Circuits That Work

In static electricity the charge sits still. In a circuit we give it a path and it flows — and a flow of charge is called a current.

A working circuit needs a complete, closed loop of conducting material from one end of the cell, through the components, and back to the other end. Break the loop anywhere and everything stops.

Conductors let charge pass through easily: copper, aluminium, steel, graphite (pencil lead), and salty water. Insulators do not: plastic, rubber, glass, dry wood, cloth and dry air. That is why a cable is copper on the inside and plastic on the outside — one carries the current, the other keeps it in and keeps you safe.

Worked example — a torch that will not light. Work through the loop in order:

1. Is the loop closed? Is the switch on? A switch is just a deliberate break in the circuit.
2. Is the cell flat? Swap it for a fresh one. Are the cells the right way round — every $+$ end must face the next cell's $-$ end?
3. Is the globe blown? Look for a broken filament, or try a globe you know works.
4. Are the contacts clean and touching? Rust, dust or a loose spring is an invisible break in the loop.

Almost every dead torch on Earth is one of those four faults — and every one of them is the same fault really: the loop is not complete, or the energy source is finished.