This chapter is about electricity, and we begin with the vocabulary the whole chapter rests on: charge, current, the ampere, a circuit, and the standard symbols. The next topics (potential difference, resistance, Ohm's law) all sit on top of these words — so getting them clean now saves you everywhere later. We will deliberately not develop voltage or resistance here; we only point at them. Today is about the flow itself.
1. Present the physical scene
Picture a torch — the kind almost every home keeps for power cuts. Inside it sit a cell (or two), a small bulb, and a sliding switch. Right now the switch is off and the bulb is dark.
You push the switch on. The bulb lights up — instantly. You push it off. Dark again. No delay either way.
Now picture the same torch with the back cap unscrewed so the cell is loose, not touching the metal strip. You push the switch. Nothing. The bulb stays dark, no matter how hard you press.
Stop scrolling. Try it in your head before reading on. In the working torch, what is the on-switch actually doing to the bulb? And why does a loose cell stop the bulb even when the switch is on?
(Answer: when the switch is on AND the cell touches its strips, there is an unbroken metal path from one end of the cell, through the wire, through the bulb, and back to the other end of the cell. Along that path tiny charged particles — electrons, the negatively charged particles inside the wire — start drifting. That moving charge is what heats the bulb's filament until it glows. The switch simply joins or breaks the path. A loose cell breaks the path at the cell itself, so even with the switch on there is no complete loop, and nothing flows.)
You can now name the scene we are about to explain: a bulb lights only when charge is flowing through it, and charge flows only when there is a complete, unbroken conducting path.