The loop that lights a lamp

The idea

Connecting each terminal of a lamp to a terminal of a cell forms an electrical circuit, a complete path for electric current; the lamp glows only when current passes through the circuit.

Cell, lamp, wires

You have a cell, a lamp and some wires. The lamp has two terminals and so does the cell.

How many wires do you think you need to make the lamp glow using the cell and wires?

• One wire
• Two wires
• Three wires

Join them up

Tap a terminal, then another one, to join them with a wire. Tap a wired terminal to take its wire off. Find a way to make the lamp glow.

A complete path

Wherever you put the wires, the lamp glowed only when both of its terminals were joined, each to a different terminal of the cell. The wires, lamp and cell make a closed loop.

Connecting each terminal of a lamp to a terminal of a cell forms an electrical circuit, a complete path for electric current. The lamp glows only when current passes through the circuit.

One wire is not enough. Break the loop anywhere and the glow is gone.

Notes

A lamp joined to a cell, terminal to terminal, forms an electrical circuit: a complete path for current. The lamp glows only when current passes through the circuit.

Check yourself

A lamp is joined to only the positive terminal of a cell by one wire. Will it glow?

What is an electrical circuit?

Which wiring makes the lamp glow?

The lamp glows only when _ passes through the circuit.

• Yes, one wire is enough. With only one wire there is no complete path back to the cell.
• No, the path is not complete — correct. Yes! The current needs a complete path, out of the cell and back into it.
• Only if the cell is new. A new cell does not help: the path is still open.
• A complete path for electric current — correct. Yes! A closed loop through the cell and the lamp.
• A wire that never ends. The wire can be short. What matters is that the path is complete.
• A kind of cell. A cell is only one part of the circuit.
• Both lamp terminals joined to the cell's + terminal. Both wires end at the same cell terminal. The path never returns to the cell's other terminal.
• One lamp terminal to the cell's +, the other lamp terminal to the cell's – — correct. Yes! This gives a complete path.
• Both lamp terminals joined to the cell's – terminal. Both wires end at the same cell terminal, so there is no complete path.
• water. Water has nothing to do with it.
• electric current — correct. Yes! No current, no glow.
• light from the room. The lamp makes light; it does not need light to glow.