Switches and Controlling a Circuit

A switch is a gap in a circuit that you can open and close whenever you like.

• Switch closed → the loop is complete → electricity flows → the bulb lights. We say the circuit is closed, or on.
• Switch open → there is a gap in the loop → nothing flows → the bulb is dark. The circuit is open, or off.

A simple switch is nothing more than two pieces of metal (conductors) that you push together or pull apart. Every light switch in your house is exactly this, hidden behind a plastic cover.

You can also change how the circuit behaves:

• Add a second cell and the bulb glows brighter, because more energy is being pushed around the loop.
• Use a flat cell and the bulb glows dimly or not at all.

Worked example — the four-bulb loop.

A learner builds a single loop containing a cell and four bulbs in a row. All four glow. Then one bulb burns out — its thin wire inside snaps.

How many bulbs are still glowing? None. The burnt-out bulb has become a gap in the only loop there is, so the path is broken for every bulb at once.

This is why old Christmas lights would all go out when a single bulb failed. It is also why an electrician's first question is never "which appliance is broken?" but "where is the break?"