100:40 is the ratio of their radii when iron and copper wires of equal length are found to have the same resistance.
Rcu=Rfe
ρl/Acu=ρl/Afe
ρcu=1.68*10^-8 And ρfe=10*10^-8
we get rFe/rCu=√(1.68/25)
=0.40
=100:40
Resistance is a property of a conductor whereby it opposes the flow of electric current through it. It is determined by the relationship between the applied voltage and the electric current that passes through it.
Conductors are substances with extremely low resistance, whereas insulators are substances with substantial resistance. The resistance increases as the current flow decreases. The type of material and its dimensions affect resistance (area of cross section)
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