A path for electric current
An electric bulb glows only when electric current has a complete path to follow. This path is an electric circuit. A simple circuit can contain a cell, connecting wires, a bulb and a switch.
The current travels through the circuit when all its parts are joined correctly. A gap anywhere in the path stops the current.
The electric cell
An electric cell changes stored chemical energy into electrical energy. It has two terminals:
the small raised metal cap is the positive terminal, marked
+,the flat metal disc is the negative terminal, marked
−.
The two terminals must be connected through a circuit. Joining them directly with a wire creates a short circuit. The wire and cell can become hot, so this must never be done.
Two or more cells connected together form a battery. Cells must be placed in the correct direction inside a torch or another device.
How a bulb glows
Inside a small electric bulb is a very thin wire called the filament. The filament becomes hot and gives out light when current passes through it. The two ends of the filament connect to the bulb's two terminals: the metal tip at the bottom and the metal casing at the side.
If the filament breaks, the path inside the bulb is incomplete. Such a fused bulb cannot glow even when the rest of the circuit is correct.
Open and closed circuits
Circuit | Path | Result |
|---|---|---|
Closed circuit | Complete, with no gap | Current flows and the bulb glows |
Open circuit | Broken by a gap | Current does not flow |
A switch controls the gap. An ON switch closes the circuit; an OFF switch opens it. A simple switch can be made using two drawing pins and a metal safety pin fixed to a board.
closed: cell ─── switch ON ─── bulb ─── back to cell
open: cell ─── switch OFF bulb ─── back to cellRemember. A bulb can be connected in either direction, but both of its terminals must be part of the complete circuit.
Conductors and insulators
A material that allows electric current to pass through it is a conductor. Metals such as copper, aluminium and iron are good conductors. This is why connecting wires have a metal core.
A material that does not allow current to pass through it easily is an insulator. Rubber, dry wood, glass and most plastics are insulators. The plastic covering of a wire protects us from touching its conducting metal core.
To test an object, place it in the gap of a working circuit. If the bulb glows, the object conducts electricity. First check that the cell and bulb work; otherwise a dark bulb does not prove that the object is an insulator.
Using electricity safely
Use only cells for classroom circuit activities.
Never connect wires or objects to a wall socket.
Never touch electrical switches with wet hands.
Do not use a cell that is leaking or damaged.
Ask an adult to replace damaged wires or plugs.
Summary
A closed circuit provides a complete path for current; an open circuit contains a gap.
A cell has positive and negative terminals, and a bulb has two terminals joined to its filament.
A switch opens or closes a circuit.
Metals are usually conductors, while rubber and plastic are commonly used as insulators.