Short circuit

Curricula: KS3, cycle 4, Sek I

Open the full lab ↗

Aim

See what happens in a short circuit and how the protection works.

The bench

A 6 V source and a 6 V / 3 W lamp; an open switch is connected in parallel with the lamp. The lamp is lit.

Procedure

1. Close the switch.
2. Look at the lamp and at the source's power indicator.
3. Open the switch and switch the source on with the power button.

Expected result

The closed switch shorts the terminals of the lamp: the current bypasses the lamp, and it goes out. The current is limited only by the internal resistance of the source (0.5 Ω) and the cables, and reaches about 10 A. The source's overload protection switches it off at a current above 3 A: the power button switches off and the indicator flashes red. The source will work again only after it is switched on by hand.

Questions

1. Why is the current so large in a short circuit?
2. What device in a home's wiring plays the role of this protection?

Answers

1. The closed switch bypasses the lamp, whose hot filament of about 12 Ω kept the current near 0.5 A. What remains is the internal resistance of the source, 0.5 Ω, and the cables, so \(I = U / R\) gives at most 6 V / 0.5 Ω = 12 A, and about 10 A with the cables.
2. A circuit breaker in the distribution board, or a fuse in older wiring. Like the source's protection, a circuit breaker switches off at an overload or a short circuit and has to be switched on again by hand.