Electricity is the flow of charge carriers in a circuit, through a transmission medium. The charge carriers are free valence electrons, whose conduction determines whether we have flow of current or not. When an electric field is applied, the electrons gain energy to move from the valence band to the conduction band, and thus the flow of electricity takes place. In the case of metals, the valence and conduction bands overlap and valence electrons move freely, but a mobile flow is established on application of an external electric field. In the case of insulators, electrons are unable to move to the conduction band (because the bands are already completely filled) and hence the circuit is not complete. Thus, conduction is essential for an electric circuit.
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