A basic electrical circuit consists of a battery, a light bulb, a switch and wires connecting them. As soon as the switch is closed, even the light bulb is one mile away, the light bulb will be on IMMEDIATELY. This means that the electrical current (made of moving electrons) starts to run everywhere in the wire as soon as the switch is closed. Explain:

a) the origin of these free moving electrons (where do they come from),
b) the force(s) that pushes free electrons moving towards the same direction (to form electrical current),
c) the average speed of these free electron (when the circuit is closed).