R3.

Laws of Resistance

The resistance of a material varies depending upon the properties of the material and environmental conditions. Laws of resistance gives the four factors where the material depends.

First Law

The First Law states that ” conductive material is directly proportional to the length of the material”. According to this law, the resistance of the material increases with the increase in the length of the material and decreases with the decrease in the length of the material. .i.e.

R ∝ L—–(1)

Second Law

The Second Law states that ” the conducting material is inversely proportional to the cross-sectional area of the material”. According to this law, its material increases with the decrease in the cross-sectional area of the conductor and decreases with an increase in the cross-sectional area. With this, we can conclude that a thin wire has a larger resistance value compared to a broad wire of a larger cross-sectional area. .i.e. R ∝ 1/A —-(2).

Third Law

The Third Law states that ” the conducting material depends on the nature of the material”. According to this law, the resistance value of the material varies depending upon the type of material. Two wires made up of different materials and having the same length and cross-sectional area will have different values. Some materials offer good electrical conducting are have lesser values.

Fourth Law

The Fourth Law states that “the conducting material depends on its temperature”. According to this law when the temperature of a metallic conductor is increased, it’s value also increases.

From the first, second and third law, the resistance of a material can be given as R ∝ L/A

i.e R = ρL/A

where ρ is known as the resistivity constant or the coefficient of resistance. It is also known as the specific resistance of the material. Its units are Ohm-meter. Thus, knowing the length, cross-sectional area and material of the wire, it can be calculated.

Silver is the best conductor but due to its high cost, it is not preferred for household circuitry. For most of the household applications, copper and aluminum wires are used as they are less expensive and also provides a suitable conductivity. Resistivity indicates the conducting ability of the material. An increase in temperature increases the resistivity values of the material. Thus resistivity depends on the electronic structure and temperature of the material.

The material with less resistance value offers good conductivity. Resistors are the common and highly used components of an electrical circuit. They are available with different values. Resistors available in the market have color bands or strips painted on them. The value of a resistor can be known by using these colored bands. Insulators are the materials that have the infinite resistance value thus no current flows through an insulator material. Calculate the resistance of a silver wire which has a potential difference of 500 volts and a current of 12 ampere flows through it.