What is a Electrical Resistance? Definition, Unit, Formula, Measurement

When an electric current flows through a bulb or any conductor, the conductor offers some obstruction to the current and this obstruction is known as electrical resistance and is denoted by R. Every material has an electrical resistance and this is the reason why conductors give out heat when current passes through it. In the next few sections, let us study electrical resistance in detail.

What is Electrical Resistance?

According to Ohm’s law, there is a relation between the current flowing through a conductor and the potential difference across it. It is given by,

 I V = IR


V is the potential difference measured across the conductor (in volts)I is the current through the conductor (in amperes)R is the constant of proportionality called resistance (in ohms)

The electrical resistance of a circuit is the ratio between the voltage applied to the current flowing through it.

Rearranging the above relation,

R = [latex] \frac VI [/latex]

The unit of electrical resistance is ohms.

ohm = [latex] \frac {1~ volt}{ 1 ~ampere}[/latex]Electric charge flows easily through some materials than others. The electrical resistance measures how much the flow of this electric charge is restricted within the circuit.

Factors Affecting Electrical Resistance

The electrical resistance of a conductor is dependent on the following factors:

  • The cross-sectional area of the conductor
  • Length of the conductor
  • The material of the conductor
  • The temperature of the conducting material

Electrical resistance is directly proportional to length (L) of the conductor and inversely proportional to the cross-sectional area (A). It is given by the following relation.

R = [latex] \frac{ρL}{A}[/latex],

where ρ is the resistivity of the material (measured in Ωm, ohm meter)

Resistivity is a qualitative measurement of a material’s ability to resist flowing electric current. Obviously, insulators will have a higher value of resistivity than that of conductors. The resistivities of a few materials are given below for a comparison. Materials with a low value of resistivity conduct electricity very well.

  • Silver – 1.00×10−8
  • Copper – 1.68×10−8
  • Aluminium – 2.82×10−8
  • Wood – 1.00×1014
  • Air – 2.30×1016
  • Teflon – 1.00×1023

What Is Resistivity?

Electric resistivity is defined as the electrical resistance offered per unit length and unit cross-sectional area at a specific temperature and is denoted by ρ. Electrical resistance is also known as specific electrical resistance. The SI unit of electrical resistivity is ohms.metre. Following is the formula of electrical resistivity:

[latex]\rho =\frac{E}{J}[/latex]


  • ρ is the resistivity of the material in Ω.m
  • E is the electric field in V.m-1
  • J is the current density in A.m-2

Difference Between Resistance And Resistivity

DefinitionWhen the flow of electrons is opposed in a material is known as resistanceWhen resistance is offered
FormulaR = [latex] \frac VI [/latex][latex]\rho =\frac{E}{J}[/latex]
SI unitOhmsOhms.metre
DependenceDependent on the length and cross-sectional area of the conductor and temperatureTemperature

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