-Resistance is a property of conductor due to which it resists the flow of electric current through it. Component that is used to resist the flow of electric current in a circuit is called resistor.
Flow of electrons in a conductor is electric current.
-The particles of conductor create hindrance to flow of electrons because of attraction between them. This hindrance is the cause of resistance in the flow of electricity.
-Variable Resistance: The component of an electric circuit which is used to regulate the current; without changing the voltage from the source; is called variable resistance.
-Rheostat: This is a device which is used in a circuit to provide variable resistance.
Factors that affect resistance:-
-Resistance in a conductor depends on nature, length and area of cross section of the conductor.
Nature of material: Some materials create least hindrance and hence are called good conductors. Silver is the best conductor of electricity. While some other materials create more hindrance in the flow of electric current, i.e. flow of electrons through them. Such materials are called bad conductors. Bad conductors are also known as insulators. Hard plastic is the one of the best insulators of electricity.
Length of conductor: Resistance R is directly proportional to the length of the conductor. This means, Resistance increases with increase in length of the conductor. This is the cause that long electric wires create more resistance to the electric current.
Thus, Resistance (R) ∝ length of conductor (l)
Area of cross section: Resistance R is inversely proportional to the area of cross section ( A) of the conductor. This means R will decrease with increase in the area of conductor and vice versa. More area of conductor facilitates the flow of electric current through more area and thus decreases the resistance. This is the cause that thick copper wire creates less resistance to the electric current.
Thus, resistance∝1Area of cross section of conductor (A)
Or, R∝`1/"A"` --------(ii)
From equation (i) and (ii)
Or, R=`ρ"l"/"A"` -----------(iii)
Where ρ (rho) is the proportionality constant. It is called the electrical resistivity of the material of conductors.
From equation (iii)
Or, ρ=`"RA"/"l"` ----------(iv)
The SI unit of resistivity: Since, the SI unit of R is Ω, SI unit of Area is m2 and SI unit of length is m. Hence
Unit of resistivity (ρ) = `Ω xx (m^2)/(m) =Ωm`
-It is a characteristic property of the material. The metals and alloys have very low resistivity in the range of 10–8 Ω m to 10–6 Ω m. They are good conductors of electricity.
-Insulators like rubber and glass have resistivity of the order of 1012 to 1017 Ω m. Both the resistance and resistivity of a material vary with temperature.