Why does electric current flow into circuits?

It may not be correct to say that an electric current will "flow into" circuits. Electrons move in a circuit in response to an applied voltage. And these electrons are alread in the circuit and available to support current flow if a voltage is applied.

It may be more correct to say that electrons leave the negative terminal of a voltage source, and electrons enter the positive terminal of that voltage source. The electrons in the circuit that are availble to support current flow will "shift over" to create the current flow. Remember that the phenomenon of current flow in a wire is the "shifting over" of electrons in the wire. It's not about electrons going into one end of a wire and those same electrons coming out the other end.

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