It is the flow of electrons.in metals only electrons can flow with minimum opposition,in metals we can easily take out the free electrons with minimum potential.While in case of semi conductors it is some what hard to take the valency electrons out,by adding some impurities we can improve conductivity of semi conductors.In case of insulators it was very hard to extract the electrons because of high bonding forces with nucleus.
i= q / v amps
current density is nothing but ratio of amount of charge flowing through conductor to area of cross section of conductor.
j=i / A =dq / dt /A amps/sec/square meter
current flow will depend on voltage across the conductor, temperature of conductor,specific resistance of conductor,length of conductor,area of cross section of conductor.
resistance = of ρ * l /a
where ρ = specific resistance,
l = length of conductor,
a= area of conductor,
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