The one-way lamp

The idea

Current passes through an LED in one direction only, so it glows only when its positive terminal (longer wire) is connected to the battery's positive terminal and its negative terminal (shorter wire) to the negative terminal.

A fussy lamp

Unlike the old lamp, an LED has a longer wire (+) and a shorter wire (–). Suppose you join the longer wire to the cell's negative terminal.

Will it glow?

• Yes, the same as the old lamp
• No, it stays dark
• Only a faint glow

Both ways round

The LED is joined to the cell. Use the turn the LED round button and watch the picture and the message.

One-way traffic

Current passes through an LED in one direction only, so it glows only when its positive terminal (longer wire) is connected to the battery's positive terminal and its negative terminal (shorter wire) to the negative terminal.

Always take care to connect an LED correctly in a circuit to make it glow.

• LED's longer wire joined to | LED
• the cell's positive terminal | glows
• the cell's negative terminal | stays dark

Notes

An LED lets current pass in one direction only. It glows only when its longer wire goes to the positive terminal and its shorter wire to the negative terminal.

Check yourself

An LED's longer wire is joined to the cell's negative terminal. Will it glow?

Why does an LED stay dark when joined the wrong way round?

Which lamp glows whichever way round it is joined, if the circuit is complete?

To make an LED glow, join its longer wire to the cell's _ and its shorter wire to the cell's _.

• Yes. Current cannot pass the LED this way round.
• No — correct. Yes! Current passes through an LED in one direction only.
• Only if the cell is new. A new cell does not change the direction rule.
• Current passes through an LED in one direction only — correct. Yes! The wrong way round, no current passes.
• Its filament breaks. An LED has no filament.
• The cell has no energy. The cell is fine. The LED blocks the current this way.
• An LED. An LED glows only when it is joined the right way round.
• An incandescent lamp — correct. Yes! It does not matter which terminal is joined to the +.
• Neither. The incandescent lamp glows either way round.
• negative terminal, positive terminal. That is the wrong way round. The LED stays dark.
• positive terminal, negative terminal — correct. Yes! Longer wire to +, shorter wire to –.
• positive terminal, positive terminal. The two wires must go to different terminals.