What must occur before the current flows through a circuit?

As current is the rate of flow of electric charges

• As I=Q/t

so,there must be free electrons for the flow of electric current in a circuit.Then when voltage is applied at the terminals of circuit the free electrons acquire an average velocity called as drift velocity in the opposite direction to that of electric field (-E).Now the free electrons modify there random motion and a steady current begin to flow in a circuit.