Electricity:- Part 3

On the basis of electrical resistance, all the substances divided into three groups:- 

* Good conductors- Those substances which have very low electrical resistance is called good conductors. 

  E.g. copper, aluminium, etc.

* Resistors- Those substances which have comparatively high electrical resistance is called resistors. 

  E.g. alloys like Nichrome, Manganin, and Constantan (or Eureka) etc. 

* Insulators- Those substances which have infinitely high electrical resistance is called insulators. 

  E.g. rubber, wood, diamond, glass, etc. 

* Ohmic resistance- The resistance of such materials which obey ohm's law called Ohmic resistance. 

  E.g. resistance of Manganin wire, incandescent light bulbs, resistors, etc.

* Non-Ohmic resistance- the resistance of such materials which do not obey ohm's law are called Non-Ohmic resistance.

  E.g. resistance of diode, the resistance of GaAs( gallium arsenide), vacuum tubes, semiconductors etc. 

The resistance of different materials 

-> Resistance is directly proportional to the length of the conductor 

           i.e. R ∝ l --- (I) 

-> Resistance is inversely proportional to the cross-sectional area of the conductor             i.e. R ∝ 1/A ---(ii)

Combining equation (i) & (ii), We get 

           R ∝ l/A 

           R = ρl/A   where ρ = resistivity of the material and ρ is read as “rho”.

* Nature of conductor- The electrical resistance of a conductor also depends on the nature of the conductor. 

  e.g. We take similar wires having equal length and diameter of copper metal and nichrome alloy, then we find the resistance of nichrome wire is about 60 times more than the resistance of copper wire. 

* Temperature- The resistance of all pure metals increases on increasing the temperature and decreases on lowering the temperature. 

Note- A thick wire has less resistance and a thin wire has more resistance.

-> The resistance of a wire is inversely proportional to the square of the diameter. 

As we know that

                      

Conductance  

-> The reciprocal of resistance of a conductor is called conductance. 

So,

                    

       

Thus, Resistivity is defined as the resistance offered by a wire of that material of unit length and unit cross-sectional area. Its SI unit is Ωm. 

Note- The resistivity of substance does not depend on its length and thickness. 

 it depends on the nature of the substance and temperature. A good conductor has less resistivity, whereas a bad conductor or insulator has high resistivity.

Q) Why Alloys are used in electric toaster & heater and pure metals don’t? 

-> The resistivity of an alloy is much higher than that of pure metal So it doesn't oxidise (or burn) readily at high temperature (red hot). Thus, alloys are used in electric toaster & heater and pure metals don’t. 

* Tungsten is used in ancient lamps or bulbs.

* Silver has the minimum value of resistivity, therefore, it is the best conductor of electricity. 

* Specific conductance or Conductivity- The reciprocal of resistivity of a conductor is called conductivity. its SI unit is mho/m or mho m^-1 or siemen/m or siemen m^-1. 

Q) What does it mean the resistivity of copper is 1.69 × 10^-8 Ωm? 

-> It means that when we take a copper wire or rod of length is 1 m long and the area of cross-section is 1 m² then its resistance will be 1.69 × 10^-8 Ω.

Combination of resistance 

-> When two or more resistance are connected ends to end consecutively, called

 connected in series.

 

                    

-> When two or more resistance are connected between the same point called connected in parallel.

                  

The formula of Series Combination 

-> In series combination, the current passes through the different resistances are the same but the voltage is different. 

The formula of Parallel Combination 

-> In Parallel combination, the current passes through the different resistances are different but the voltage is the same.