Closed and open circuit

Curricula: NGSS 4, KS2 (Year 4), cycle 3, Sek I

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Aim

Make sure that current flows and the lamp lights only in a closed circuit.

The bench

A 4.5 V DC voltage source, a switch (closed) and a 4.5 V / 1 W lamp, connected in a single circuit. The lamp is lit.

Procedure

1. Move the switch lever to open the circuit.
2. Close the switch again.
3. Turn the source off with the power button and turn it on again.

Expected result

The lamp is lit only when the switch is closed and the source is on. A break anywhere in the circuit puts the lamp out.

Questions

1. Why does the lamp go out even though the source stays on?
2. Where else in the circuit could a switch be placed to control the lamp in the same way?

Answers

1. The open switch makes a gap in the circuit. Current flows only around a closed loop, so with the gap no current reaches the lamp and it goes out.
2. Anywhere in the same loop: on the wire from the source to the lamp, or on the wire from the lamp back to the source. The circuit has only one path, so a gap at any point stops the current.