It may be defined as the property of a substance due to which it opposes the flow of electricity through it. Metals (as a class), acids and salts solutions are good conductors of electricity. Amongst pure metals, silver, copper and aluminium are very good conductors. This, as discussed earlier, is due to the presence of a large number of free or loosely-attached electrons in their atoms. These vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide and other atoms and electrons, thereby producing heat.
Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like bakelite, mica, glass, rubber, p.v.c. (polyvinyl chloride) and dry wood etc. Amongst good insulators can be included fibrous substances such as paper and cotton when dry, mineral oils free from acids and water, ceramics like hard porcelain and asbestos and many other plastics besides p.v.c. It is helpful to remember that electric friction is similar to friction in Mechanics.
The Unit of Resistance
The practical unit of resistance is ohm. A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals. For insulators whose resistances are very high, a much bigger unit is used i.e.,
mega-ohm = 106 ohm
kilo-ohm = 103 ohm
milli-ohm = 10-3 ohm
micro-ohm = 10-6 ohm are used.
The symbol for ohm is Ω.
Multiples and Sub-multiples of Ohm
Prefix Its meaning Abbreviation Equal to
Mega- One million =M Ω =106 Ω
Kilo - One thousand =k Ω =103 Ω
Milli- One thousandth =m Ω =10-3Ω
Micro- One millionth =μ Ω =10-6 Ω
Laws of Resistance-
The resistance R offered by a conductor depends on the following factors :
(i) It varies directly as its length, L.
(ii) It varies inversely as the cross-section A of the conductor.
(iii) It depends on the nature of the material.
(iv) It also depends on the temperature of the conductor.
Neglecting the last factor for the time being, we can say that
R ∝ L ..............(i)
R ∝1/A ..............(ii)
From equ. i & ii
R = ρ L/A ...(iii)
Here-
R=Resistance of conductor or material in Ohms
L=Length of conductor In meter or cm
A=Cross section area of conductor in meter square Or cm square
where ρ is a constant depending on the nature of the material of the conductor and is known as its specific resistance or resistivity.
